-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE2 man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. Neither has
the pcre2demo program. There are separate text files for the pcre2grep and
pcre2test commands.
-----------------------------------------------------------------------------


PCRE2(3)                   Library Functions Manual                   PCRE2(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


INTRODUCTION

       PCRE2 is the name used for a revised API for the PCRE library, which is
       a  set  of  functions,  written in C, that implement regular expression
       pattern matching using the same syntax and semantics as Perl, with just
       a few differences. After nearly two decades,  the  limitations  of  the
       original  API  were  making development increasingly difficult. The new
       API is more extensible, and it was simplified by abolishing  the  sepa-
       rate  "study" optimizing function; in PCRE2, patterns are automatically
       optimized where possible. Since forking from PCRE1, the code  has  been
       extensively  refactored and new features introduced. The old library is
       now obsolete and is no longer maintained.

       As well as Perl-style regular expression patterns, some  features  that
       appeared  in  Python and the original PCRE before they appeared in Perl
       are available using the Python syntax. There is also support  for  some
       .NET  and  Oniguruma syntax items, and there are options for requesting
       minor changes that give better ECMAScript (JavaScript) compatibility.

       The source code for PCRE2 can be compiled to support strings of  8-bit,
       16-bit, or 32-bit code units, which means that up to three separate li-
       braries  may  be  installed, one for each code unit size. The size of a
       code unit is not related to the bit size of the underlying hardware. In
       a 64-bit environment that also supports 32-bit  applications,  versions
       of  PCRE2  that  are  compiled  in  both 64-bit and 32-bit modes may be
       needed.

       The original work to extend PCRE to 16-bit and 32-bit  code  units  was
       done by Zoltan Herczeg and Christian Persch, respectively. In all three
       cases,  strings  can  be  interpreted  either as one character per code
       unit, or as UTF-encoded Unicode, with support for Unicode general cate-
       gory properties. Unicode support is optional at build time (but is  the
       default). However, processing strings as UTF code units must be enabled
       explicitly at run time. The version of Unicode in use can be discovered
       by running

         pcre2test -C

       The  three  libraries  contain  identical sets of functions, with names
       ending in _8,  _16,  or  _32,  respectively  (for  example,  pcre2_com-
       pile_8()).  However,  by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or
       32, a program that uses just one code unit width can be  written  using
       generic names such as pcre2_compile(), and the documentation is written
       assuming that this is the case.

       In addition to the Perl-compatible matching function, PCRE2 contains an
       alternative  function that matches the same compiled patterns in a dif-
       ferent way. In certain circumstances, the alternative function has some
       advantages.  For a discussion of the two matching algorithms,  see  the
       pcre2matching page.

       Details  of  exactly which Perl regular expression features are and are
       not supported by  PCRE2  are  given  in  separate  documents.  See  the
       pcre2pattern  and  pcre2compat  pages. There is a syntax summary in the
       pcre2syntax page.

       Some features of PCRE2 can be included, excluded, or changed  when  the
       library  is  built. The pcre2_config() function makes it possible for a
       client to discover which features are  available.  The  features  them-
       selves are described in the pcre2build page. Documentation about build-
       ing  PCRE2 for various operating systems can be found in the README and
       NON-AUTOTOOLS-BUILD files in the source distribution.

       The libraries contains a number of undocumented internal functions  and
       data  tables  that  are  used by more than one of the exported external
       functions, but which are not intended  for  use  by  external  callers.
       Their  names  all begin with "_pcre2", which hopefully will not provoke
       any name clashes. In some environments, it is possible to control which
       external symbols are exported when a shared library is  built,  and  in
       these cases the undocumented symbols are not exported.


SECURITY CONSIDERATIONS

       If  you  are using PCRE2 in a non-UTF application that permits users to
       supply arbitrary patterns for compilation, you should  be  aware  of  a
       feature that allows users to turn on UTF support from within a pattern.
       For  example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8
       mode, which interprets patterns and subjects as strings of  UTF-8  code
       units instead of individual 8-bit characters. This causes both the pat-
       tern  and  any data against which it is matched to be checked for UTF-8
       validity. If the data string is very long, such a check might use  suf-
       ficiently  many  resources as to cause your application to lose perfor-
       mance.

       One way of guarding against this possibility is to use  the  pcre2_pat-
       tern_info()  function  to  check  the  compiled  pattern's  options for
       PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF  option  when
       calling  pcre2_compile().  This causes a compile time error if the pat-
       tern contains a UTF-setting sequence.

       The use of Unicode properties for character types such as \d  can  also
       be  enabled  from within the pattern, by specifying "(*UCP)". This fea-
       ture can be disallowed by setting the PCRE2_NEVER_UCP option.

       If your application is one that supports UTF, be  aware  that  validity
       checking  can  take time. If the same data string is to be matched many
       times, you can use the PCRE2_NO_UTF_CHECK option  for  the  second  and
       subsequent matches to avoid running redundant checks.

       The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead
       to  problems,  because  it  may leave the current matching point in the
       middle of a multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C  op-
       tion can be used by an application to lock out the use of \C, causing a
       compile-time  error  if it is encountered. It is also possible to build
       PCRE2 with the use of \C permanently disabled.

       Another way that performance can be hit is by running  a  pattern  that
       has  a  very  large search tree against a string that will never match.
       Nested unlimited repeats in a pattern are a common example. PCRE2  pro-
       vides  some  protection  against  this: see the pcre2_set_match_limit()
       function in the pcre2api page.  There  is  a  similar  function  called
       pcre2_set_depth_limit() that can be used to restrict the amount of mem-
       ory that is used.


USER DOCUMENTATION

       The  user  documentation for PCRE2 comprises a number of different sec-
       tions. In the "man" format, each of these is a separate "man page".  In
       the  HTML  format, each is a separate page, linked from the index page.
       In the plain  text  format,  the  descriptions  of  the  pcre2grep  and
       pcre2test programs are in files called pcre2grep.txt and pcre2test.txt,
       respectively.  The remaining sections, except for the pcre2demo section
       (which is a program listing), and the short pages for individual  func-
       tions,  are  concatenated in pcre2.txt, for ease of searching. The sec-
       tions are as follows:

         pcre2              this document
         pcre2-config       show PCRE2 installation configuration information
         pcre2api           details of PCRE2's native C API
         pcre2build         building PCRE2
         pcre2callout       details of the pattern callout feature
         pcre2compat        discussion of Perl compatibility
         pcre2convert       details of pattern conversion functions
         pcre2demo          a demonstration C program that uses PCRE2
         pcre2grep          description of the pcre2grep command (8-bit only)
         pcre2jit           discussion of just-in-time optimization support
         pcre2limits        details of size and other limits
         pcre2matching      discussion of the two matching algorithms
         pcre2partial       details of the partial matching facility
         pcre2pattern       syntax and semantics of supported regular
                              expression patterns
         pcre2perform       discussion of performance issues
         pcre2posix         the POSIX-compatible C API for the 8-bit library
         pcre2sample        discussion of the pcre2demo program
         pcre2serialize     details of pattern serialization
         pcre2syntax        quick syntax reference
         pcre2test          description of the pcre2test command
         pcre2unicode       discussion of Unicode and UTF support

       In the "man" and HTML formats, there is also a short page  for  each  C
       library function, listing its arguments and results.


AUTHORS

       The  current  maintainers  of PCRE2 are Nicholas Wilson and Zoltan Her-
       czeg.

       PCRE2 was written by Philip Hazel, of the University Computing Service,
       Cambridge, England. Many others have also contributed.

       To contact the maintainers, please use the  GitHub  issues  tracker  or
       PCRE2    mailing    list,   as   described   at   the   project   page:
       https://github.com/PCRE2Project/pcre2


REVISION

       Last updated: 22 February 2025
       Copyright (c) 1997-2021 University of Cambridge.


PCRE2 10.48                    22 February 2025                       PCRE2(3)
------------------------------------------------------------------------------


PCRE2API(3)                Library Functions Manual                PCRE2API(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

       #include <pcre2.h>

       PCRE2  is  a  new API for PCRE, starting at release 10.0. This document
       contains a description of all its native functions. See the pcre2 docu-
       ment for an overview of all the PCRE2 documentation.


PCRE2 NATIVE API BASIC FUNCTIONS

       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
         pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize,
         pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
         const pcre2_code *code, pcre2_general_context *gcontext);

       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext,
         int *workspace, PCRE2_SIZE wscount);

       void pcre2_match_data_free(pcre2_match_data *match_data);


PCRE2 NATIVE API AUXILIARY MATCH FUNCTIONS

       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_match_data_size(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_match_data_heapframes_size(
         pcre2_match_data *match_data);

       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);


PCRE2 NATIVE API GENERAL CONTEXT FUNCTIONS

       pcre2_general_context *pcre2_general_context_create(
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       pcre2_general_context *pcre2_general_context_copy(
         pcre2_general_context *gcontext);

       void pcre2_general_context_free(pcre2_general_context *gcontext);


PCRE2 NATIVE API COMPILE CONTEXT FUNCTIONS

       pcre2_compile_context *pcre2_compile_context_create(
         pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
         pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       int pcre2_set_bsr(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
         const uint8_t *tables);

       int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext,
         uint32_t extra_options);

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
         PCRE2_SIZE value);

       int pcre2_set_max_pattern_compiled_length(
         pcre2_compile_context *ccontext, PCRE2_SIZE value);

       int pcre2_set_max_varlookbehind(pcre2_compile_contest *ccontext,
         uint32_t value);

       int pcre2_set_newline(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
         int (*guard_function)(uint32_t, void *), void *user_data);

       int pcre2_set_optimize(pcre2_compile_context *ccontext,
         uint32_t directive);


PCRE2 NATIVE API MATCH CONTEXT FUNCTIONS

       pcre2_match_context *pcre2_match_context_create(
         pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
         pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       int pcre2_set_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_callout_block *, void *),
         void *callout_data);

       int pcre2_set_substitute_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_substitute_callout_block *, void *),
         void *callout_data);

       int pcre2_set_substitute_case_callout(pcre2_match_context *mcontext,
         PCRE2_SIZE (*callout_function)(PCRE2_SPTR, PCRE2_SIZE,
                                        PCRE2_UCHAR *, PCRE2_SIZE,
                                        int, void *),
         void *callout_data);

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
         PCRE2_SIZE value);

       int pcre2_set_heap_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_depth_limit(pcre2_match_context *mcontext,
         uint32_t value);


PCRE2 NATIVE API STRING EXTRACTION FUNCTIONS

       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR *buffer,
         PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR **bufferptr,
         PCRE2_SIZE *bufflen);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_SIZE *length);

       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_SIZE *length);

       int pcre2_substring_nametable_scan(const pcre2_code *code,
         PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);

       int pcre2_substring_number_from_name(const pcre2_code *code,
         PCRE2_SPTR name);

       void pcre2_substring_list_free(PCRE2_UCHAR **list);

       int pcre2_substring_list_get(pcre2_match_data *match_data,
         PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);


PCRE2 NATIVE API STRING SUBSTITUTION FUNCTION

       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext, PCRE2_SPTR replacement,
         PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer,
         PCRE2_SIZE *outlengthptr);


PCRE2 NATIVE API JIT FUNCTIONS

       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(size_t startsize,
         size_t maxsize, pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
         pcre2_jit_callback callback_function, void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);


PCRE2 NATIVE API SERIALIZATION FUNCTIONS

       int32_t pcre2_serialize_decode(pcre2_code **codes,
         int32_t number_of_codes, const uint8_t *bytes,
         pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(const pcre2_code **codes,
         int32_t number_of_codes, uint8_t **serialized_bytes,
         PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);


PCRE2 NATIVE API AUXILIARY FUNCTIONS

       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);

       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
         PCRE2_SIZE bufflen);

       const uint8_t *pcre2_maketables(pcre2_general_context *gcontext);

       void pcre2_maketables_free(pcre2_general_context *gcontext,
         const uint8_t *tables);

       int pcre2_pattern_info(const pcre2_code *code, uint32_t what,
         void *where);

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       int pcre2_config(uint32_t what, void *where);


PCRE2 NATIVE API OBSOLETE FUNCTIONS

       int pcre2_set_recursion_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_recursion_memory_management(
         pcre2_match_context *mcontext,
         void *(*private_malloc)(size_t, void *),
         void (*private_free)(void *, void *), void *memory_data);

       These functions became obsolete at release 10.30 and are retained  only
       for  backward  compatibility.  They should not be used in new code. The
       first is replaced by pcre2_set_depth_limit(); the second is  no  longer
       needed and has no effect (it always returns zero).


PCRE2 EXPERIMENTAL PATTERN CONVERSION FUNCTIONS

       pcre2_convert_context *pcre2_convert_context_create(
         pcre2_general_context *gcontext);

       pcre2_convert_context *pcre2_convert_context_copy(
         pcre2_convert_context *cvcontext);

       void pcre2_convert_context_free(pcre2_convert_context *cvcontext);

       int pcre2_set_glob_escape(pcre2_convert_context *cvcontext,
         uint32_t escape_char);

       int pcre2_set_glob_separator(pcre2_convert_context *cvcontext,
         uint32_t separator_char);

       int pcre2_pattern_convert(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, PCRE2_UCHAR **buffer,
         PCRE2_SIZE *blength, pcre2_convert_context *cvcontext);

       void pcre2_converted_pattern_free(PCRE2_UCHAR *converted_pattern);

       These  functions  provide  a  way of converting non-PCRE2 patterns into
       patterns that can be processed by pcre2_compile(). This facility is ex-
       perimental and may be changed in future releases. At  present,  "globs"
       and  POSIX  basic  and  extended patterns can be converted. Details are
       given in the pcre2convert documentation.


PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES

       There are three PCRE2 libraries, supporting 8-bit, 16-bit,  and  32-bit
       code  units,  respectively.  However,  there  is  just one header file,
       pcre2.h.  This contains the function prototypes and  other  definitions
       for all three libraries. One, two, or all three can be installed simul-
       taneously.  On  Unix-like  systems the libraries are called libpcre2-8,
       libpcre2-16, and libpcre2-32, and they can also co-exist with the orig-
       inal PCRE libraries.  Every PCRE2 function  comes  in  three  different
       forms, one for each library, for example:

         pcre2_compile_8()
         pcre2_compile_16()
         pcre2_compile_32()

       There are also three different sets of data types:

         PCRE2_UCHAR8, PCRE2_UCHAR16, PCRE2_UCHAR32
         PCRE2_SPTR8,  PCRE2_SPTR16,  PCRE2_SPTR32

       The  UCHAR  types define unsigned code units of the appropriate widths.
       For example, PCRE2_UCHAR16 is usually defined as `uint16_t'.  The  SPTR
       types are pointers to constants of the equivalent UCHAR types, that is,
       they are pointers to vectors of unsigned code units.

       Character  strings  are  passed  to a PCRE2 library as sequences of un-
       signed integers in code units of the appropriate width. The length of a
       string may be given as a number of code units, or  the  string  may  be
       specified as zero-terminated.

       Many  applications use only one code unit width. For their convenience,
       macros are defined whose names are the generic forms such as pcre2_com-
       pile() and  PCRE2_SPTR.  These  macros  use  the  value  of  the  macro
       PCRE2_CODE_UNIT_WIDTH  to generate the appropriate width-specific func-
       tion and macro names.  PCRE2_CODE_UNIT_WIDTH is not defined by default.
       An application must define it to be  8,  16,  or  32  before  including
       pcre2.h in order to make use of the generic names.

       Applications  that use more than one code unit width can be linked with
       more than one PCRE2 library, but must define  PCRE2_CODE_UNIT_WIDTH  to
       be  0  before  including pcre2.h, and then use the real function names.
       Any code that is to be included in an environment where  the  value  of
       PCRE2_CODE_UNIT_WIDTH  is  unknown  should  also  use the real function
       names. (Unfortunately, it is not possible in C code to save and restore
       the value of a macro.)

       If PCRE2_CODE_UNIT_WIDTH is not defined  before  including  pcre2.h,  a
       compiler error occurs.

       When  using  multiple  libraries  in an application, you must take care
       when processing any particular pattern to use  only  functions  from  a
       single  library.   For example, if you want to run a match using a pat-
       tern that was compiled with pcre2_compile_16(), you  must  do  so  with
       pcre2_match_16(), not pcre2_match_8() or pcre2_match_32().

       In  the  function summaries above, and in the rest of this document and
       other PCRE2 documents, functions and data  types  are  described  using
       their generic names, without the _8, _16, or _32 suffix.


PCRE2 API OVERVIEW

       PCRE2  has  its  own  native  API, which is described in this document.
       There are also some wrapper functions for the 8-bit library that corre-
       spond to the POSIX regular expression API, but they do not give  access
       to  all  the  functionality of PCRE2 and they are not thread-safe. They
       are described in the pcre2posix documentation. Both these APIs define a
       set of C function calls.

       The native API C data types, function prototypes,  option  values,  and
       error codes are defined in the header file pcre2.h, which also contains
       definitions of PCRE2_MAJOR and PCRE2_MINOR, the major and minor release
       numbers  for the library. Applications can use these to include support
       for different releases of PCRE2.

       In a Windows environment, if you want to statically link an application
       program against a non-dll PCRE2 library, you must  define  PCRE2_STATIC
       before including pcre2.h.

       The  functions pcre2_compile() and pcre2_match() are used for compiling
       and matching regular expressions in a Perl-compatible manner. A  sample
       program that demonstrates the simplest way of using them is provided in
       the file called pcre2demo.c in the PCRE2 source distribution. A listing
       of  this  program  is  given  in  the  pcre2demo documentation, and the
       pcre2sample documentation describes how to compile and run it.

       The compiling and matching functions recognize various options that are
       passed as bits in an options argument. There are also some more compli-
       cated parameters such as custom memory  management  functions  and  re-
       source  limits  that  are  passed  in "contexts" (which are just memory
       blocks, described below). Simple applications do not need to  make  use
       of contexts.

       Just-in-time  (JIT)  compiler  support  is an optional feature of PCRE2
       that can be built in  appropriate  hardware  environments.  It  greatly
       speeds  up  the matching performance of many patterns. Programs can re-
       quest that it be used if available by calling pcre2_jit_compile() after
       a pattern has been successfully compiled by pcre2_compile(). This  does
       nothing if JIT support is not available.

       More  complicated  programs  might  need  to make use of the specialist
       functions   pcre2_jit_stack_create(),    pcre2_jit_stack_free(),    and
       pcre2_jit_stack_assign()  in order to control the JIT code's memory us-
       age.

       JIT matching is automatically used by pcre2_match() if it is available,
       unless the PCRE2_NO_JIT option is set. There is also a direct interface
       for JIT matching, which gives improved performance at  the  expense  of
       less  sanity  checking. The JIT-specific functions are discussed in the
       pcre2jit documentation.

       A second matching function, pcre2_dfa_match(), which is  not  Perl-com-
       patible,  is  also  provided.  This  uses a different algorithm for the
       matching. The alternative algorithm finds all possible  matches  (at  a
       given  point  in  the subject), and scans the subject just once (unless
       there are lookaround assertions). However, this algorithm does not  re-
       turn  captured substrings. A description of the two matching algorithms
       and their advantages and disadvantages is given  in  the  pcre2matching
       documentation. There is no JIT support for pcre2_dfa_match().

       In  addition  to  the  main compiling and matching functions, there are
       convenience functions for extracting captured substrings from a subject
       string that has been matched by pcre2_match(). They are:

         pcre2_substring_copy_byname()
         pcre2_substring_copy_bynumber()
         pcre2_substring_get_byname()
         pcre2_substring_get_bynumber()
         pcre2_substring_list_get()
         pcre2_substring_length_byname()
         pcre2_substring_length_bynumber()
         pcre2_substring_nametable_scan()
         pcre2_substring_number_from_name()

       pcre2_substring_free() and pcre2_substring_list_free()  are  also  pro-
       vided,  to  free  memory used for extracted strings. If either of these
       functions is called with a NULL argument, the function returns  immedi-
       ately without doing anything.

       The  function  pcre2_substitute()  can be called to match a pattern and
       return a copy of the subject string with substitutions for  parts  that
       were matched.

       Functions  whose  names begin with pcre2_serialize_ are used for saving
       compiled patterns on disc or elsewhere, and reloading them later.

       Finally, there are functions for finding out information about  a  com-
       piled  pattern  (pcre2_pattern_info()) and about the configuration with
       which PCRE2 was built (pcre2_config()) and that it is using.

       Functions with names ending with _free() are used  for  freeing  memory
       blocks  of  various  sorts.  In all cases, if one of these functions is
       called with a NULL argument, it does nothing.


STRING LENGTHS AND OFFSETS

       The PCRE2 API uses string lengths and  offsets  into  strings  of  code
       units  in  several  places. These values are always of type PCRE2_SIZE,
       which is an unsigned integer type, currently always defined as  size_t.
       The  largest  value  that  can  be  stored  in  such  a  type  (that is
       ~(PCRE2_SIZE)0) is reserved as a special indicator for  zero-terminated
       strings  and  unset offsets.  Therefore, the longest string that can be
       handled is one less than this maximum. Note that string lengths are al-
       ways given in code units. Only in the 8-bit library is  such  a  length
       the same as the number of bytes in the string.


NEWLINES

       PCRE2 supports five different conventions for indicating line breaks in
       strings:  a  single  CR (carriage return) character, a single LF (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The Unicode newline  sequences
       are  the  three just mentioned, plus the single characters VT (vertical
       tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each of the first three conventions is used by at least  one  operating
       system as its standard newline sequence. When PCRE2 is built, a default
       can be specified.  If it is not, the default is set to LF, which is the
       Unix standard. However, the newline convention can be changed by an ap-
       plication  when calling pcre2_compile(), or it can be specified by spe-
       cial text at the start of the pattern itself; this overrides any  other
       settings.  See the pcre2pattern page for details of the special charac-
       ter sequences.

       In the PCRE2 documentation the word "newline"  is  used  to  mean  "the
       character or pair of characters that indicate a line break". The choice
       of  newline convention affects the handling of the dot, circumflex, and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF is a recognized line ending sequence, the match position  advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre2_match() options below.

       The  choice of newline convention does not affect the interpretation of
       the \n or \r escape sequences, nor does it affect what \R matches; this
       has its own separate convention.


MULTITHREADING

       In a multithreaded application it is important to keep  thread-specific
       data  separate  from data that can be shared between threads. The PCRE2
       library code itself is thread-safe: it contains  no  static  or  global
       variables. The API is designed to be fairly simple for non-threaded ap-
       plications  while at the same time ensuring that multithreaded applica-
       tions can use it.

       There are several different blocks of data that are used to pass infor-
       mation between the application and the PCRE2 libraries.

   The compiled pattern

       A pointer to the compiled form of a pattern is  returned  to  the  user
       when pcre2_compile() is successful. The data in the compiled pattern is
       fixed,  and  does not change when the pattern is matched. Therefore, it
       is thread-safe, that is, the same compiled pattern can be used by  more
       than one thread simultaneously. For example, an application can compile
       all its patterns at the start, before forking off multiple threads that
       use  them.  However,  if the just-in-time (JIT) optimization feature is
       being used, it needs separate memory stack areas for each  thread.  See
       the pcre2jit documentation for more details.

       In  a more complicated situation, where patterns are compiled only when
       they are first needed, but are still shared between  threads,  pointers
       to  compiled  patterns  must  be protected from simultaneous writing by
       multiple threads. This is somewhat tricky to do correctly. If you  know
       that  writing  to  a pointer is atomic in your environment, you can use
       logic like this:

         Get a read-only (shared) lock (mutex) for pointer
         if (pointer == NULL)
           {
           Get a write (unique) lock for pointer
           if (pointer == NULL) pointer = pcre2_compile(...
           }
         Release the lock
         Use pointer in pcre2_match()

       Of course, testing for compilation errors should also  be  included  in
       the code.

       The  reason  for checking the pointer a second time is as follows: Sev-
       eral threads may have acquired the shared lock and tested  the  pointer
       for being NULL, but only one of them will be given the write lock, with
       the  rest kept waiting. The winning thread will compile the pattern and
       store the result.  After this thread releases the write  lock,  another
       thread  will  get it, and if it does not retest pointer for being NULL,
       will recompile the pattern and overwrite the pointer, creating a memory
       leak and possibly causing other issues.

       In an environment where writing to a pointer may  not  be  atomic,  the
       above  logic  is not sufficient. The thread that is doing the compiling
       may be descheduled after writing only part of the pointer, which  could
       cause  other  threads  to use an invalid value. Instead of checking the
       pointer itself, a separate "pointer is valid" flag (that can be updated
       atomically) must be used:

         Get a read-only (shared) lock (mutex) for pointer
         if (!pointer_is_valid)
           {
           Get a write (unique) lock for pointer
           if (!pointer_is_valid)
             {
             pointer = pcre2_compile(...
             pointer_is_valid = TRUE
             }
           }
         Release the lock
         Use pointer in pcre2_match()

       If JIT is being used, but the JIT compilation is not being done immedi-
       ately (perhaps waiting to see if the pattern  is  used  often  enough),
       similar  logic  is required. JIT compilation updates a value within the
       compiled code block, so a thread must gain unique write access  to  the
       pointer     before    calling    pcre2_jit_compile().    Alternatively,
       pcre2_code_copy() or pcre2_code_copy_with_tables() can be used  to  ob-
       tain  a  private  copy of the compiled code before calling the JIT com-
       piler.

   Context blocks

       The next main section below introduces the idea of "contexts" in  which
       PCRE2 functions are called. A context is nothing more than a collection
       of parameters that control the way PCRE2 operates. Grouping a number of
       parameters together in a context is a convenient way of passing them to
       a  PCRE2  function without using lots of arguments. The parameters that
       are stored in contexts are in some sense  "advanced  features"  of  the
       API. Many straightforward applications will not need to use contexts.

       In a multithreaded application, if the parameters in a context are val-
       ues  that  are  never  changed, the same context can be used by all the
       threads. However, if any thread needs to change any value in a context,
       it must make its own thread-specific copy.

   Match blocks

       The matching functions need a block of memory for storing  the  results
       of a match. This includes details of what was matched, as well as addi-
       tional  information  such as the name of a (*MARK) setting. Each thread
       must provide its own copy of this memory.


PCRE2 CONTEXTS

       Some PCRE2 functions have a lot of parameters, many of which  are  used
       only  by  specialist  applications,  for example, those that use custom
       memory management or non-standard character tables.  To  keep  function
       argument  lists  at a reasonable size, and at the same time to keep the
       API extensible, "uncommon" parameters are passed to  certain  functions
       in  a  context instead of directly. A context is just a block of memory
       that holds the parameter values.  Applications that do not need to  ad-
       just any of the context parameters can pass NULL when a context pointer
       is required.

       There  are  three different types of context: a general context that is
       relevant for several PCRE2 operations, a compile-time  context,  and  a
       match-time context.

   The general context

       At  present,  this context just contains pointers to (and data for) ex-
       ternal memory management functions that are called from several  places
       in  the  PCRE2  library.  The  context  is  named `general' rather than
       specifically `memory' because in future other fields may be  added.  If
       you  do not want to supply your own custom memory management functions,
       you do not need to bother with a general context. A general context  is
       created by:

       pcre2_general_context *pcre2_general_context_create(
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       The  two  function pointers specify custom memory management functions,
       whose prototypes are:

         void *private_malloc(PCRE2_SIZE, void *);
         void  private_free(void *, void *);

       Whenever code in PCRE2 calls these functions, the final argument is the
       value of memory_data. Either of the first two arguments of the creation
       function may be NULL, in which case the system memory management  func-
       tions  malloc()  and free() are used. (This is not currently useful, as
       there are no other fields in a general context,  but  in  future  there
       might  be.)  The private_malloc() function is used (if supplied) to ob-
       tain memory for storing the context, and all three values are saved  as
       part of the context.

       Whenever  PCRE2  creates a data block of any kind, the block contains a
       pointer to the free() function that matches the malloc() function  that
       was  used.  When  the  time  comes  to free the block, this function is
       called.

       A general context can be copied by calling:

       pcre2_general_context *pcre2_general_context_copy(
         pcre2_general_context *gcontext);

       The memory used for a general context should be freed by calling:

       void pcre2_general_context_free(pcre2_general_context *gcontext);

       If this function is passed a  NULL  argument,  it  returns  immediately
       without doing anything.

   The compile context

       A  compile context is required if you want to provide an external func-
       tion for stack checking during compilation or  to  change  the  default
       values of any of the following compile-time parameters:

         What \R matches (Unicode newlines or CR, LF, CRLF only)
         PCRE2's character tables
         The newline character sequence
         The compile time nested parentheses limit
         The maximum length of the pattern string
         The extra options bits (none set by default)
         Which performance optimizations the compiler should apply

       A  compile context is also required if you are using custom memory man-
       agement.  If none of these apply, just pass NULL as the  context  argu-
       ment of pcre2_compile().

       A  compile context is created, copied, and freed by the following func-
       tions:

       pcre2_compile_context *pcre2_compile_context_create(
         pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
         pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       A compile context is created with default values  for  its  parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       int pcre2_set_bsr(pcre2_compile_context *ccontext,
         uint32_t value);

       The  value  must  be PCRE2_BSR_ANYCRLF, to specify that \R matches only
       CR, LF, or CRLF, or PCRE2_BSR_UNICODE, to specify that \R  matches  any
       Unicode line ending sequence. The value is used by the JIT compiler and
       by   the   two   interpreted   matching  functions,  pcre2_match()  and
       pcre2_dfa_match().

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
         const uint8_t *tables);

       The value must be the result of a  call  to  pcre2_maketables(),  whose
       only argument is a general context. This function builds a set of char-
       acter tables in the current locale.

       int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext,
         uint32_t extra_options);

       As  PCRE2  has developed, almost all the 32 option bits that are avail-
       able in the options argument of pcre2_compile() have been used  up.  To
       avoid  running  out, the compile context contains a set of extra option
       bits which are used for some newer, assumed rarer, options. This  func-
       tion  sets  those bits. It always sets all the bits (either on or off).
       It does not modify any existing setting. The available options are  de-
       fined in the section entitled "Extra compile options" below.

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
         PCRE2_SIZE value);

       This  sets a maximum length, in code units, for any pattern string that
       is compiled with this context. If the pattern is longer,  an  error  is
       generated.   This facility is provided so that applications that accept
       patterns from external sources can limit their size. The default is the
       largest number that a PCRE2_SIZE variable can  hold,  which  is  effec-
       tively unlimited.

       int pcre2_set_max_pattern_compiled_length(
         pcre2_compile_context *ccontext, PCRE2_SIZE value);

       This  sets  a maximum size, in bytes, for the memory needed to hold the
       compiled version of a pattern that is compiled with  this  context.  If
       the  pattern needs more memory, an error is generated. This facility is
       provided so  that  applications  that  accept  patterns  from  external
       sources  can  limit  the  amount of memory they use. The default is the
       largest number that a PCRE2_SIZE variable can  hold,  which  is  effec-
       tively unlimited.

       int pcre2_set_max_varlookbehind(pcre2_compile_contest *ccontext,
         uint32_t value);

       This  sets  a  maximum length for the number of characters matched by a
       variable-length lookbehind assertion. The default is set when PCRE2  is
       built,  with  the ultimate default being 255, the same as Perl. Lookbe-
       hind assertions without a bounding length are not supported.

       int pcre2_set_newline(pcre2_compile_context *ccontext,
         uint32_t value);

       This specifies which characters or character sequences are to be recog-
       nized as newlines. The value must be one of PCRE2_NEWLINE_CR  (carriage
       return only), PCRE2_NEWLINE_LF (linefeed only), PCRE2_NEWLINE_CRLF (the
       two-character  sequence  CR followed by LF), PCRE2_NEWLINE_ANYCRLF (any
       of the above), PCRE2_NEWLINE_ANY (any  Unicode  newline  sequence),  or
       PCRE2_NEWLINE_NUL (the NUL character, that is a binary zero).

       A pattern can override the value set in the compile context by starting
       with a sequence such as (*CRLF). See the pcre2pattern page for details.

       When  a  pattern  is  compiled  with  the  PCRE2_EXTENDED  or PCRE2_EX-
       TENDED_MORE option, the newline convention affects the  recognition  of
       the  end  of internal comments starting with #. The value is saved with
       the compiled pattern for subsequent use by the JIT compiler and by  the
       two     interpreted     matching     functions,    pcre2_match()    and
       pcre2_dfa_match().

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
         uint32_t value);

       This parameter adjusts the limit, set  when  PCRE2  is  built  (default
       250),  on  the  depth  of  parenthesis nesting in a pattern. This limit
       stops rogue patterns using up too much system  stack  when  being  com-
       piled.  The limit applies to parentheses of all kinds, not just captur-
       ing parentheses.

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
         int (*guard_function)(uint32_t, void *), void *user_data);

       There is at least one application that runs PCRE2 in threads with  very
       limited  system  stack,  where running out of stack is to be avoided at
       all costs. The parenthesis limit above cannot take account of how  much
       stack  is  actually  available during compilation. For a finer control,
       you can supply a  function  that  is  called  whenever  pcre2_compile()
       starts  to compile a parenthesized part of a pattern. This function can
       check the actual stack size (or anything else  that  it  wants  to,  of
       course).

       The  first  argument to the callout function gives the current depth of
       nesting, and the second is user data that is set up by the  last  argu-
       ment   of  pcre2_set_compile_recursion_guard().  The  callout  function
       should return zero if all is well, or non-zero to force an error.

       int pcre2_set_optimize(pcre2_compile_context *ccontext,
         uint32_t directive);

       PCRE2 can apply various performance optimizations  during  compilation,
       in  order to make matching faster. For example, the compiler might con-
       vert  some  regex  constructs  into  an  equivalent   construct   which
       pcre2_match()  can  execute faster. By default, all available optimiza-
       tions are enabled. However, in rare cases, one might  wish  to  disable
       specific optimizations. For example, if it is known that some optimiza-
       tions  cannot benefit a certain regex, it might be desirable to disable
       them, in order to speed up compilation.

       The permitted values of directive are as follows:

         PCRE2_OPTIMIZATION_FULL

       Enable all optional performance  optimizations.  This  is  the  default
       value.

         PCRE2_OPTIMIZATION_NONE

       Disable all optional performance optimizations.

         PCRE2_AUTO_POSSESS
         PCRE2_AUTO_POSSESS_OFF

       Enable/disable  "auto-possessification" of variable quantifiers such as
       * and +.  This optimization, for example, turns a+b into a++b in  order
       to  avoid  backtracks into a+ that can never be successful. However, if
       callouts are in use, auto-possessification means that some callouts are
       never taken. You can disable this optimization if you want the matching
       functions to do a full, unoptimized search and run all the callouts.

         PCRE2_DOTSTAR_ANCHOR
         PCRE2_DOTSTAR_ANCHOR_OFF

       Enable/disable an optimization that is applied when  .*  is  the  first
       significant  item in a top-level branch of a pattern, and all the other
       branches also start with .* or with \A or \G or ^. Such  a  pattern  is
       automatically  anchored if PCRE2_DOTALL is set for all the .* items and
       PCRE2_MULTILINE is not set for any ^ items. Otherwise,  the  fact  that
       any  match must start either at the start of the subject or following a
       newline is remembered. Like other optimizations, this can  cause  call-
       outs to be skipped.

       Dotstar  anchor  optimization is automatically disabled for .* if it is
       inside an atomic group or a capture group that  is  the  subject  of  a
       backreference, or if the pattern contains (*PRUNE) or (*SKIP).

         PCRE2_START_OPTIMIZE
         PCRE2_START_OPTIMIZE_OFF

       Enable/disable optimizations which cause matching functions to scan the
       subject string for specific code unit values before attempting a match.
       For  example, if it is known that an unanchored match must start with a
       specific value, the matching code searches the subject for that  value,
       and  fails  immediately  if it cannot find it, without actually running
       the main matching function. This means that  a  special  item  such  as
       (*COMMIT)  at  the  start  of a pattern is not considered until after a
       suitable starting point for the match has been found.  Also, when call-
       outs or (*MARK) items are in use, these  "start-up"  optimizations  can
       cause  them  to  be  skipped if the pattern is never actually used. The
       start-up optimizations are in effect a pre-scan  of  the  subject  that
       takes place before the pattern is run.

       Disabling start-up optimizations ensures that in cases where the result
       is  "no match", the callouts do occur, and that items such as (*COMMIT)
       and (*MARK) are considered at every possible starting position  in  the
       subject string.

       Disabling  start-up  optimizations may change the outcome of a matching
       operation.  Consider the pattern

         (*COMMIT)ABC

       When this is compiled, PCRE2 records the fact that a match  must  start
       with  the  character  "A".  Suppose the subject string is "DEFABC". The
       start-up optimization scans along the subject, finds "A" and  runs  the
       first  match attempt from there. The (*COMMIT) item means that the pat-
       tern must match the current starting position, which in this  case,  it
       does. However, if the same match is run without start-up optimizations,
       the  initial  scan  along the subject string does not happen. The first
       match attempt is run starting from "D" and when this  fails,  (*COMMIT)
       prevents  any further matches being tried, so the overall result is "no
       match".

       Another start-up optimization makes use  of  a  minimum  length  for  a
       matching subject, which is recorded when possible. Consider the pattern

         (*MARK:1)B(*MARK:2)(X|Y)

       The  minimum  length  for  a match is two characters. If the subject is
       "XXBB", the "starting character" optimization skips "XX", then tries to
       match "BB", which is long enough. In the process, (*MARK:2) is  encoun-
       tered  and  remembered.  When  the match attempt fails, the next "B" is
       found, but there is only one character left, so there are no  more  at-
       tempts,  and  "no  match"  is returned with the "last mark seen" set to
       "2". Without start-up optimizations,  however,  matches  are  tried  at
       every  possible starting position, including at the end of the subject,
       where (*MARK:1) is encountered, but there is no "B", so the "last  mark
       seen"  that  is returned is "1". In this case, the optimizations do not
       affect the overall match result, which is still "no match", but they do
       affect the auxiliary information that is returned.

   The match context

       A match context is required if you want to:

         Set up a callout function
         Set an offset limit for matching an unanchored pattern
         Change the limit on the amount of heap used when matching
         Change the backtracking match limit
         Change the backtracking depth limit
         Set custom memory management specifically for the match

       If none of these apply, just pass  NULL  as  the  context  argument  of
       pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match().

       A  match  context  is created, copied, and freed by the following func-
       tions:

       pcre2_match_context *pcre2_match_context_create(
         pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
         pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       A match context is created with  default  values  for  its  parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       int pcre2_set_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_callout_block *, void *),
         void *callout_data);

       This  sets  up a callout function for PCRE2 to call at specified points
       during a matching operation. Details are given in the pcre2callout doc-
       umentation.

       int pcre2_set_substitute_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_substitute_callout_block *, void *),
         void *callout_data);

       This sets up a callout function for PCRE2 to call after each  substitu-
       tion made by pcre2_substitute(). Details are given in the section enti-
       tled "Creating a new string with substitutions" below.

       int pcre2_set_substitute_case_callout(pcre2_match_context *mcontext,
         PCRE2_SIZE (*callout_function)(PCRE2_SPTR, PCRE2_SIZE,
                                        PCRE2_UCHAR *, PCRE2_SIZE,
                                        int, void *),
         void *callout_data);

       This  sets up a callout function for PCRE2 to call when performing case
       transformations inside pcre2_substitute(). Details  are  given  in  the
       section entitled "Creating a new string with substitutions" below.

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
         PCRE2_SIZE value);

       The  offset_limit parameter limits how far an unanchored search can ad-
       vance in the subject string. The  default  value  is  PCRE2_UNSET.  The
       pcre2_match()  and  pcre2_dfa_match()  functions return PCRE2_ERROR_NO-
       MATCH if a match with a starting point before or at the given offset is
       not found. The pcre2_substitute() function makes no more substitutions.

       For example, if the pattern /abc/ is matched against "123abc"  with  an
       offset  limit  less  than 3, the result is PCRE2_ERROR_NOMATCH. A match
       can never be  found  if  the  startoffset  argument  of  pcre2_match(),
       pcre2_dfa_match(),  or  pcre2_substitute()  is  greater than the offset
       limit set in the match context.

       When using this facility, you must set the  PCRE2_USE_OFFSET_LIMIT  op-
       tion when calling pcre2_compile() so that when JIT is in use, different
       code  can  be  compiled. If a match is started with a non-default match
       limit when PCRE2_USE_OFFSET_LIMIT is not set, an error is generated.

       The offset limit facility can be used to track progress when  searching
       large  subject  strings or to limit the extent of global substitutions.
       See also the PCRE2_FIRSTLINE option, which requires a  match  to  start
       before  or  at  the first newline that follows the start of matching in
       the subject. If this is set with an offset limit, a match must occur in
       the first line and also  within  the  offset  limit.  In  other  words,
       whichever limit comes first is used.

       int pcre2_set_heap_limit(pcre2_match_context *mcontext,
         uint32_t value);

       The heap_limit parameter specifies, in units of kibibytes (1024 bytes),
       the  maximum  amount  of heap memory that pcre2_match() may use to hold
       backtracking information when running an interpretive match. This limit
       also applies to pcre2_dfa_match(), which may use the heap when process-
       ing patterns with a lot of nested pattern recursion or  lookarounds  or
       atomic groups. This limit does not apply to matching with the JIT opti-
       mization,  which  has  its  own  memory  control  arrangements (see the
       pcre2jit documentation for more details). If the limit is reached,  the
       negative  error  code  PCRE2_ERROR_HEAPLIMIT  is  returned. The default
       limit can be set when PCRE2 is built; if it is not, the default is  set
       very large and is essentially unlimited.

       A value for the heap limit may also be supplied by an item at the start
       of a pattern of the form

         (*LIMIT_HEAP=ddd)

       where  ddd  is a decimal number. However, such a setting is ignored un-
       less ddd is less than the limit set by the caller of pcre2_match()  or,
       if no such limit is set, less than the default.

       The  pcre2_match() function always needs some heap memory, so setting a
       value of zero guarantees a "heap limit exceeded" error. Details of  how
       pcre2_match()  uses  the  heap are given in the pcre2perform documenta-
       tion.

       For pcre2_dfa_match(), a vector on the system stack is used  when  pro-
       cessing  pattern recursions, lookarounds, or atomic groups, and only if
       this is not big enough is heap memory used. In  this  case,  setting  a
       value of zero disables the use of the heap.

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
         uint32_t value);

       The match_limit parameter provides a means of preventing PCRE2 from us-
       ing  up  too many computing resources when processing patterns that are
       not going to match, but which have a very large number of possibilities
       in their search trees. The classic  example  is  a  pattern  that  uses
       nested unlimited repeats.

       There  is an internal counter in pcre2_match() that is incremented each
       time round its main matching loop. If  this  value  reaches  the  match
       limit, pcre2_match() returns the negative value PCRE2_ERROR_MATCHLIMIT.
       This  has  the  effect  of limiting the amount of backtracking that can
       take place. For patterns that are not anchored, the count restarts from
       zero for each position in the subject string. This limit  also  applies
       to pcre2_dfa_match(), though the counting is done in a different way.

       When  pcre2_match()  is  called  with  a  pattern that was successfully
       processed by pcre2_jit_compile(), the way in which matching is executed
       is entirely different. However, there is still the possibility of  run-
       away matching that goes on for a very long time, and so the match_limit
       value  is  also used in this case (but in a different way) to limit how
       long the matching can continue.

       The default value for the limit can be set when PCRE2 is built; the de-
       fault is 10 million, which handles all but the most  extreme  cases.  A
       value  for the match limit may also be supplied by an item at the start
       of a pattern of the form

         (*LIMIT_MATCH=ddd)

       where ddd is a decimal number. However, such a setting is  ignored  un-
       less  ddd  is less than the limit set by the caller of pcre2_match() or
       pcre2_dfa_match() or, if no such limit is set, less than the default.

       int pcre2_set_depth_limit(pcre2_match_context *mcontext,
         uint32_t value);

       This  parameter  limits   the   depth   of   nested   backtracking   in
       pcre2_match().   Each time a nested backtracking point is passed, a new
       memory frame is used to remember the state of matching at  that  point.
       Thus,  this  parameter  indirectly  limits the amount of memory that is
       used in a match. However, because the size of each memory frame depends
       on the number of capturing parentheses, the actual memory limit  varies
       from  pattern to pattern. This limit was more useful in versions before
       10.30, where function recursion was used for backtracking.

       The depth limit is not relevant, and is ignored, when matching is  done
       using JIT compiled code. However, it is supported by pcre2_dfa_match(),
       which  uses it to limit the depth of nested internal recursive function
       calls that implement atomic groups, lookaround assertions, and  pattern
       recursions. This limits, indirectly, the amount of system stack that is
       used.  It  was  more useful in versions before 10.32, when stack memory
       was used for local workspace vectors for recursive function calls. From
       version 10.32, only local variables are allocated on the stack  and  as
       each call uses only a few hundred bytes, even a small stack can support
       quite a lot of recursion.

       If  the depth of internal recursive function calls is great enough, lo-
       cal workspace vectors are allocated on the heap from version 10.32  on-
       wards,  so  the  depth  limit also indirectly limits the amount of heap
       memory that is used. A recursive pattern such as /(.(?2))((?1)|)/, when
       matched to a very long string using pcre2_dfa_match(), can use a  great
       deal  of memory. However, it is probably better to limit heap usage di-
       rectly by calling pcre2_set_heap_limit().

       The default value for the depth limit can be set when PCRE2  is  built;
       if  it  is not, the default is set to the same value as the default for
       the  match  limit.   If  the  limit  is  exceeded,   pcre2_match()   or
       pcre2_dfa_match() returns PCRE2_ERROR_DEPTHLIMIT. A value for the depth
       limit  may also be supplied by an item at the start of a pattern of the
       form

         (*LIMIT_DEPTH=ddd)

       where ddd is a decimal number. However, such a setting is  ignored  un-
       less  ddd  is less than the limit set by the caller of pcre2_match() or
       pcre2_dfa_match() or, if no such limit is set, less than the default.


CHECKING BUILD-TIME OPTIONS

       int pcre2_config(uint32_t what, void *where);

       The function pcre2_config() makes it possible for  a  PCRE2  client  to
       find  the  value  of  certain  configuration parameters and to discover
       which optional features have been compiled into the PCRE2 library.  The
       pcre2build documentation has more details about these features.

       The  first  argument  for pcre2_config() specifies which information is
       required. The second argument is a pointer to memory into which the in-
       formation is placed. If NULL is passed, the function returns the amount
       of memory that is needed for the requested information. For calls  that
       return  numerical  values, the value is in bytes; when requesting these
       values, where should point to appropriately aligned memory.  For  calls
       that  return  strings,  the required length is given in code units, not
       counting the terminating zero.

       When requesting information, the returned value from pcre2_config()  is
       non-negative  on success, or the negative error code PCRE2_ERROR_BADOP-
       TION if the value in the first argument is not recognized. The  follow-
       ing information is available:

         PCRE2_CONFIG_BSR

       The  output  is a uint32_t integer whose value indicates what character
       sequences the \R  escape  sequence  matches  by  default.  A  value  of
       PCRE2_BSR_UNICODE  means  that  \R  matches any Unicode line ending se-
       quence; a value of PCRE2_BSR_ANYCRLF means that \R matches only CR, LF,
       or CRLF. The default can be overridden when a pattern is compiled.

         PCRE2_CONFIG_COMPILED_WIDTHS

       The output is a uint32_t integer whose lower bits indicate  which  code
       unit  widths  were  selected  when PCRE2 was built. The 1-bit indicates
       8-bit support, and the 2-bit and 4-bit indicate 16-bit and 32-bit  sup-
       port, respectively.

         PCRE2_CONFIG_DEPTHLIMIT

       The  output  is a uint32_t integer that gives the default limit for the
       depth of nested backtracking in pcre2_match() or the  depth  of  nested
       recursions,  lookarounds,  and atomic groups in pcre2_dfa_match(). Fur-
       ther details are given with pcre2_set_depth_limit() above.

         PCRE2_CONFIG_EFFECTIVE_LINKSIZE

       The output is a uint32_t integer that contains the number of bytes  the
       library  uses for internal linkage in compiled regular expressions. Its
       value is derived from the value that was provided  at  build  time  and
       that is described below by PCRE2_CONFIG_LINKSIZE.

         PCRE2_CONFIG_HEAPLIMIT

       The  output is a uint32_t integer that gives, in kibibytes, the default
       limit  for  the  amount  of  heap  memory  used  by  pcre2_match()   or
       pcre2_dfa_match().      Further      details     are     given     with
       pcre2_set_heap_limit() above.

         PCRE2_CONFIG_JIT

       The output is a uint32_t integer that is set  to  one  if  support  for
       just-in-time  compiling is included in the library; otherwise it is set
       to zero. Note that having the support in the library does not guarantee
       that JIT will be used for any given match, and neither does it  guaran-
       tee  that  JIT will actually be able to function, because it may not be
       able to allocate executable memory in some  environments.  There  is  a
       special call to pcre2_jit_compile() that can be used to check this. See
       the pcre2jit documentation for more details.

         PCRE2_CONFIG_JITTARGET

       The where argument should point to a code-unit-aligned buffer. All pre-
       vious  versions  of  PCRE2 have required no more than 128 code units of
       buffer capacity. However, this requirement  is  not  guaranteed  to  be
       maintained,  so  applications should call pcre2_config() with where set
       to NULL to receive the required buffer size, then assert or allocate  a
       suitably-size buffer for a second call to pcre2_config(). The buffer is
       filled  with  a  string  that contains the name of the architecture for
       which the JIT compiler is configured at  build  time,  for  example,  a
       64-bit ARM CPU that supports the Armv8.1 extension writes "ARM-64 (LSE)
       64bit  (little  endian  + unaligned)". If JIT support is not available,
       PCRE2_ERROR_BADOPTION is returned; otherwise the number of  code  units
       used  is  returned.  This is the length of the string plus one unit for
       the terminating zero.

         PCRE2_CONFIG_LINKSIZE

       The output is a uint32_t integer that contains the number of bytes  the
       library  was instructed to use for internal linkage in compiled regular
       expressions.  When PCRE2 is configured, the value can be set to  2,  3,
       or 4, with the default being 2 for most libraries.

       The  actual  number of bytes used depends on the size of the code units
       that the library supports and can be  higher.  See  PCRE2_CONFIG_EFFEC-
       TIVE_LINKSIZE above for details.

       The default value of 2 for the 8-bit and 16-bit libraries is sufficient
       for  all but the most massive patterns, since it allows the size of the
       compiled pattern to be up to 65535  code  units.  Larger  values  allow
       larger  regular  expressions to be compiled by those two libraries, but
       at the expense of slower matching.

         PCRE2_CONFIG_MATCHLIMIT

       The output is a uint32_t integer that gives the default match limit for
       pcre2_match(). Further details are given  with  pcre2_set_match_limit()
       above.

         PCRE2_CONFIG_NEWLINE

       The  output  is  a  uint32_t  integer whose value specifies the default
       character sequence that is recognized as meaning "newline". The  values
       are:

         PCRE2_NEWLINE_CR       Carriage return (CR)
         PCRE2_NEWLINE_LF       Linefeed (LF)
         PCRE2_NEWLINE_CRLF     Carriage return, linefeed (CRLF)
         PCRE2_NEWLINE_ANY      Any Unicode line ending
         PCRE2_NEWLINE_ANYCRLF  Any of CR, LF, or CRLF
         PCRE2_NEWLINE_NUL      The NUL character (binary zero)

       The  default  should  normally  correspond to the standard sequence for
       your operating system.

         PCRE2_CONFIG_NEVER_BACKSLASH_C

       The output is a uint32_t integer that is set to one if the  use  of  \C
       was  permanently  disabled when PCRE2 was built; otherwise it is set to
       zero.

         PCRE2_CONFIG_PARENSLIMIT

       The output is a uint32_t integer that gives the maximum depth of  nest-
       ing of parentheses (of any kind) in a pattern. This limit is imposed to
       cap  the  amount of system stack used when a pattern is compiled. It is
       specified when PCRE2 is built; the default is 250. This limit does  not
       take into account the stack that may already be used by the calling ap-
       plication.   For  finer  control  over  compilation  stack  usage,  see
       pcre2_set_compile_recursion_guard().

         PCRE2_CONFIG_STACKRECURSE

       This parameter is obsolete and should not be used in new code. The out-
       put is a uint32_t integer that is always set to zero.

         PCRE2_CONFIG_TABLES_LENGTH

       The output is a uint32_t integer that gives the length of PCRE2's char-
       acter processing tables in bytes. For details of these tables  see  the
       section on locale support below.

         PCRE2_CONFIG_UNICODE_VERSION

       The where argument should point to a code-unit-aligned buffer. All pre-
       vious  versions  of  PCRE2  have required no more than 24 code units of
       buffer capacity. However, applications should call pcre2_config()  with
       where  set  to NULL to receive the required buffer size, then assert or
       allocate a suitably-size buffer for a second call to pcre2_config(). If
       PCRE2 has been compiled without Unicode support, the buffer  is  filled
       with  the  text "Unicode not supported". Otherwise, the Unicode version
       string (for example, "8.0.0") is written. The number of code units used
       is returned. This is the length of the string plus  one  unit  for  the
       terminating zero.

         PCRE2_CONFIG_UNICODE

       The  output is a uint32_t integer that is set to one if Unicode support
       is available; otherwise it is set to zero. Unicode support implies  UTF
       support.

         PCRE2_CONFIG_VERSION

       The where argument should point to a code-unit-aligned buffer. All pre-
       vious  versions  of  PCRE2  have required no more than 24 code units of
       buffer capacity. However, applications should call pcre2_config()  with
       where  set  to NULL to receive the required buffer size, then assert or
       allocate a suitably-size buffer for a second  call  to  pcre2_config().
       The  buffer  is  filled with the PCRE2 version string, zero-terminated.
       The number of code units used is returned. This is the  length  of  the
       string plus one unit for the terminating zero.


COMPILING A PATTERN

       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
         pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);

       The  pcre2_compile() function compiles a pattern into an internal form.
       The pattern is defined by a pointer to a string of  code  units  and  a
       length in code units. If the pattern is zero-terminated, the length can
       be  specified  as  PCRE2_ZERO_TERMINATED. A NULL pattern pointer with a
       length of zero is treated as an empty  string  (NULL  with  a  non-zero
       length  causes  an  error  return). The function returns a pointer to a
       block of memory that contains the compiled pattern and related data, or
       NULL if an error occurred.

       If the compile context argument ccontext is NULL, memory for  the  com-
       piled  pattern  is  obtained  by calling malloc(). Otherwise, it is ob-
       tained from the same memory function that was used for the compile con-
       text. The caller must free the memory by calling pcre2_code_free() when
       it is no longer needed.  If pcre2_code_free() is called with a NULL ar-
       gument, it returns immediately, without doing anything.

       The function pcre2_code_copy() makes a copy of the compiled code in new
       memory, using the same memory allocator as was used for  the  original.
       However,  if  the  code has been processed by the JIT compiler (see be-
       low), the JIT information cannot be copied (because it is  position-de-
       pendent).   The  new copy can initially be used only for non-JIT match-
       ing, though it can be passed to  pcre2_jit_compile()  if  required.  If
       pcre2_code_copy() is called with a NULL argument, it returns NULL.

       The pcre2_code_copy() function provides a way for individual threads in
       a  multithreaded  application  to acquire a private copy of shared com-
       piled code.  However, it does not make a copy of the  character  tables
       used  by  the compiled pattern; the new pattern code points to the same
       tables as the original code.  (See "Locale Support" below  for  details
       of  these  character  tables.) In many applications the same tables are
       used throughout, so this behaviour is appropriate. Nevertheless,  there
       are occasions when a copy of a compiled pattern and the relevant tables
       are  needed.  The pcre2_code_copy_with_tables() provides this facility.
       Copies of both the code and the tables are  made,  with  the  new  code
       pointing  to the new tables. The memory for the new tables is automati-
       cally freed when pcre2_code_free() is called for the new  copy  of  the
       compiled  code.  If pcre2_code_copy_with_tables() is called with a NULL
       argument, it returns NULL.

       NOTE: When one of the matching functions is  called,  pointers  to  the
       compiled pattern and the subject string are set in the match data block
       so  that  they  can be referenced by the substring extraction functions
       after a successful match.  After running a match, you must not  free  a
       compiled  pattern or a subject string until after all operations on the
       match data block have taken place, unless, in the case of  the  subject
       string,  you  have used the PCRE2_COPY_MATCHED_SUBJECT option, which is
       described in the section entitled "Option bits for  pcre2_match()"  be-
       low.

       The  options argument for pcre2_compile() contains various bit settings
       that affect the compilation. It should be zero if none of them are  re-
       quired.  The  available  options  are described below. Some of them (in
       particular, those that are compatible with Perl,  but  some  others  as
       well)  can  also  be set and unset from within the pattern (see the de-
       tailed description in the pcre2pattern documentation).

       For those options that can be different in different parts of the  pat-
       tern,  the contents of the options argument specifies their settings at
       the start of compilation. The  PCRE2_ANCHORED,  PCRE2_ENDANCHORED,  and
       PCRE2_NO_UTF_CHECK  options  can be set at the time of matching as well
       as at compile time.

       Some additional options and less frequently required compile-time para-
       meters (for example, the newline setting) can be provided in a  compile
       context (as described above).

       If errorcode or erroroffset is NULL, pcre2_compile() returns NULL imme-
       diately.  Otherwise,  the  variables to which these point are set to an
       error code and an offset (number of code units) within the pattern, re-
       spectively, when pcre2_compile() returns NULL because a compilation er-
       ror has occurred.

       There are over 100 positive error codes that pcre2_compile() may return
       if it finds an error in the pattern. There are also some negative error
       codes that are used for invalid UTF strings when validity  checking  is
       in   force.   These   are  the  same  as  given  by  pcre2_match()  and
       pcre2_dfa_match(), and are described in the pcre2unicode documentation.
       There is no separate documentation for the positive  error  codes,  be-
       cause  the  textual  error  messages  that  are obtained by calling the
       pcre2_get_error_message() function (see "Obtaining a textual error mes-
       sage" below) should be  self-explanatory.  Macro  names  starting  with
       PCRE2_ERROR_  are defined for both positive and negative error codes in
       pcre2.h. When compilation is successful errorcode is  set  to  a  value
       that  returns  the message "no error" if passed to pcre2_get_error_mes-
       sage().

       The value returned in erroroffset is an indication of where in the pat-
       tern an error occurred. When there is no error,  zero  is  returned.  A
       non-zero  value  is  not  necessarily the furthest point in the pattern
       that was read. For example, after the error  "lookbehind  assertion  is
       not  fixed length", the error offset points to the start of the failing
       assertion. For an invalid UTF-8 or UTF-16 string, the offset is that of
       the first code unit of the failing character.

       Some errors are not detected until the whole pattern has been  scanned;
       in  these  cases,  the offset passed back is the length of the pattern.
       Note that the offset is in code units, not characters, even  in  a  UTF
       mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char-
       acter.

       This  code  fragment shows a typical straightforward call to pcre2_com-
       pile():

         pcre2_code *re;
         PCRE2_SIZE erroffset;
         int errorcode;
         re = pcre2_compile(
           "^A.*Z",                /* the pattern */
           PCRE2_ZERO_TERMINATED,  /* the pattern is zero-terminated */
           0,                      /* default options */
           &errorcode,             /* for error code */
           &erroffset,             /* for error offset */
           NULL);                  /* no compile context */


   Main compile options

       The following names for option bits are defined in the  pcre2.h  header
       file:

         PCRE2_ANCHORED

       If this bit is set, the pattern is forced to be "anchored", that is, it
       is  constrained to match only at the first matching point in the string
       that is being searched (the "subject string"). This effect can also  be
       achieved  by appropriate constructs in the pattern itself, which is the
       only way to do it in Perl.

         PCRE2_ALLOW_EMPTY_CLASS

       By default, for compatibility with Perl, a closing square bracket  that
       immediately  follows  an opening one is treated as a data character for
       the class. When  PCRE2_ALLOW_EMPTY_CLASS  is  set,  it  terminates  the
       class, which therefore contains no characters and so can never match.

         PCRE2_ALT_BSUX

       This  option  request  alternative  handling of three escape sequences,
       which makes PCRE2's behaviour more like  ECMAscript  (aka  JavaScript).
       When it is set:

       (1) \U matches an upper case "U" character; by default \U causes a com-
       pile time error (Perl uses \U to upper case subsequent characters).

       (2) \u matches a lower case "u" character unless it is followed by four
       hexadecimal  digits,  in  which case the hexadecimal number defines the
       code point to match. By default, \u causes a compile time  error  (Perl
       uses it to upper case the following character).

       (3)  \x matches a lower case "x" character unless it is followed by two
       hexadecimal digits, in which case the hexadecimal  number  defines  the
       code  point  to  match. By default, as in Perl, a hexadecimal number is
       always expected after \x, but it may have one or two digits.

       ECMAscript 6 added additional functionality to \u. This can be accessed
       using the PCRE2_EXTRA_ALT_BSUX extra option  (see  "Extra  compile  op-
       tions" below).  Note that this alternative escape handling applies only
       to  patterns.  Neither  of  these options affects the processing of re-
       placement strings passed to pcre2_substitute().

         PCRE2_ALT_CIRCUMFLEX

       In  multiline  mode  (when  PCRE2_MULTILINE  is  set),  the  circumflex
       metacharacter  matches at the start of the subject (unless PCRE2_NOTBOL
       is set), and also after any internal  newline.  However,  it  does  not
       match after a newline at the end of the subject, for compatibility with
       Perl.  If  you want a multiline circumflex also to match after a termi-
       nating newline, you must set PCRE2_ALT_CIRCUMFLEX.

         PCRE2_ALT_EXTENDED_CLASS

       Alters the parsing of character classes to follow the  extended  syntax
       described by Unicode UTS#18. The PCRE2_ALT_EXTENDED_CLASS option has no
       impact  on the behaviour of the Perl-specific "(?[...])" syntax for ex-
       tended classes, but instead enables the alternative syntax of  extended
       class  behaviour  inside  ordinary  "[...]"  character classes. See the
       pcre2pattern documentation for details of the  character  classes  sup-
       ported.

         PCRE2_ALT_VERBNAMES

       By  default, for compatibility with Perl, the name in any verb sequence
       such as (*MARK:NAME) is any sequence of characters that  does  not  in-
       clude  a closing parenthesis. The name is not processed in any way, and
       it is not possible to include a closing parenthesis in the  name.  How-
       ever,  if  the PCRE2_ALT_VERBNAMES option is set, normal backslash pro-
       cessing is applied to verb names and only an unescaped  closing  paren-
       thesis  terminates the name. A closing parenthesis can be included in a
       name either as \) or between  \Q  and  \E.  If  the  PCRE2_EXTENDED  or
       PCRE2_EXTENDED_MORE  option  is set with PCRE2_ALT_VERBNAMES, unescaped
       white space in verb names is skipped and #-comments are recognized, ex-
       actly as in the rest of the pattern.

         PCRE2_AUTO_CALLOUT

       If this bit  is  set,  pcre2_compile()  automatically  inserts  callout
       items,  all  with  number 255, before each pattern item, except immedi-
       ately before or after an explicit callout in the pattern.  For  discus-
       sion of the callout facility, see the pcre2callout documentation.

         PCRE2_CASELESS

       If  this  bit is set, letters in the pattern match both upper and lower
       case letters in the subject. It is equivalent to Perl's /i option,  and
       it  can be changed within a pattern by a (?i) option setting. If either
       PCRE2_UTF or PCRE2_UCP is set, Unicode  properties  are  used  for  all
       characters  with more than one other case, and for all characters whose
       code points are greater than U+007F.

       Note that there are two ASCII characters, K and S, that, in addition to
       their lower case ASCII equivalents,  are  case-equivalent  with  U+212A
       (Kelvin sign) and U+017F (long S) respectively. If you do not want this
       case  equivalence,  you  can  suppress  it by setting PCRE2_EXTRA_CASE-
       LESS_RESTRICT.

       One language family, Turkish and Azeri, has its own  case-insensitivity
       rules,  which  can  be  selected by setting PCRE2_EXTRA_TURKISH_CASING.
       This alters the behaviour of the 'i', 'I', U+0130 (capital I  with  dot
       above), and U+0131 (small dotless i) characters.

       For lower valued characters with only one other case, a lookup table is
       used  for  speed. When neither PCRE2_UTF nor PCRE2_UCP is set, a lookup
       table is used for all code points less than 256, and higher code points
       (available only in 16-bit or 32-bit mode) are treated as not having an-
       other case.

       From release 10.45 PCRE2_CASELESS also affects what some of the letter-
       related Unicode property escapes (\p and \P) match. The  properties  Lu
       (upper case letter), Ll (lower case letter), and Lt (title case letter)
       are all treated as LC (cased letter) when PCRE2_CASELESS is set.

         PCRE2_DOLLAR_ENDONLY

       If  this bit is set, a dollar metacharacter in the pattern matches only
       at the end of the subject string. Without this option,  a  dollar  also
       matches  immediately before a newline at the end of the string (but not
       before any other newlines). The PCRE2_DOLLAR_ENDONLY option is  ignored
       if  PCRE2_MULTILINE  is  set.  There is no equivalent to this option in
       Perl, and no way to set it within a pattern.

         PCRE2_DOTALL

       If this bit is set, a dot metacharacter  in  the  pattern  matches  any
       character,  including  one  that  indicates a newline. However, it only
       ever matches one character, even if newlines are coded as CRLF. Without
       this option, a dot does not match when the current position in the sub-
       ject is at a newline. This option is equivalent to  Perl's  /s  option,
       and it can be changed within a pattern by a (?s) option setting. A neg-
       ative  class such as [^a] always matches newline characters, and the \N
       escape sequence always matches a non-newline character, independent  of
       the setting of PCRE2_DOTALL.

         PCRE2_DUPNAMES

       If  this  bit is set, names used to identify capture groups need not be
       unique.  This can be helpful for certain types of pattern  when  it  is
       known  that  only  one instance of the named group can ever be matched.
       There are more details of named capture  groups  below;  see  also  the
       pcre2pattern documentation.

         PCRE2_ENDANCHORED

       If  this  bit is set, the end of any pattern match must be right at the
       end of the string being searched (the "subject string"). If the pattern
       match succeeds by reaching (*ACCEPT), but does not reach the end of the
       subject, the match fails at the current starting point. For  unanchored
       patterns,  a  new  match is then tried at the next starting point. How-
       ever, if the match succeeds by reaching the end of the pattern, but not
       the end of the subject, backtracking occurs and  an  alternative  match
       may be found. Consider these two patterns:

         .(*ACCEPT)|..
         .|..

       If  matched against "abc" with PCRE2_ENDANCHORED set, the first matches
       "c" whereas the second matches "bc". The  effect  of  PCRE2_ENDANCHORED
       can  also  be achieved by appropriate constructs in the pattern itself,
       which is the only way to do it in Perl.

       For DFA matching with pcre2_dfa_match(), PCRE2_ENDANCHORED applies only
       to the first (that is, the  longest)  matched  string.  Other  parallel
       matches,  which are necessarily substrings of the first one, must obvi-
       ously end before the end of the subject.

         PCRE2_EXTENDED

       If this bit is set, most white space characters in the pattern are  to-
       tally  ignored except when escaped, inside a character class, or inside
       a \Q...\E sequence. However, white space  is  not  allowed  within  se-
       quences  such  as  (?> that introduce various parenthesized groups, nor
       within numerical quantifiers such as {1,3}. Ignorable  white  space  is
       permitted  between  an  item  and  a following quantifier and between a
       quantifier and a following + that indicates  possessiveness.  PCRE2_EX-
       TENDED  is equivalent to Perl's /x option, and it can be changed within
       a pattern by a (?x) option setting.

       When PCRE2 is compiled without Unicode support,  PCRE2_EXTENDED  recog-
       nizes  as  white space only those characters with code points less than
       256 that are flagged as white space in its low-character table. The ta-
       ble is normally created by pcre2_maketables(), which uses the isspace()
       function to identify space characters. In most ASCII environments,  the
       relevant  characters  are  those  with code points 0x0009 (tab), 0x000A
       (linefeed), 0x000B (vertical tab), 0x000C (formfeed), 0x000D  (carriage
       return), and 0x0020 (space).

       When PCRE2 is compiled with Unicode support, in addition to these char-
       acters,  five  more Unicode "Pattern White Space" characters are recog-
       nized by PCRE2_EXTENDED. These are U+0085 (next line), U+200E (left-to-
       right mark), U+200F (right-to-left mark), U+2028 (line separator),  and
       U+2029  (paragraph  separator).  This  set of characters is the same as
       recognized by Perl's /x option. Note that the horizontal  and  vertical
       space  characters that are matched by the \h and \v escapes in patterns
       are a much bigger set.

       As well as ignoring most white space, PCRE2_EXTENDED also causes  char-
       acters  between  an  unescaped # outside a character class and the next
       newline, inclusive, to be ignored, which makes it possible  to  include
       comments inside complicated patterns. Note that the end of this type of
       comment  is a literal newline sequence in the pattern; escape sequences
       that happen to represent a newline do not count.

       Which characters are interpreted as newlines can be specified by a set-
       ting in the compile context that is passed to pcre2_compile() or  by  a
       special  sequence at the start of the pattern, as described in the sec-
       tion entitled "Newline conventions" in the pcre2pattern  documentation.
       A default is defined when PCRE2 is built.

         PCRE2_EXTENDED_MORE

       This  option  has  the  effect of PCRE2_EXTENDED, but, in addition, un-
       escaped space and horizontal tab characters are ignored inside a  char-
       acter  class. Note: only these two characters are ignored, not the full
       set of pattern white space characters that are ignored outside a  char-
       acter  class.  PCRE2_EXTENDED_MORE  is equivalent to Perl's /xx option,
       and it can be changed within a pattern by a (?xx) option setting.

         PCRE2_FIRSTLINE

       If this option is set, the start of an unanchored pattern match must be
       before or at the first newline in  the  subject  string  following  the
       start  of  matching, though the matched text may continue over the new-
       line. If startoffset is non-zero, the limiting newline is not necessar-
       ily the first newline in the  subject.  For  example,  if  the  subject
       string is "abc\nxyz" (where \n represents a single-character newline) a
       pattern  match for "yz" succeeds with PCRE2_FIRSTLINE if startoffset is
       greater than 3. See also PCRE2_USE_OFFSET_LIMIT, which provides a  more
       general  limiting  facility.  If  PCRE2_FIRSTLINE is set with an offset
       limit, a match must occur in the first line and also within the  offset
       limit. In other words, whichever limit comes first is used. This option
       has no effect for anchored patterns.

         PCRE2_LITERAL

       If this option is set, all meta-characters in the pattern are disabled,
       and  it is treated as a literal string. Matching literal strings with a
       regular expression engine is not the most efficient way of doing it. If
       you are doing a lot of literal matching and  are  worried  about  effi-
       ciency, you should consider using other approaches. The only other main
       options  that  are  allowed  with  PCRE2_LITERAL  are:  PCRE2_ANCHORED,
       PCRE2_ENDANCHORED, PCRE2_AUTO_CALLOUT, PCRE2_CASELESS, PCRE2_FIRSTLINE,
       PCRE2_MATCH_INVALID_UTF,  PCRE2_NO_START_OPTIMIZE,  PCRE2_NO_UTF_CHECK,
       PCRE2_UTF,  and  PCRE2_USE_OFFSET_LIMIT.  The  extra  options PCRE2_EX-
       TRA_MATCH_LINE and PCRE2_EXTRA_MATCH_WORD are also supported. Any other
       options cause an error.

         PCRE2_MATCH_INVALID_UTF

       This option forces PCRE2_UTF (see below) and also enables  support  for
       matching  by  pcre2_match() in subject strings that contain invalid UTF
       sequences.  Note, however, that the 16-bit and 32-bit  PCRE2  libraries
       process  strings as sequences of uint16_t or uint32_t code points. They
       cannot find valid UTF sequences within an arbitrary string of bytes un-
       less such sequences are suitably aligned. This  facility  is  not  sup-
       ported  for  DFA matching. For details, see the pcre2unicode documenta-
       tion.

         PCRE2_MATCH_UNSET_BACKREF

       If this option is set,  a  backreference  to  an  unset  capture  group
       matches  an  empty  string (by default this causes the current matching
       alternative to fail).  A pattern such as (\1)(a) succeeds when this op-
       tion is set (assuming it can find an "a" in the  subject),  whereas  it
       fails  by  default,  for  Perl compatibility. Setting this option makes
       PCRE2 behave more like ECMAscript (aka JavaScript).

         PCRE2_MULTILINE

       By default, for the purposes of matching "start of line"  and  "end  of
       line",  PCRE2  treats the subject string as consisting of a single line
       of characters, even if it actually contains  newlines.  The  "start  of
       line"  metacharacter  (^)  matches only at the start of the string, and
       the "end of line" metacharacter ($) matches only  at  the  end  of  the
       string,  or  before a terminating newline (except when PCRE2_DOLLAR_EN-
       DONLY is set). Note, however, that unless PCRE2_DOTALL is set, the "any
       character" metacharacter (.) does not match at a newline.  This  behav-
       iour (for ^, $, and dot) is the same as Perl.

       When  PCRE2_MULTILINE  it is set, the "start of line" and "end of line"
       constructs match immediately following or immediately  before  internal
       newlines  in  the  subject string, respectively, as well as at the very
       start and end. This is equivalent to Perl's /m option, and  it  can  be
       changed within a pattern by a (?m) option setting. Note that the "start
       of line" metacharacter does not match after a newline at the end of the
       subject,  for compatibility with Perl.  However, you can change this by
       setting the PCRE2_ALT_CIRCUMFLEX option. If there are no newlines in  a
       subject  string,  or  no  occurrences  of  ^ or $ in a pattern, setting
       PCRE2_MULTILINE has no effect.

         PCRE2_NEVER_BACKSLASH_C

       This option locks out the use of \C in the pattern that is  being  com-
       piled.   This  escape  can  cause  unpredictable  behaviour in UTF-8 or
       UTF-16 modes, because it may leave the current matching  point  in  the
       middle of a multi-code-unit character. This option may be useful in ap-
       plications that process patterns from external sources. Note that there
       is also a build-time option that permanently locks out the use of \C.

         PCRE2_NEVER_UCP

       This  option  locks  out the use of Unicode properties for handling \B,
       \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes, as
       described for the PCRE2_UCP option below. In  particular,  it  prevents
       the  creator of the pattern from enabling this facility by starting the
       pattern with (*UCP). This option may be  useful  in  applications  that
       process   patterns   from  external  sources.  The  option  combination
       PCRE2_UCP and PCRE2_NEVER_UCP causes an error.

         PCRE2_NEVER_UTF

       This option locks out interpretation of the pattern as  UTF-8,  UTF-16,
       or UTF-32, depending on which library is in use. In particular, it pre-
       vents  the  creator of the pattern from switching to UTF interpretation
       by starting the pattern with (*UTF). This option may be useful  in  ap-
       plications that process patterns from external sources. The combination
       of PCRE2_UTF and PCRE2_NEVER_UTF causes an error.

         PCRE2_NO_AUTO_CAPTURE

       If this option is set, it disables the use of numbered capturing paren-
       theses  in the pattern. Any opening parenthesis that is not followed by
       ? behaves as if it were followed by ?: but named parentheses can  still
       be used for capturing (and they acquire numbers in the usual way). This
       is  the  same as Perl's /n option.  Note that, when this option is set,
       references to capture groups  (backreferences  or  recursion/subroutine
       calls)  may  only refer to named groups, though the reference can be by
       name or by number.

         PCRE2_NO_AUTO_POSSESS

       If this (deprecated) option is  set,  it  disables  "auto-possessifica-
       tion",  which is an optimization that, for example, turns a+b into a++b
       in order to avoid backtracks into a+ that can never be successful. How-
       ever, if callouts are in use,  auto-possessification  means  that  some
       callouts  are  never  taken.  You  can  set this option if you want the
       matching functions to do a full unoptimized  search  and  run  all  the
       callouts, but it is mainly provided for testing purposes.

       If   a   compile  context  is  available,  it  is  recommended  to  use
       pcre2_set_optimize() with the directive  PCRE2_AUTO_POSSESS_OFF  rather
       than    the    compile    option   PCRE2_NO_AUTO_POSSESS.   Note   that
       PCRE2_NO_AUTO_POSSESS takes precedence  over  the  pcre2_set_optimize()
       optimization directives PCRE2_AUTO_POSSESS and PCRE2_AUTO_POSSESS_OFF.

         PCRE2_NO_DOTSTAR_ANCHOR

       If this (deprecated) option is set, it disables an optimization that is
       applied  when .* is the first significant item in a top-level branch of
       a pattern, and all the other branches also start with .* or with \A  or
       \G or ^. The optimization is automatically disabled for .* if it is in-
       side  an atomic group or a capture group that is the subject of a back-
       reference, or if the pattern contains (*PRUNE) or (*SKIP). When the op-
       timization is not disabled, such a pattern is automatically anchored if
       PCRE2_DOTALL is set for all the .* items and PCRE2_MULTILINE is not set
       for any ^ items. Otherwise, the fact that any match must  start  either
       at  the start of the subject or following a newline is remembered. Like
       other optimizations, this can cause callouts to be skipped.  (If a com-
       pile context is available, it is  recommended  to  use  pcre2_set_opti-
       mize() with the directive PCRE2_DOTSTAR_ANCHOR_OFF instead.)

         PCRE2_NO_START_OPTIMIZE

       This  is  an  option whose main effect is at matching time. It does not
       change what pcre2_compile() generates, but it does affect the output of
       the  JIT  compiler.  Setting  this  option  is  equivalent  to  calling
       pcre2_set_optimize()    with    the    directive   parameter   set   to
       PCRE2_START_OPTIMIZE_OFF.

       There are a number of optimizations that may occur at the  start  of  a
       match,  in  order  to speed up the process. For example, if it is known
       that an unanchored match must start with a specific  code  unit  value,
       the  matching code searches the subject for that value, and fails imme-
       diately if it cannot find it, without actually running the main  match-
       ing  function.  The  start-up optimizations are in effect a pre-scan of
       the subject that takes place before the pattern is run.

       Disabling the start-up optimizations may cause performance  to  suffer.
       However,  this  may be desirable for patterns which contain callouts or
       items such as (*COMMIT) and  (*MARK).  See  the  above  description  of
       PCRE2_START_OPTIMIZE_OFF for further details.

         PCRE2_NO_UTF_CHECK

       When  PCRE2_UTF  is set, the validity of the pattern as a UTF string is
       automatically checked. There are  discussions  about  the  validity  of
       UTF-8  strings,  UTF-16 strings, and UTF-32 strings in the pcre2unicode
       document. If an invalid UTF sequence is found, pcre2_compile()  returns
       a negative error code.

       If  you  know  that your pattern is a valid UTF string, and you want to
       skip  this  check  for   performance   reasons,   you   can   set   the
       PCRE2_NO_UTF_CHECK option. When it is set, the effect of passing an in-
       valid  UTF  string as a pattern is undefined. It may cause your program
       to crash or loop.

       Note  that  this  option  can  also  be  passed  to  pcre2_match()  and
       pcre2_dfa_match(),  to  suppress  UTF  validity checking of the subject
       string.

       Note also that setting PCRE2_NO_UTF_CHECK at compile time does not dis-
       able the error that is given if an escape sequence for an invalid  Uni-
       code  code  point is encountered in the pattern. In particular, the so-
       called "surrogate" code points (0xd800 to 0xdfff) are invalid.  If  you
       want  to  allow  escape  sequences  such  as  \x{d800}  you can set the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra option, as described  in  the
       section  entitled "Extra compile options" below.  However, this is pos-
       sible only in UTF-8 and UTF-32 modes, because these values are not rep-
       resentable in UTF-16.

         PCRE2_UCP

       This option has two effects. Firstly, it change the way PCRE2 processes
       \B, \b, \D, \d, \S, \s,  \W,  \w,  and  some  of  the  POSIX  character
       classes.  By  default,  only  ASCII  characters  are recognized, but if
       PCRE2_UCP is set, Unicode properties are used to  classify  characters.
       There  are  some PCRE2_EXTRA options (see below) that add finer control
       to this behaviour. More details are given in  the  section  on  generic
       character types in the pcre2pattern page.

       The  second  effect of PCRE2_UCP is to force the use of Unicode proper-
       ties for upper/lower casing operations, even when PCRE2_UTF is not set.
       This makes it possible to process strings in  the  16-bit  UCS-2  code.
       This  option  is available only if PCRE2 has been compiled with Unicode
       support (which is the default).

       The PCRE2_EXTRA_CASELESS_RESTRICT option (see above) restricts caseless
       matching such that ASCII characters match  only  ASCII  characters  and
       non-ASCII  characters  match  only  non-ASCII characters. The PCRE2_EX-
       TRA_TURKISH_CASING option (see above) alters the matching  of  the  'i'
       characters  to  follow  their behaviour in Turkish and Azeri languages.
       For further  details  on  PCRE2_EXTRA_CASELESS_RESTRICT  and  PCRE2_EX-
       TRA_TURKISH_CASING, see the pcre2unicode page.

         PCRE2_UNGREEDY

       This  option  inverts  the "greediness" of the quantifiers so that they
       are not greedy by default, but become greedy if followed by "?". It  is
       not  compatible  with Perl. It can also be set by a (?U) option setting
       within the pattern.

         PCRE2_USE_OFFSET_LIMIT

       This option must be set for pcre2_compile() if pcre2_set_offset_limit()
       is going to be used to set a non-default offset limit in a  match  con-
       text  for  matches  that  use this pattern. An error is generated if an
       offset limit is set without this option. For more details, see the  de-
       scription  of  pcre2_set_offset_limit()  in  the section that describes
       match contexts. See also the PCRE2_FIRSTLINE option above.

         PCRE2_UTF

       This option causes PCRE2 to regard both the  pattern  and  the  subject
       strings  that  are  subsequently processed as strings of UTF characters
       instead of single-code-unit strings. It  is  available  when  PCRE2  is
       built  to  include  Unicode  support (which is the default). If Unicode
       support is not available, the use of this option provokes an error. De-
       tails of how PCRE2_UTF changes the behaviour of PCRE2 are given in  the
       pcre2unicode  page.  In  particular,  note  that  it  changes  the  way
       PCRE2_CASELESS works.

   Extra compile options

       The option bits that can be set in a compile  context  by  calling  the
       pcre2_set_compile_extra_options() function are as follows:

         PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK

       Since release 10.38 PCRE2 has forbidden the use of \K within lookaround
       assertions, following Perl's lead. This option is provided to re-enable
       the previous behaviour (act in positive lookarounds, ignore in negative
       ones) in case anybody is relying on it.

         PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES

       This  option  applies when compiling a pattern in UTF-8 or UTF-32 mode.
       It is forbidden in UTF-16 mode, and ignored in non-UTF  modes.  Unicode
       "surrogate" code points in the range 0xd800 to 0xdfff are used in pairs
       in  UTF-16  to  encode  code points with values in the range 0x10000 to
       0x10ffff. The surrogates cannot therefore  be  represented  in  UTF-16.
       They can be represented in UTF-8 and UTF-32, but are defined as invalid
       code  points,  and  cause  errors  if  encountered in a UTF-8 or UTF-32
       string that is being checked for validity by PCRE2.

       These values also cause errors if encountered in escape sequences  such
       as \x{d912} within a pattern. However, it seems that some applications,
       when using PCRE2 to check for unwanted characters in UTF-8 strings, ex-
       plicitly   test   for   the  surrogates  using  escape  sequences.  The
       PCRE2_NO_UTF_CHECK option does not disable the error that  occurs,  be-
       cause it applies only to the testing of input strings for UTF validity.

       If  the extra option PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES is set, surro-
       gate code point values in UTF-8 and UTF-32 patterns no  longer  provoke
       errors  and are incorporated in the compiled pattern. However, they can
       only match subject characters if the matching function is  called  with
       PCRE2_NO_UTF_CHECK set.

         PCRE2_EXTRA_ALT_BSUX

       The  original option PCRE2_ALT_BSUX causes PCRE2 to process \U, \u, and
       \x in the way that ECMAscript (aka JavaScript) does.  Additional  func-
       tionality was defined by ECMAscript 6; setting PCRE2_EXTRA_ALT_BSUX has
       the  effect  of PCRE2_ALT_BSUX, but in addition it recognizes \u{hhh..}
       as a hexadecimal character code, where hhh.. is any number of hexadeci-
       mal digits.

         PCRE2_EXTRA_ASCII_BSD

       This option forces \d to match only ASCII digits, even  when  PCRE2_UCP
       is  set.   It can be changed within a pattern by means of the (?aD) op-
       tion setting.

         PCRE2_EXTRA_ASCII_BSS

       This option forces \s to match only ASCII space characters,  even  when
       PCRE2_UCP  is  set.  It can be changed within a pattern by means of the
       (?aS) option setting.

         PCRE2_EXTRA_ASCII_BSW

       This option forces \w to match only ASCII word  characters,  even  when
       PCRE2_UCP  is  set.  It can be changed within a pattern by means of the
       (?aW) option setting.

         PCRE2_EXTRA_ASCII_DIGIT

       This option forces the POSIX character classes [:digit:] and [:xdigit:]
       to match only ASCII digits, even when  PCRE2_UCP  is  set.  It  can  be
       changed within a pattern by means of the (?aT) option setting.

         PCRE2_EXTRA_ASCII_POSIX

       This option forces all the POSIX character classes, including [:digit:]
       and  [:xdigit:], to match only ASCII characters, even when PCRE2_UCP is
       set. It can be changed within a pattern by means of  the  (?aP)  option
       setting,  but note that this also sets PCRE2_EXTRA_ASCII_DIGIT in order
       to ensure that (?-aP) unsets all ASCII restrictions for POSIX classes.

         PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL

       This is a dangerous option. Use with care. By default, an  unrecognized
       escape  such  as \j or a malformed one such as \x{2z} causes a compile-
       time error when detected by pcre2_compile(). Perl is somewhat inconsis-
       tent in handling such items: for example, \j is treated  as  a  literal
       "j",  and non-hexadecimal digits in \x{} are just ignored, though warn-
       ings are given in both cases if Perl's warning switch is enabled.  How-
       ever,  a  malformed  octal  number  after \o{ always causes an error in
       Perl.

       If the PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL  extra  option  is  passed  to
       pcre2_compile(),  all  unrecognized  or  malformed escape sequences are
       treated as single-character escapes. For example, \j is a  literal  "j"
       and  \x{2z}  is treated as the literal string "x{2z}". Setting this op-
       tion means that typos in patterns may go undetected and have unexpected
       results. Also note that a sequence such as [\N{] is  interpreted  as  a
       malformed  attempt  at [\N{...}] and so is treated as [N{] whereas [\N]
       gives an error because an unqualified \N is a valid escape sequence but
       is not supported in a character class. To reiterate: this is a  danger-
       ous option. Use with great care.

         PCRE2_EXTRA_CASELESS_RESTRICT

       When  either  PCRE2_UCP  or PCRE2_UTF is set, caseless matching follows
       Unicode rules, which allow for more than two cases per character. There
       are two case-equivalent character sets that contain both ASCII and non-
       ASCII characters. The ASCII letter S is case-equivalent to U+017f (long
       S) and the ASCII letter K is case-equivalent to U+212a  (Kelvin  sign).
       This  option  disables  recognition of case-equivalences that cross the
       ASCII/non-ASCII boundary. In a caseless match, both characters must ei-
       ther be ASCII or non-ASCII. The option can be changed within a  pattern
       by the (*CASELESS_RESTRICT) or (?r) option settings.

         PCRE2_EXTRA_ESCAPED_CR_IS_LF

       There  are  some  legacy applications where the escape sequence \r in a
       pattern is expected to match a newline. If this option is set, \r in  a
       pattern  is  converted to \n so that it matches a LF (linefeed) instead
       of a CR (carriage return) character. The option does not affect a  lit-
       eral  CR in the pattern, nor does it affect CR specified as an explicit
       code point such as \x{0D}.

         PCRE2_EXTRA_MATCH_LINE

       This option is provided for use by  the  -x  option  of  pcre2grep.  It
       causes  the  pattern  only to match complete lines. This is achieved by
       automatically inserting the code for "^(?:" at the start  of  the  com-
       piled  pattern  and ")$" at the end. Thus, when PCRE2_MULTILINE is set,
       the matched line may be in the middle of the subject string.  This  op-
       tion can be used with PCRE2_LITERAL.

         PCRE2_EXTRA_MATCH_WORD

       This  option  is  provided  for  use  by the -w option of pcre2grep. It
       causes the pattern only to match strings that have a word  boundary  at
       the  start and the end. This is achieved by automatically inserting the
       code for "\b(?:" at the start of the compiled pattern and ")\b" at  the
       end.  The option may be used with PCRE2_LITERAL. However, it is ignored
       if PCRE2_EXTRA_MATCH_LINE is also set.

         PCRE2_EXTRA_NO_BS0

       If this option is set (note that its final character is the digit 0) it
       locks out the use of the sequence \0 unless at  least  one  more  octal
       digit follows.

         PCRE2_EXTRA_PYTHON_OCTAL

       If  this  option  is set, PCRE2 follows Python's rules for interpreting
       octal escape sequences. The rules for handling sequences such  as  \14,
       which  could  be an octal number or a back reference are different. De-
       tails are given in the pcre2pattern documentation.

         PCRE2_EXTRA_NEVER_CALLOUT

       If this option is set, PCRE2 treats callouts in the pattern as a syntax
       error, returning PCRE2_ERROR_CALLOUT_CALLER_DISABLED. This is useful if
       the  application  knows  that  a  callout  will  not  be  provided   to
       pcre2_match(),  so  that  callouts  in the pattern are not silently ig-
       nored.

         PCRE2_EXTRA_TURKISH_CASING

       This option alters case-equivalence of the 'i' letters  to  follow  the
       alphabet used by Turkish and Azeri languages. The option can be changed
       within a pattern by the (*TURKISH_CASING) start-of-pattern setting. Ei-
       ther the UTF or UCP options must be set. In the 8-bit library, UTF must
       be  set.  This  option cannot be combined with PCRE2_EXTRA_CASELESS_RE-
       STRICT.


JUST-IN-TIME (JIT) COMPILATION

       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(size_t startsize,
         size_t maxsize, pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
         pcre2_jit_callback callback_function, void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);

       These functions provide support for  JIT  compilation,  which,  if  the
       just-in-time  compiler  is available, further processes a compiled pat-
       tern into machine code that executes much faster than the pcre2_match()
       interpretive matching function. Full details are given in the  pcre2jit
       documentation.

       JIT  compilation  is  a heavyweight optimization. It can take some time
       for patterns to be analyzed, and for one-off matches  and  simple  pat-
       terns  the benefit of faster execution might be offset by a much slower
       compilation time.  Most (but not all) patterns can be optimized by  the
       JIT compiler.


LOCALE SUPPORT

       const uint8_t *pcre2_maketables(pcre2_general_context *gcontext);

       void pcre2_maketables_free(pcre2_general_context *gcontext,
         const uint8_t *tables);

       PCRE2  handles caseless matching, and determines whether characters are
       letters, digits, or whatever, by reference to a set of tables,  indexed
       by character code point. However, this applies only to characters whose
       code  points  are  less than 256. By default, higher-valued code points
       never match escapes such as \w or \d.

       When PCRE2 is built with Unicode support (the default), certain Unicode
       character properties can be tested with \p and \P,  or,  alternatively,
       the PCRE2_UCP option can be set when a pattern is compiled; this causes
       \w  and friends to use Unicode property support instead of the built-in
       tables.  PCRE2_UCP also causes upper/lower casing operations on charac-
       ters with code points greater than 127 to use Unicode properties. These
       effects apply even when PCRE2_UTF is not set. There are, however,  some
       PCRE2_EXTRA  options (see above) that can be used to modify or suppress
       them.

       The use of locales with Unicode is discouraged.  If  you  are  handling
       characters  with  code  points  greater than 127, you should either use
       Unicode support, or use locales, but not try to mix the two.

       PCRE2 contains a built-in set of character tables that are used by  de-
       fault.   These  are sufficient for many applications. Normally, the in-
       ternal tables recognize only ASCII characters. However, when  PCRE2  is
       built, it is possible to cause the internal tables to be rebuilt in the
       default "C" locale of the local system, which may cause them to be dif-
       ferent.

       The  built-in tables can be overridden by tables supplied by the appli-
       cation that calls PCRE2. These may be created  in  a  different  locale
       from  the  default.  As more and more applications change to using Uni-
       code, the need for this locale support is expected to die away.

       External tables are built by calling the  pcre2_maketables()  function,
       in the relevant locale. The only argument to this function is a general
       context,  which  can  be used to pass a custom memory allocator. If the
       argument is NULL, the system malloc() is used. The result can be passed
       to pcre2_compile() as often as necessary, by creating a compile context
       and calling pcre2_set_character_tables()  to  set  the  tables  pointer
       therein.

       For  example,  to  build  and  use  tables that are appropriate for the
       French locale (where accented characters with values greater  than  127
       are treated as letters), the following code could be used:

         setlocale(LC_CTYPE, "fr_FR");
         tables = pcre2_maketables(NULL);
         ccontext = pcre2_compile_context_create(NULL);
         pcre2_set_character_tables(ccontext, tables);
         re = pcre2_compile(..., ccontext);

       The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
       if you are using Windows, the name for the French locale is "french".

       The pointer that is passed (via the compile context) to pcre2_compile()
       is saved with the compiled pattern, and the same tables are used by the
       matching functions. Thus,  for  any  single  pattern,  compilation  and
       matching  both happen in the same locale, but different patterns can be
       processed in different locales.

       It is the caller's responsibility to ensure that the memory  containing
       the tables remains available while they are still in use. When they are
       no  longer  needed, you can discard them using pcre2_maketables_free(),
       which should pass as its first parameter the same global  context  that
       was used to create the tables.

   Saving locale tables

       The  tables  described above are just a sequence of binary bytes, which
       makes them independent of hardware characteristics such  as  endianness
       or  whether  the processor is 32-bit or 64-bit. A copy of the result of
       pcre2_maketables() can therefore be saved in a file  or  elsewhere  and
       re-used  later, even in a different program or on another computer. The
       size of the tables (number  of  bytes)  must  be  obtained  by  calling
       pcre2_config()   with  the  PCRE2_CONFIG_TABLES_LENGTH  option  because
       pcre2_maketables()  does  not  return  this  value.   Note   that   the
       pcre2_dftables program, which is part of the PCRE2 build system, can be
       used stand-alone to create a file that contains a set of binary tables.
       See the pcre2build documentation for details.


INFORMATION ABOUT A COMPILED PATTERN

       int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where);

       The  pcre2_pattern_info()  function returns general information about a
       compiled pattern. For information about callouts, see the next section.
       The first argument for pcre2_pattern_info() is a pointer  to  the  com-
       piled pattern. The second argument specifies which piece of information
       is  required,  and the third argument is a pointer to a variable to re-
       ceive the data. If the third argument is NULL, the  first  argument  is
       ignored,  and  the  function  returns the size in bytes of the variable
       that is required for the information requested. Otherwise, the yield of
       the function is zero for success, or one of the following negative num-
       bers:

         PCRE2_ERROR_NULL           the argument code was NULL
         PCRE2_ERROR_BADMAGIC       the "magic number" was not found
         PCRE2_ERROR_BADOPTION      the value of what was invalid
         PCRE2_ERROR_UNSET          the requested field is not set

       The "magic number" is placed at the start of each compiled pattern as a
       simple check against passing an arbitrary memory  pointer.  Here  is  a
       typical  call of pcre2_pattern_info(), to obtain the length of the com-
       piled pattern:

         int rc;
         size_t length;
         rc = pcre2_pattern_info(
           re,               /* result of pcre2_compile() */
           PCRE2_INFO_SIZE,  /* what is required */
           &length);         /* where to put the data */

       The possible values for the second argument are defined in pcre2.h, and
       are as follows:

         PCRE2_INFO_ALLOPTIONS
         PCRE2_INFO_ARGOPTIONS
         PCRE2_INFO_EXTRAOPTIONS

       Return copies of the pattern's options. The third argument should point
       to a uint32_t variable. PCRE2_INFO_ARGOPTIONS returns exactly  the  op-
       tions  that  were  passed to pcre2_compile(), whereas PCRE2_INFO_ALLOP-
       TIONS returns the compile options as modified by any  top-level  (*XXX)
       option  settings  such  as  (*UTF)  at the start of the pattern itself.
       PCRE2_INFO_EXTRAOPTIONS returns the extra options that were set in  the
       compile  context by calling the pcre2_set_compile_extra_options() func-
       tion.

       For example, if the pattern /(*UTF)abc/ is compiled with the  PCRE2_EX-
       TENDED  option,  the result for PCRE2_INFO_ALLOPTIONS is PCRE2_EXTENDED
       and PCRE2_UTF.  Option settings such as (?i) that can change  within  a
       pattern do not affect the result of PCRE2_INFO_ALLOPTIONS, even if they
       appear  right  at the start of the pattern. (This was different in some
       earlier releases.)

       A pattern compiled without PCRE2_ANCHORED is automatically anchored  by
       PCRE2 if the first significant item in every top-level branch is one of
       the following:

         ^     unless PCRE2_MULTILINE is set
         \A    always
         \G    always
         .*    sometimes - see below

       When  .* is the first significant item, anchoring is possible only when
       all the following are true:

         .* is not in an atomic group
         .* is not in a capture group that is the subject
              of a backreference
         PCRE2_DOTALL is in force for .*
         Neither (*PRUNE) nor (*SKIP) appears in the pattern
         PCRE2_NO_DOTSTAR_ANCHOR is not set
         Dotstar anchoring has not been disabled with PCRE2_DOTSTAR_ANCHOR_OFF

       For patterns that are auto-anchored, the PCRE2_ANCHORED bit is  set  in
       the options returned for PCRE2_INFO_ALLOPTIONS.

         PCRE2_INFO_BACKREFMAX

       Return  the  number  of  the  highest backreference in the pattern. The
       third argument should point  to  a  uint32_t  variable.  Named  capture
       groups  acquire  numbers  as well as names, and these count towards the
       highest backreference. Backreferences such as \4 or  \g{12}  match  the
       captured characters of the given group, but in addition, the check that
       a capture group is set in a conditional group such as (?(3)a|b) is also
       a backreference.  Zero is returned if there are no backreferences.

         PCRE2_INFO_BSR

       The  output  is a uint32_t integer whose value indicates what character
       sequences the \R escape sequence matches. A value of  PCRE2_BSR_UNICODE
       means  that  \R  matches  any  Unicode line ending sequence; a value of
       PCRE2_BSR_ANYCRLF means that \R matches only CR, LF, or CRLF.

         PCRE2_INFO_CAPTURECOUNT

       Return the highest capture group number in  the  pattern.  In  patterns
       where (?| is not used, this is also the total number of capture groups.
       The third argument should point to a uint32_t variable.

         PCRE2_INFO_DEPTHLIMIT

       If  the  pattern set a backtracking depth limit by including an item of
       the form (*LIMIT_DEPTH=nnnn) at the start, the value is  returned.  The
       third argument should point to a uint32_t integer. If no such value has
       been  set, the call to pcre2_pattern_info() returns the error PCRE2_ER-
       ROR_UNSET. Note that this limit will only be used during matching if it
       is less than the limit set or defaulted by  the  caller  of  the  match
       function.

         PCRE2_INFO_FIRSTBITMAP

       In  the absence of a single first code unit for a non-anchored pattern,
       pcre2_compile() may construct a 256-bit table that defines a fixed  set
       of  values for the first code unit in any match. For example, a pattern
       that starts with [abc] results in a table with  three  bits  set.  When
       code  unit  values greater than 255 are supported, the flag bit for 255
       means "any code unit of value 255 or above". If such a table  was  con-
       structed,  a pointer to it is returned. Otherwise NULL is returned. The
       third argument should point to a const uint8_t * variable.

         PCRE2_INFO_FIRSTCODETYPE

       Return information about the first code unit of any matched string, for
       a non-anchored pattern. The third argument should point to  a  uint32_t
       variable.  If there is a fixed first value, for example, the letter "c"
       from a pattern such as (cat|cow|coyote), 1 is returned, and  the  value
       can  be  retrieved using PCRE2_INFO_FIRSTCODEUNIT. If there is no fixed
       first value, but it is known that a match can occur only at  the  start
       of  the  subject  or following a newline in the subject, 2 is returned.
       Otherwise, and for anchored patterns, 0 is returned.

         PCRE2_INFO_FIRSTCODEUNIT

       Return the value of the first code unit of any  matched  string  for  a
       pattern  where  PCRE2_INFO_FIRSTCODETYPE returns 1; otherwise return 0.
       The third argument should point to a uint32_t variable.  In  the  8-bit
       library,  the  value is always less than 256. In the 16-bit library the
       value can be up to 0xffff. In the 32-bit library  in  UTF-32  mode  the
       value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32
       mode.

         PCRE2_INFO_FRAMESIZE

       Return the size (in bytes) of the data frames that are used to remember
       backtracking  positions  when the pattern is processed by pcre2_match()
       without the use of JIT. The third argument should  point  to  a  size_t
       variable. The frame size depends on the number of capturing parentheses
       in the pattern. Each additional capture group adds two PCRE2_SIZE vari-
       ables.

         PCRE2_INFO_HASBACKSLASHC

       Return  1 if the pattern contains any instances of \C, otherwise 0. The
       third argument should point to a uint32_t variable.

         PCRE2_INFO_HASCRORLF

       Return 1 if the pattern contains any explicit  matches  for  CR  or  LF
       characters,  otherwise 0. The third argument should point to a uint32_t
       variable. An explicit match is either a literal CR or LF character,  or
       \r  or  \n  or  one  of  the equivalent hexadecimal or octal escape se-
       quences.

         PCRE2_INFO_HEAPLIMIT

       If the pattern set a heap memory limit by including an item of the form
       (*LIMIT_HEAP=nnnn) at the start, the value is returned. The third argu-
       ment should point to a uint32_t integer. If no such value has been set,
       the call to pcre2_pattern_info() returns the  error  PCRE2_ERROR_UNSET.
       Note  that  this  limit will only be used during matching if it is less
       than the limit set or defaulted by the caller of the match function.

         PCRE2_INFO_JCHANGED

       Return 1 if the (?J) or (?-J) option setting is used  in  the  pattern,
       otherwise  0.  The  third argument should point to a uint32_t variable.
       (?J) and (?-J) set and unset the local PCRE2_DUPNAMES  option,  respec-
       tively.

         PCRE2_INFO_JITSIZE

       If  the  compiled  pattern was successfully processed by pcre2_jit_com-
       pile(), return the size of the  JIT  compiled  code,  otherwise  return
       zero. The third argument should point to a size_t variable.

         PCRE2_INFO_LASTCODETYPE

       Returns  1 if there is a rightmost literal code unit that must exist in
       any matched string, other than at its start. The third argument  should
       point to a uint32_t variable. If there is no such value, 0 is returned.
       When  1  is returned, the code unit value itself can be retrieved using
       PCRE2_INFO_LASTCODEUNIT. For anchored patterns, a last literal value is
       recorded only if it follows something of variable length. For  example,
       for  the pattern /^a\d+z\d+/ the returned value is 1 (with "z" returned
       from PCRE2_INFO_LASTCODEUNIT), but for /^a\dz\d/ the returned value  is
       0.

         PCRE2_INFO_LASTCODEUNIT

       Return  the value of the rightmost literal code unit that must exist in
       any matched string, other than  at  its  start,  for  a  pattern  where
       PCRE2_INFO_LASTCODETYPE returns 1. Otherwise, return 0. The third argu-
       ment should point to a uint32_t variable.

         PCRE2_INFO_MATCHEMPTY

       Return  1  if the pattern might match an empty string, otherwise 0. The
       third argument should point to a uint32_t variable. When a pattern con-
       tains recursive subroutine calls it is not always possible to determine
       whether or not it can match an empty string. PCRE2 takes a cautious ap-
       proach and returns 1 in such cases.

         PCRE2_INFO_MATCHLIMIT

       If the pattern set a match limit by  including  an  item  of  the  form
       (*LIMIT_MATCH=nnnn)  at the start, the value is returned. The third ar-
       gument should point to a uint32_t integer. If no such  value  has  been
       set, the call to pcre2_pattern_info() returns the error PCRE2_ERROR_UN-
       SET.  Note  that  this limit will only be used during matching if it is
       less than the limit set or defaulted by the caller of the  match  func-
       tion.

         PCRE2_INFO_MAXLOOKBEHIND

       A  lookbehind  assertion moves back a certain number of characters (not
       code units) when it starts to process each of its  branches.  This  re-
       quest  returns  the largest of these backward moves. The third argument
       should point to a uint32_t integer. The simple assertions \b and \B re-
       quire a one-character lookbehind and cause PCRE2_INFO_MAXLOOKBEHIND  to
       return  1  in  the absence of anything longer. \A also registers a one-
       character lookbehind, though it does not actually inspect the  previous
       character.

       Note that this information is useful for multi-segment matching only if
       the  pattern  contains  no nested lookbehinds. For example, the pattern
       (?<=a(?<=ba)c) returns a maximum  lookbehind  of  2,  but  when  it  is
       processed,  the  first lookbehind moves back by two characters, matches
       one character, then the nested lookbehind also moves back by two  char-
       acters.  This  puts the matching point three characters earlier than it
       was at the start.  PCRE2_INFO_MAXLOOKBEHIND is really only useful as  a
       debugging  tool. See the pcre2partial documentation for a discussion of
       multi-segment matching.

         PCRE2_INFO_MINLENGTH

       If a minimum length for matching  subject  strings  was  computed,  its
       value is returned. Otherwise the returned value is 0. This value is not
       computed  when PCRE2_NO_START_OPTIMIZE is set. The value is a number of
       characters, which in UTF mode may be different from the number of  code
       units.  The  third  argument  should  point to a uint32_t variable. The
       value is a lower bound to the length of any matching string. There  may
       not  be  any  strings  of that length that do actually match, but every
       string that does match is at least that long.

         PCRE2_INFO_NAMECOUNT
         PCRE2_INFO_NAMEENTRYSIZE
         PCRE2_INFO_NAMETABLE

       PCRE2 supports the use of named as well as numbered capturing parenthe-
       ses. The names are just an additional way of identifying the  parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre2_substring_get_byname()  are provided for extracting captured sub-
       strings by name. It is also possible to extract the data  directly,  by
       first  converting  the  name to a number in order to access the correct
       pointers in the output vector (described with pcre2_match() below).  To
       do the conversion, you need to use the name-to-number map, which is de-
       scribed by these three values.

       The  map  consists  of a number of fixed-size entries. PCRE2_INFO_NAME-
       COUNT gives the number of entries, and  PCRE2_INFO_NAMEENTRYSIZE  gives
       the  size  of each entry in code units; both of these return a uint32_t
       value. The entry size depends on the length of the longest name.

       PCRE2_INFO_NAMETABLE returns a pointer to the first entry of the table.
       This is a PCRE2_SPTR pointer to a block of code units. In the 8-bit li-
       brary, the first two bytes of each entry are the number of the  captur-
       ing  parenthesis,  most  significant byte first. In the 16-bit library,
       the pointer points to 16-bit code units, the first  of  which  contains
       the  parenthesis  number.  In the 32-bit library, the pointer points to
       32-bit code units, the first of which contains the parenthesis  number.
       The rest of the entry is the corresponding name, zero terminated.

       The  names are in alphabetical order. If (?| is used to create multiple
       capture groups with the same number, as described in the section on du-
       plicate group numbers in the pcre2pattern page, the groups may be given
       the same name, but there is only one  entry  in  the  table.  Different
       names for groups of the same number are not permitted.

       Duplicate  names  for capture groups with different numbers are permit-
       ted, but only if PCRE2_DUPNAMES is set. They appear in the table in the
       order in which they were found in the pattern. In the  absence  of  (?|
       this  is  the  order of increasing number; when (?| is used this is not
       necessarily the case because later capture groups may have  lower  num-
       bers.

       As  a  simple  example of the name/number table, consider the following
       pattern after compilation by the 8-bit library  (assume  PCRE2_EXTENDED
       is set, so white space - including newlines - is ignored):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<day>\d\d) )

       There are four named capture groups, so the table has four entries, and
       each  entry  in the table is eight bytes long. The table is as follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

         00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

       When writing code to extract data from named capture groups  using  the
       name-to-number  map,  remember that the length of the entries is likely
       to be different for each compiled pattern.

         PCRE2_INFO_NEWLINE

       The output is one of the following uint32_t values:

         PCRE2_NEWLINE_CR       Carriage return (CR)
         PCRE2_NEWLINE_LF       Linefeed (LF)
         PCRE2_NEWLINE_CRLF     Carriage return, linefeed (CRLF)
         PCRE2_NEWLINE_ANY      Any Unicode line ending
         PCRE2_NEWLINE_ANYCRLF  Any of CR, LF, or CRLF
         PCRE2_NEWLINE_NUL      The NUL character (binary zero)

       This identifies the character sequence that will be recognized as mean-
       ing "newline" while matching.

         PCRE2_INFO_SIZE

       Return the size of the compiled pattern in bytes  (for  all  three  li-
       braries).  The  third  argument should point to a size_t variable. This
       value includes the size of the general data  block  that  precedes  the
       code  units of the compiled pattern itself. The value that is used when
       pcre2_compile() is getting memory in which to place the  compiled  pat-
       tern may be slightly larger than the value returned by this option, be-
       cause  there  are  cases where the code that calculates the size has to
       over-estimate. Processing a pattern with the JIT compiler does not  al-
       ter the value returned by this option.


INFORMATION ABOUT A PATTERN'S CALLOUTS

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       A script language that supports the use of string arguments in callouts
       might  like  to  scan  all the callouts in a pattern before running the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument is a pointer to a compiled pattern, the  second  points  to  a
       callback  function,  and the third is arbitrary user data. The callback
       function is called for every callout in the pattern  in  the  order  in
       which they appear. Its first argument is a pointer to a callout enumer-
       ation  block,  and  its second argument is the user_data value that was
       passed to pcre2_callout_enumerate(). The contents of the  callout  enu-
       meration  block  are described in the pcre2callout documentation, which
       also gives further details about callouts.


SERIALIZATION AND PRECOMPILING

       It is possible to save compiled patterns on disc or elsewhere, and  re-
       load them later, subject to a number of restrictions. The host on which
       the  patterns  are  reloaded must be running the same version of PCRE2,
       with the same code unit width, and must also have the same  endianness,
       pointer  width,  and  PCRE2_SIZE  type. Before compiled patterns can be
       saved, they must be converted to a "serialized" form, which in the case
       of PCRE2 is really just a bytecode dump.  The functions whose names be-
       gin with pcre2_serialize_ are used for converting to and from the seri-
       alized form. They are described in  the  pcre2serialize  documentation.
       Note  that PCRE2 serialization does not convert compiled patterns to an
       abstract format like Java or .NET serialization.


THE MATCH DATA BLOCK

       pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize,
         pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
         const pcre2_code *code, pcre2_general_context *gcontext);

       void pcre2_match_data_free(pcre2_match_data *match_data);

       Information about a successful or unsuccessful match  is  placed  in  a
       match  data  block,  which  is  an opaque structure that is accessed by
       function calls. In particular, the match data block contains  a  vector
       of offsets into the subject string that define the matched parts of the
       subject. This is known as the ovector.

       Before  calling  pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match()
       you must create a match data block by calling one of the creation func-
       tions above. For pcre2_match_data_create(), the first argument  is  the
       number of pairs of offsets in the ovector.

       When  using  pcre2_match(), one pair of offsets is required to identify
       the string that matched the whole pattern, with an additional pair  for
       each captured substring. For example, a value of 4 creates enough space
       to  record  the matched portion of the subject plus three captured sub-
       strings.

       When using pcre2_dfa_match() there may be multiple  matched  substrings
       of  different  lengths  at  the  same point in the subject. The ovector
       should be made large enough to hold as many as are expected.

       A minimum of at least 1 pair is imposed  by  pcre2_match_data_create(),
       so  it  is  always possible to return the overall matched string in the
       case  of  pcre2_match()  or  the  longest  match   in   the   case   of
       pcre2_dfa_match().  The  maximum number of pairs is 65535; if the first
       argument of pcre2_match_data_create() is greater than  this,  65535  is
       used.

       The second argument of pcre2_match_data_create() is a pointer to a gen-
       eral  context, which can specify custom memory management for obtaining
       the memory for the match data block. If you are not using custom memory
       management, pass NULL, which causes malloc() to be used.

       For pcre2_match_data_create_from_pattern(), the  first  argument  is  a
       pointer to a compiled pattern. The ovector is created to be exactly the
       right  size  to  hold  all  the substrings a pattern might capture when
       matched using pcre2_match(). You should not use this call when matching
       with pcre2_dfa_match(). The second argument is again  a  pointer  to  a
       general  context, but in this case if NULL is passed, the memory is ob-
       tained using the same allocator that was used for the compiled  pattern
       (custom or default).

       A  match  data block can be used many times, with the same or different
       compiled patterns. You can extract information from a match data  block
       after  a  match  operation  has  finished, using functions that are de-
       scribed in the sections on matched strings and other match data below.

       When a call of pcre2_match() fails, valid  data  is  available  in  the
       match  block  only  when  the  error  is PCRE2_ERROR_NOMATCH, PCRE2_ER-
       ROR_PARTIAL, or one of the error codes for an invalid UTF  string.  Ex-
       actly what is available depends on the error, and is detailed below.

       When  one of the matching functions is called, pointers to the compiled
       pattern and the subject string are set in the match data block so  that
       they  can  be referenced by the extraction functions after a successful
       match. After running a match, you must not free a compiled pattern or a
       subject string until after all operations on the match data block  (for
       that  match)  have  taken  place,  unless,  in  the case of the subject
       string, you have used the PCRE2_COPY_MATCHED_SUBJECT option,  which  is
       described  in  the section entitled "Option bits for pcre2_match()" be-
       low.

       When a match data block itself is no longer needed, it should be  freed
       by  calling  pcre2_match_data_free(). If this function is called with a
       NULL argument, it returns immediately, without doing anything.


MEMORY USE FOR MATCH DATA BLOCKS

       PCRE2_SIZE pcre2_get_match_data_size(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_match_data_heapframes_size(
         pcre2_match_data *match_data);

       The size of a match data block depends on the size of the ovector  that
       it contains. The function pcre2_get_match_data_size() returns the size,
       in bytes, of the block that is its argument.

       When pcre2_match() runs interpretively (that is, without using JIT), it
       makes use of a vector of data frames for remembering backtracking posi-
       tions.  The size of each individual frame depends on the number of cap-
       turing  parentheses  in  the  pattern  and  can  be obtained by calling
       pcre2_pattern_info() with the PCRE2_INFO_FRAMESIZE option (see the sec-
       tion entitled "Information about a compiled pattern" above).

       Heap memory is used for the frames vector; if the initial memory  block
       turns  out  to  be  too  small during matching, it is automatically ex-
       panded. When pcre2_match() returns, the memory is not  freed,  but  re-
       mains  attached  to  the  match  data  block, for use by any subsequent
       matches that use the same block. It is  automatically  freed  when  the
       match data block itself is freed.

       You  can  find  the current size of the frames vector that a match data
       block owns by  calling  pcre2_get_match_data_heapframes_size().  For  a
       newly  created  match  data  block the size will be zero. Some types of
       match may require a lot of frames and thus a large vector; applications
       that run in environments where memory is constrained can check this and
       free the match data block if the heap frames vector has become too big.


MATCHING A PATTERN: THE TRADITIONAL FUNCTION

       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       The function pcre2_match() is called to match a subject string  against
       a  compiled pattern, which is passed in the code argument. You can call
       pcre2_match() with the same code argument as many times as you like, in
       order to find multiple matches in the subject string or to  match  dif-
       ferent subject strings with the same pattern.

       This  function is the main matching facility of the library, and it op-
       erates in a Perl-like manner. For specialist use there is also  an  al-
       ternative  matching  function,  which is described below in the section
       about the pcre2_dfa_match() function.

       Here is an example of a simple call to pcre2_match():

         pcre2_match_data *md = pcre2_match_data_create(4, NULL);
         int rc = pcre2_match(
           re,             /* result of pcre2_compile() */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           md,             /* the match data block */
           NULL);          /* a match context; NULL means use defaults */

       If the subject string is zero-terminated, the length can  be  given  as
       PCRE2_ZERO_TERMINATED. A match context must be provided if certain less
       common matching parameters are to be changed. For details, see the sec-
       tion on the match context above.

   The string to be matched by pcre2_match()

       The  subject string is passed to pcre2_match() as a pointer in subject,
       a length in length, and a starting offset in  startoffset.  The  length
       and  offset  are  in  code units, not characters.  That is, they are in
       bytes for the 8-bit library, 16-bit code units for the 16-bit  library,
       and  32-bit  code units for the 32-bit library, whether or not UTF pro-
       cessing is enabled. As a special case, if subject is NULL and length is
       zero, the subject is assumed to be an empty string. If length  is  non-
       zero, an error occurs if subject is NULL.

       If startoffset is greater than the length of the subject, pcre2_match()
       returns  PCRE2_ERROR_BADOFFSET.  When  the starting offset is zero, the
       search for a match starts at the beginning of the subject, and this  is
       by far the most common case. In UTF-8 or UTF-16 mode, the starting off-
       set  must  point to the start of a character, or to the end of the sub-
       ject (in UTF-32 mode, one code unit equals one character, so  all  off-
       sets  are  valid). Like the pattern string, the subject may contain bi-
       nary zeros.

       A non-zero starting offset is useful when searching for  another  match
       in  the  same  subject  by calling pcre2_match() again after a previous
       success.  Setting startoffset differs from  passing  over  a  shortened
       string  and  setting  PCRE2_NOTBOL in the case of a pattern that begins
       with any kind of lookbehind. For example, consider the pattern

         \Biss\B

       which finds occurrences of "iss" in the middle of  words.  (\B  matches
       only  if  the  current position in the subject is not a word boundary.)
       When  applied  to  the  string  "Mississippi"   the   first   call   to
       pcre2_match()  finds  the  first occurrence. If pcre2_match() is called
       again with just the remainder of the subject, namely "issippi", it does
       not match, because \B is always false at  the  start  of  the  subject,
       which  is  deemed  to  be a word boundary. However, if pcre2_match() is
       passed the entire string again, but with startoffset set to 4, it finds
       the second occurrence of "iss" because it is able to  look  behind  the
       starting point to discover that it is preceded by a letter.

       Finding  all  the  matches  in a subject is tricky when the pattern can
       match an empty string. PCRE2 includes a helper API to assist with this;
       see the section entitled "Iterating over all  matches"  below  for  de-
       tails.

       If a non-zero starting offset is passed when the pattern is anchored, a
       single attempt to match at the given offset is made. This can only suc-
       ceed  if  the  pattern does not require the match to be at the start of
       the subject. In other words, the anchoring must be the result  of  set-
       ting  the PCRE2_ANCHORED option or the use of .* with PCRE2_DOTALL, not
       by starting the pattern with ^ or \A.

   Option bits for pcre2_match()

       The unused bits of the options argument for pcre2_match() must be zero.
       The   only   bits    that    may    be    set    are    PCRE2_ANCHORED,
       PCRE2_COPY_MATCHED_SUBJECT,  PCRE2_DISABLE_RECURSELOOP_CHECK, PCRE2_EN-
       DANCHORED,      PCRE2_NOTBOL,       PCRE2_NOTEOL,       PCRE2_NOTEMPTY,
       PCRE2_NOTEMPTY_ATSTART,  PCRE2_NO_JIT,  PCRE2_NO_UTF_CHECK,  PCRE2_PAR-
       TIAL_HARD, and PCRE2_PARTIAL_SOFT.  Their action is described below.

       Setting PCRE2_ANCHORED or PCRE2_ENDANCHORED at match time is  not  sup-
       ported  by  the just-in-time (JIT) compiler. If it is set, JIT matching
       is  disabled  and  the  interpretive  code  in  pcre2_match()  is  run.
       PCRE2_DISABLE_RECURSELOOP_CHECK  is  ignored  by  JIT,  but  apart from
       PCRE2_NO_JIT (obviously), the remaining options are supported  for  JIT
       matching.

         PCRE2_ANCHORED

       The PCRE2_ANCHORED option limits pcre2_match() to matching at the first
       matching  position.  If  a pattern was compiled with PCRE2_ANCHORED, or
       turned out to be anchored by virtue of its contents, it cannot be  made
       unanchored at matching time. Note that setting the option at match time
       disables JIT matching.

         PCRE2_COPY_MATCHED_SUBJECT

       By  default,  a  pointer to the subject is remembered in the match data
       block so that, after a successful match, it can be  referenced  by  the
       substring  extraction  functions.  This means that the subject's memory
       must not be freed until all such operations are complete. For some  ap-
       plications  where the lifetime of the subject string is not guaranteed,
       it may be necessary to make a copy of the subject  string,  but  it  is
       wasteful  to do this unless the match is successful. After a successful
       match, if PCRE2_COPY_MATCHED_SUBJECT is set, the subject is copied  and
       the  new  pointer  is remembered in the match data block instead of the
       original subject pointer. The memory allocator that was  used  for  the
       match  block  itself  is  used.  The  copy  is automatically freed when
       pcre2_match_data_free() is called to free the match data block.  It  is
       also automatically freed if the match data block is re-used for another
       match operation.

         PCRE2_DISABLE_RECURSELOOP_CHECK

       This  option  is relevant only to pcre2_match() for interpretive match-
       ing.   It  is  ignored  when  JIT  is  used,  and  is   forbidden   for
       pcre2_dfa_match().

       The use of recursion in patterns can lead to infinite loops. In the in-
       terpretive  matcher  these  would  be eventually caught by the match or
       heap limits, but this could take a long time and/or use a lot of memory
       if the limits are large. There is therefore a check  at  the  start  of
       each  recursion.   If  the  same  group is still active from a previous
       call, and the current subject pointer is the same  as  it  was  at  the
       start  of  that group, and the furthest inspected character of the sub-
       ject has not changed, an error is generated.

       There are rare cases of matches that would complete,  but  nevertheless
       trigger  this  error.  This  option  disables the check. It is provided
       mainly for testing when comparing JIT and interpretive behaviour.

         PCRE2_ENDANCHORED

       If the PCRE2_ENDANCHORED option is set, any string  that  pcre2_match()
       matches  must be right at the end of the subject string. Note that set-
       ting the option at match time disables JIT matching.

         PCRE2_NOTBOL

       This option specifies that first character of the subject string is not
       the beginning of a line, so the  circumflex  metacharacter  should  not
       match  before  it.  Setting  this without having set PCRE2_MULTILINE at
       compile time causes circumflex never to match. This option affects only
       the behaviour of the circumflex metacharacter. It does not affect \A.

         PCRE2_NOTEOL

       This option specifies that the end of the subject string is not the end
       of a line, so the dollar metacharacter should not match it nor  (except
       in  multiline mode) a newline immediately before it. Setting this with-
       out having set PCRE2_MULTILINE at compile time causes dollar  never  to
       match. This option affects only the behaviour of the dollar metacharac-
       ter. It does not affect \Z or \z.

         PCRE2_NOTEMPTY

       An empty string is not considered to be a valid match if this option is
       set.  If  there are alternatives in the pattern, they are tried. If all
       the alternatives match the empty string, the entire  match  fails.  For
       example, if the pattern

         a?b?

       is  applied  to  a  string not beginning with "a" or "b", it matches an
       empty string at the start of the subject. With PCRE2_NOTEMPTY set, this
       match is not valid, so pcre2_match() searches further into  the  string
       for occurrences of "a" or "b".

         PCRE2_NOTEMPTY_ATSTART

       This  is  like PCRE2_NOTEMPTY, except that it locks out an empty string
       match only at the first matching position, that is, at the start of the
       subject plus the starting offset. An empty string match  later  in  the
       subject is permitted.  If the pattern is anchored, such a match can oc-
       cur only if the pattern contains \K.

         PCRE2_NO_JIT

       By   default,   if   a  pattern  has  been  successfully  processed  by
       pcre2_jit_compile(), JIT is automatically used  when  pcre2_match()  is
       called  with  options  that JIT supports. Setting PCRE2_NO_JIT disables
       the use of JIT; it forces matching to be done by the interpreter.

         PCRE2_NO_UTF_CHECK

       When PCRE2_UTF is set at compile time, the validity of the subject as a
       UTF  string  is  checked  unless  PCRE2_NO_UTF_CHECK   is   passed   to
       pcre2_match() or PCRE2_MATCH_INVALID_UTF was passed to pcre2_compile().
       The latter special case is discussed in detail in the pcre2unicode doc-
       umentation.

       In  the default case, if a non-zero starting offset is given, the check
       is applied only to that part of the subject  that  could  be  inspected
       during  matching,  and there is a check that the starting offset points
       to the first code unit of a character or to the end of the subject.  If
       there  are no lookbehind assertions in the pattern, the check starts at
       the starting offset.  Otherwise, it starts at the length of the longest
       lookbehind before the starting offset, or at the start of  the  subject
       if  there are not that many characters before the starting offset. Note
       that the sequences \b and \B are one-character lookbehinds.

       The check is carried out before any other processing takes place, and a
       negative error code is returned if the check fails. There  are  several
       UTF  error  codes  for each code unit width, corresponding to different
       problems with the code unit sequence. There are discussions  about  the
       validity  of  UTF-8  strings, UTF-16 strings, and UTF-32 strings in the
       pcre2unicode documentation.

       If you know that your subject is valid, and you want to skip this check
       for performance reasons, you can set the PCRE2_NO_UTF_CHECK option when
       calling pcre2_match(). You might want to do this  for  the  second  and
       subsequent  calls  to pcre2_match() if you are making repeated calls to
       find multiple matches in the same subject string.

       Warning: Unless PCRE2_MATCH_INVALID_UTF was set at compile  time,  when
       PCRE2_NO_UTF_CHECK  is  set  at match time the effect of passing an in-
       valid string as a subject, or an invalid value of startoffset, is unde-
       fined.  Your program may crash or loop indefinitely or give  wrong  re-
       sults.

         PCRE2_PARTIAL_HARD
         PCRE2_PARTIAL_SOFT

       These options turn on the partial matching feature. A partial match oc-
       curs  if  the  end  of  the subject string is reached successfully, but
       there are not enough subject characters to complete the match. In addi-
       tion, either at least one character must have  been  inspected  or  the
       pattern  must  contain  a  lookbehind,  or the pattern must be one that
       could match an empty string.

       If this situation arises when PCRE2_PARTIAL_SOFT  (but  not  PCRE2_PAR-
       TIAL_HARD) is set, matching continues by testing any remaining alterna-
       tives.  Only  if  no complete match can be found is PCRE2_ERROR_PARTIAL
       returned instead of PCRE2_ERROR_NOMATCH.  In  other  words,  PCRE2_PAR-
       TIAL_SOFT  specifies  that  the  caller is prepared to handle a partial
       match, but only if no complete match can be found.

       If PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In  this
       case,  if  a  partial match is found, pcre2_match() immediately returns
       PCRE2_ERROR_PARTIAL, without considering  any  other  alternatives.  In
       other words, when PCRE2_PARTIAL_HARD is set, a partial match is consid-
       ered to be more important than an alternative complete match.

       There is a more detailed discussion of partial and multi-segment match-
       ing, with examples, in the pcre2partial documentation.


NEWLINE HANDLING WHEN MATCHING

       When  PCRE2 is built, a default newline convention is set; this is usu-
       ally the standard convention for the operating system. The default  can
       be  overridden  in a compile context by calling pcre2_set_newline(). It
       can also be overridden by starting a pattern string with, for  example,
       (*CRLF),  as  described  in  the  section on newline conventions in the
       pcre2pattern page. During matching, the newline choice affects the  be-
       haviour  of the dot, circumflex, and dollar metacharacters. It may also
       alter the way the match starting position is  advanced  after  a  match
       failure for an unanchored pattern.

       When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is
       set  as  the  newline convention, and a match attempt for an unanchored
       pattern fails when the current starting position is at a CRLF sequence,
       and the pattern contains no explicit matches for CR or  LF  characters,
       the  match  position  is  advanced by two characters instead of one, in
       other words, to after the CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected. For example, if the pattern is .+A (and the PCRE2_DOTALL  op-
       tion  is  not set), it does not match the string "\r\nA" because, after
       failing at the start, it skips both the CR and the LF before  retrying.
       However,  the  pattern  [\r\n]A does match that string, because it con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those characters in the pattern, or one of the \r or \n  or  equivalent
       octal or hexadecimal escape sequences. Implicit matches such as [^X] do
       not  count, nor does \s, even though it includes CR and LF in the char-
       acters that it matches.

       Notwithstanding the above, anomalous effects may still occur when  CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.


HOW PCRE2_MATCH() RETURNS A STRING AND CAPTURED SUBSTRINGS

       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       In  general, a pattern matches a certain portion of the subject, and in
       addition, further substrings from the subject  may  be  picked  out  by
       parenthesized  parts  of  the  pattern.  Following the usage in Jeffrey
       Friedl's book, this is called "capturing"  in  what  follows,  and  the
       phrase  "capture  group" (Perl terminology) is used for a fragment of a
       pattern that picks out a substring. PCRE2 supports several other  kinds
       of parenthesized group that do not cause substrings to be captured. The
       pcre2_pattern_info()  function can be used to find out how many capture
       groups there are in a compiled pattern.

       You can use auxiliary functions for accessing  captured  substrings  by
       number or by name, as described in sections below.

       Alternatively, you can make direct use of the vector of PCRE2_SIZE val-
       ues,  called  the  ovector,  which  contains  the  offsets  of captured
       strings.  It  is  part  of  the  match  data   block.    The   function
       pcre2_get_ovector_pointer()  returns  the  address  of the ovector, and
       pcre2_get_ovector_count() returns the number of pairs of values it con-
       tains.

       Within the ovector, the first in each pair of values is set to the off-
       set of the first code unit of a substring, and the second is set to the
       offset of the first code unit after the end of a substring. These  val-
       ues  are always code unit offsets, not character offsets. That is, they
       are byte offsets in the 8-bit library, 16-bit offsets in the 16-bit li-
       brary, and 32-bit offsets in the 32-bit library.

       After a partial match  (error  return  PCRE2_ERROR_PARTIAL),  only  the
       first  pair  of  offsets  (that is, ovector[0] and ovector[1]) are set.
       They identify the part of the subject that was partially  matched.  See
       the pcre2partial documentation for details of partial matching.

       After  a  fully  successful match, the first pair of offsets identifies
       the portion of the subject string that was matched by the  entire  pat-
       tern.  The  next  pair is used for the first captured substring, and so
       on. The value returned by pcre2_match() is one more  than  the  highest
       numbered  pair  that  has been set. For example, if two substrings have
       been captured, the returned value is 3. If there are no  captured  sub-
       strings, the return value from a successful match is 1, indicating that
       just the first pair of offsets has been set.

       If  a  pattern  uses the \K escape sequence within a positive lookahead
       assertion, the reported start of a successful match can be greater than
       the end of the match. For example, if the pattern (?=ab\K)  is  matched
       against  "ab",  the start and end offset values for the match are 2 and
       0.

       If a capture group is matched repeatedly within a single  match  opera-
       tion, it is the last portion of the subject that it matched that is re-
       turned.

       If the ovector is too small to hold all the captured substring offsets,
       as  much  as possible is filled in, and the function returns a value of
       zero. If captured substrings are not of interest, pcre2_match() may  be
       called with a match data block whose ovector is of minimum length (that
       is, one pair).

       It  is  possible for capture group number n+1 to match some part of the
       subject when group n has not been used at  all.  For  example,  if  the
       string "abc" is matched against the pattern (a|(z))(bc) the return from
       the  function  is 4, and groups 1 and 3 are matched, but 2 is not. When
       this happens, both values in the offset pairs corresponding  to  unused
       groups are set to PCRE2_UNSET.

       Offset  values  that  correspond to unused groups at the end of the ex-
       pression are also set to PCRE2_UNSET. For example, if the string  "abc"
       is  matched  against  the pattern (abc)(x(yz)?)? groups 2 and 3 are not
       matched. The return from the function is 2, because  the  highest  used
       capture group number is 1. The offsets for the second and third capture
       groups  (assuming  the  vector  is  large enough, of course) are set to
       PCRE2_UNSET.

       Elements in the ovector that do not correspond to capturing parentheses
       in the pattern are never changed. That is, if a pattern contains n cap-
       turing parentheses, no more than ovector[0] to ovector[2n+1] are set by
       pcre2_match(). The other elements retain whatever  values  they  previ-
       ously  had.  After  a failed match attempt, the contents of the ovector
       are unchanged.


OTHER INFORMATION ABOUT A MATCH

       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);

       As well as the offsets in the ovector, other information about a  match
       is  retained  in the match data block and can be retrieved by the above
       functions in appropriate circumstances. If they  are  called  at  other
       times, the result is undefined.

       After  a  successful match, a partial match (PCRE2_ERROR_PARTIAL), or a
       failure to match (PCRE2_ERROR_NOMATCH), a mark name may  be  available.
       The  function pcre2_get_mark() can be called to access this name, which
       can be specified in the pattern by  any  of  the  backtracking  control
       verbs, not just (*MARK). The same function applies to all the verbs. It
       returns a pointer to the zero-terminated name, which is within the com-
       piled pattern. If no name is available, NULL is returned. The length of
       the  name  (excluding  the terminating zero) is stored in the code unit
       that precedes the name. You should use this length instead  of  relying
       on the terminating zero if the name might contain a binary zero.

       After  a  successful  match, the name that is returned is the last mark
       name encountered on the matching path through the pattern. Instances of
       backtracking verbs without names do not count. Thus,  for  example,  if
       the matching path contains (*MARK:A)(*PRUNE), the name "A" is returned.
       After a "no match" or a partial match, the last encountered name is re-
       turned. For example, consider this pattern:

         ^(*MARK:A)((*MARK:B)a|b)c

       When  it  matches "bc", the returned name is A. The B mark is "seen" in
       the first branch of the group, but it is not on the matching  path.  On
       the  other  hand,  when  this pattern fails to match "bx", the returned
       name is B.

       Warning: By default, certain start-of-match optimizations are  used  to
       give  a  fast "no match" result in some situations. For example, if the
       anchoring is removed from the pattern above, there is an initial  check
       for  the presence of "c" in the subject before running the matching en-
       gine. This check fails for "bx", causing a match failure without seeing
       any marks. You can disable the start-of-match optimizations by  setting
       the  PCRE2_NO_START_OPTIMIZE  option for pcre2_compile() or by starting
       the pattern with (*NO_START_OPT).

       After a successful match, a partial match, or one of  the  invalid  UTF
       errors  (for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar() can
       be called. After a successful or partial match it returns the code unit
       offset of the character at which the match started. For  a  non-partial
       match,  this can be different to the value of ovector[0] if the pattern
       contains the \K escape sequence. After a partial match,  however,  this
       value  is  always the same as ovector[0] because \K does not affect the
       result of a partial match.

       After a UTF check failure, pcre2_get_startchar() can be used to  obtain
       the code unit offset of the invalid UTF character. Details are given in
       the pcre2unicode page.


ERROR RETURNS FROM pcre2_match()

       If  pcre2_match() fails, it returns a negative number. This can be con-
       verted to a text string by calling the pcre2_get_error_message()  func-
       tion  (see  "Obtaining a textual error message" below).  Negative error
       codes are also returned by other functions,  and  are  documented  with
       them.  The codes are given names in the header file. If UTF checking is
       in force and an invalid UTF subject string is detected, one of a number
       of UTF-specific negative error codes is returned. Details are given  in
       the  pcre2unicode  page. The following are the other errors that may be
       returned by pcre2_match():

         PCRE2_ERROR_NOMATCH

       The subject string did not match the pattern.

         PCRE2_ERROR_PARTIAL

       The subject string did not match, but it did match partially.  See  the
       pcre2partial documentation for details of partial matching.

         PCRE2_ERROR_BADMAGIC

       PCRE2 stores a 4-byte "magic number" at the start of the compiled code,
       to  catch  the case when it is passed a junk pointer. This is the error
       that is returned when the magic number is not present.

         PCRE2_ERROR_BADMODE

       This error is given when a compiled pattern is passed to a function  in
       a  library  of a different code unit width, for example, a pattern com-
       piled by the 8-bit library is passed to  a  16-bit  or  32-bit  library
       function.

         PCRE2_ERROR_BADOFFSET

       The value of startoffset was greater than the length of the subject.

         PCRE2_ERROR_BADOPTION

       An unrecognized bit was set in the options argument.

         PCRE2_ERROR_BADUTFOFFSET

       The UTF code unit sequence that was passed as a subject was checked and
       found  to be valid (the PCRE2_NO_UTF_CHECK option was not set), but the
       value of startoffset did not point to the beginning of a UTF  character
       or the end of the subject.

         PCRE2_ERROR_CALLOUT

       This  error  is never generated by pcre2_match() itself. It is provided
       for use by callout  functions  that  want  to  cause  pcre2_match()  or
       pcre2_callout_enumerate()  to  return a distinctive error code. See the
       pcre2callout documentation for details.

         PCRE2_ERROR_DEPTHLIMIT

       The nested backtracking depth limit was reached.

         PCRE2_ERROR_HEAPLIMIT

       The heap limit was reached.

         PCRE2_ERROR_INTERNAL

       An unexpected internal error has occurred. This error could  be  caused
       by a bug in PCRE2 or by overwriting of the compiled pattern.

         PCRE2_ERROR_JIT_STACKLIMIT

       This error is returned when a pattern that was successfully studied us-
       ing JIT is being matched, but the memory available for the just-in-time
       processing  stack  is  not large enough. See the pcre2jit documentation
       for more details.

         PCRE2_ERROR_MATCHLIMIT

       The backtracking match limit was reached.

         PCRE2_ERROR_NOMEMORY

       Heap memory is used to remember  backtracking  points.  This  error  is
       given  when  the  memory allocation function (default or custom) fails.
       Note that a different error, PCRE2_ERROR_HEAPLIMIT,  is  given  if  the
       amount of memory needed exceeds the heap limit. PCRE2_ERROR_NOMEMORY is
       also  returned  if PCRE2_COPY_MATCHED_SUBJECT is set and memory alloca-
       tion fails.

         PCRE2_ERROR_NULL

       Either the code, subject, or match_data argument was passed as NULL.

         PCRE2_ERROR_RECURSELOOP

       This error is returned when  pcre2_match()  detects  a  recursion  loop
       within  the  pattern. Specifically, it means that either the whole pat-
       tern or a capture group has been called recursively for the second time
       at the same position in the subject string. Some simple  patterns  that
       might  do  this are detected and faulted at compile time, but more com-
       plicated cases, in particular mutual recursions between  two  different
       groups, cannot be detected until matching is attempted.


OBTAINING A TEXTUAL ERROR MESSAGE

       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
         PCRE2_SIZE bufflen);

       A  text  message  for  an  error code from any PCRE2 function (compile,
       match, or auxiliary) can be obtained  by  calling  pcre2_get_error_mes-
       sage().  The  code  is passed as the first argument, with the remaining
       two arguments specifying a code unit buffer  and  its  length  in  code
       units,  into  which the text message is placed. The message is returned
       in code units of the appropriate width for the library  that  is  being
       used.

       The  returned message is terminated with a trailing zero, and the func-
       tion returns the number of code  units  used,  excluding  the  trailing
       zero. If the error number is unknown, the negative error code PCRE2_ER-
       ROR_BADDATA  is  returned.  If  the buffer is too small, the message is
       truncated (but still with a trailing zero), and the negative error code
       PCRE2_ERROR_NOMEMORY is returned.  None of the messages is very long; a
       buffer size of 120 code units is ample.


ITERATING OVER ALL MATCHES

       int pcre2_next_match(pcre2_match_data *match_data,
         PCRE2_SIZE *pstart_offset, uint32_t *poptions);

       A common task for applications is to implement "global" matching behav-
       iour, for example, replacing all matches in the subject; splitting  the
       subject  on  all matches; or simply counting the number of matches. The
       pcre2_next_match() function helps with this task by providing  the  ap-
       propriate  parameters for the next match attempt (available since PCRE2
       10.47).

       First, a match attempt should be made using one of the  matching  func-
       tions  (pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match()).  Then,
       pcre2_next_match() can be called, providing the same match_data parame-
       ter.

       It returns 0 ("false") if there is no need to make a further match  at-
       tempt,  or 1 ("true") if another match should be attempted. Returning 1
       does not imply that there is another match,  only  that  another  match
       should be attempted (which may return PCRE2_ERROR_NOMATCH).

       The  *pstart_offset  and  *poptions  are set if the function returns 1.
       The *pstart_offset should be passed to the next match attempt directly,
       and the *poptions should be passed to the next match attempt by combin-
       ing with the application's match options using OR.

       There is some code that demonstrates how to do this  in  the  pcre2demo
       sample program. The general pattern is:

         uint32_t app_options = ...;
         uint32_t global_options = 0;
         PCRE2_SIZE start_offset = 0;
         while (1)
           {
           int rc = pcre2_match(re, subject, subject_len, start_offset,
                                app_options | global_options, match_data,
                                match_context);

           if (rc == PCRE2_ERROR_NOMATCH) break; /* no match, and no more attempts */
           if (rc < 0) { ... exit }

           ...handle the match

           if (!pcre2_next_match(match_data, &start_offset, &global_options))
             break; /* no more attempts */
           }

       The guarantees provided by pcre2_next_match() are that the start_offset
       will  advance,  so  the  loop will definitely terminate. The conditions
       which ensure this are that either:  (a)  pcre2_next_match()  returns  0
       (false);  or  (b)  the returned *pstart_offset is strictly greater than
       the previous start_offset; or (c) if the previous match was a  success-
       ful match of the empty string then the returned *pstart_offset is equal
       to   the   previous   ovector[1],   and   *poptions   will  be  set  to
       PCRE2_NOTEMPTY_ATSTART to prevent another empty match  from  being  re-
       turned.

       A  loop  implemented as shown above will always terminate, unless there
       is a bug in PCRE2. As a measure of  "defensive  programming",  applica-
       tions  are  encouraged to add an assertion or check to break their loop
       if it does not make progress (and report the issue as a bug).

       If   an   application   does   not   use   the   flag   PCRE2_EXTRA_AL-
       LOW_LOOKAROUND_BSK, then each match is "well-behaved" and satisfies:

         start_offset <= ovector[0] <= ovector[1].

       In    this    case,   the   matches   found   by   pcre2_match()   with
       pcre2_next_match() will be sorted, non-overlapping (possibly touching),
       and with no duplicates.

       Otherwise, if PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK is used, then the  guar-
       antees  are  considerably  weaker. We do not guarantee that the matches
       will always advance: only that the start_offset will. The matches found
       by pcre2_match() with pcre2_next_match() will be a finite sequence  (as
       pcre2_next_match()  ensures  that  start_offset advances, so the search
       will terminate). The matches can however be  overlapping,  can  contain
       duplicates, and (in truly pathological examples) may not even be sorted
       by ovector[0]. Additionally, each match itself can end before it starts
       (ovector[1]  <  ovector[0]).  We recommend that applications do not set
       PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK.


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_SIZE *length);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR *buffer,
         PCRE2_SIZE *bufflen);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR **bufferptr,
         PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       Captured substrings can be accessed directly by using  the  ovector  as
       described above.  For convenience, auxiliary functions are provided for
       extracting   captured  substrings  as  new,  separate,  zero-terminated
       strings. A substring that contains a binary zero is correctly extracted
       and has a further zero added on the end, but  the  result  is  not,  of
       course, a C string.

       The functions in this section identify substrings by number. The number
       zero refers to the entire matched substring, with higher numbers refer-
       ring  to  substrings  captured by parenthesized groups. After a partial
       match, only substring zero is available.  An  attempt  to  extract  any
       other  substring  gives the error PCRE2_ERROR_PARTIAL. The next section
       describes similar functions for extracting captured substrings by name.

       If a pattern uses the \K escape sequence within  a  positive  lookahead
       assertion, the reported start of a successful match can be greater than
       the  end  of the match. For example, if the pattern (?=ab\K) is matched
       against "ab", the start and end offset values for the match are  2  and
       0.  In  this  situation,  calling these functions with a zero substring
       number extracts a zero-length empty string.

       You can find the length in code units of a captured  substring  without
       extracting  it  by calling pcre2_substring_length_bynumber(). The first
       argument is a pointer to the match data block, the second is the  group
       number,  and the third is a pointer to a variable into which the length
       is placed. If you just want to know whether or not  the  substring  has
       been captured, you can pass the third argument as NULL.

       The  pcre2_substring_copy_bynumber()  function  copies  a captured sub-
       string into a supplied buffer,  whereas  pcre2_substring_get_bynumber()
       copies  it  into  new memory, obtained using the same memory allocation
       function that was used for the match data block. The  first  two  argu-
       ments  of  these  functions are a pointer to the match data block and a
       capture group number.

       The final arguments of pcre2_substring_copy_bynumber() are a pointer to
       the buffer and a pointer to a variable that contains its length in code
       units.  This is updated to contain the actual number of code units used
       for the extracted substring, excluding the terminating zero.

       For pcre2_substring_get_bynumber() the third and fourth arguments point
       to variables that are updated with a pointer to the new memory and  the
       number  of  code units that comprise the substring, again excluding the
       terminating zero. When the substring is no longer  needed,  the  memory
       should be freed by calling pcre2_substring_free().

       The  return  value  from  all these functions is zero for success, or a
       negative error code. If the pattern match  failed,  the  match  failure
       code  is returned.  If a substring number greater than zero is used af-
       ter a partial match, PCRE2_ERROR_PARTIAL is  returned.  Other  possible
       error codes are:

         PCRE2_ERROR_NOMEMORY

       The  buffer  was  too small for pcre2_substring_copy_bynumber(), or the
       attempt to get memory failed for pcre2_substring_get_bynumber().

         PCRE2_ERROR_NOSUBSTRING

       There is no substring with that number in the  pattern,  that  is,  the
       number is greater than the number of capturing parentheses.

         PCRE2_ERROR_UNAVAILABLE

       The substring number, though not greater than the number of captures in
       the pattern, is greater than the number of slots in the ovector, so the
       substring could not be captured.

         PCRE2_ERROR_UNSET

       The  substring  did  not  participate in the match. For example, if the
       pattern is (abc)|(def) and the subject is "def", and the  ovector  con-
       tains at least two capturing slots, substring number 1 is unset.


EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS

       int pcre2_substring_list_get(pcre2_match_data *match_data,
         PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);

       void pcre2_substring_list_free(PCRE2_UCHAR **list);

       The  pcre2_substring_list_get()  function  extracts  all available sub-
       strings and builds a list of pointers to  them.  It  also  (optionally)
       builds  a  second list that contains their lengths (in code units), ex-
       cluding a terminating zero that is added to each of them. All  this  is
       done in a single block of memory that is obtained using the same memory
       allocation function that was used to get the match data block.

       This  function  must be called only after a successful match. If called
       after a partial match, the error code PCRE2_ERROR_PARTIAL is returned.

       The address of the memory block is returned via listptr, which is  also
       the start of the list of string pointers. The end of the list is marked
       by  a  NULL pointer. The address of the list of lengths is returned via
       lengthsptr. If your strings do not contain binary zeros and you do  not
       therefore need the lengths, you may supply NULL as the lengthsptr argu-
       ment  to  disable  the  creation of a list of lengths. The yield of the
       function is zero if all went well, or PCRE2_ERROR_NOMEMORY if the  mem-
       ory  block could not be obtained. When the list is no longer needed, it
       should be freed by calling pcre2_substring_list_free().

       If this function encounters a substring that is unset, which can happen
       when capture group number n+1 matches some part  of  the  subject,  but
       group  n has not been used at all, it returns an empty string. This can
       be distinguished from a genuine zero-length substring by inspecting the
       appropriate offset in the ovector, which contain PCRE2_UNSET for  unset
       substrings, or by calling pcre2_substring_length_bynumber().


EXTRACTING CAPTURED SUBSTRINGS BY NAME

       int pcre2_substring_number_from_name(const pcre2_code *code,
         PCRE2_SPTR name);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_SIZE *length);

       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       To  extract a substring by name, you first have to find associated num-
       ber.  For example, for this pattern:

         (a+)b(?<xxx>\d+)...

       the number of the capture group called "xxx" is 2. If the name is known
       to be unique (PCRE2_DUPNAMES was not set), you can find the number from
       the name by calling pcre2_substring_number_from_name(). The first argu-
       ment is the compiled pattern, and the second is the name. The yield  of
       the  function  is the group number, PCRE2_ERROR_NOSUBSTRING if there is
       no group with that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if  there  is
       more  than one group with that name.  Given the number, you can extract
       the substring directly from the ovector, or use one of  the  "bynumber"
       functions described above.

       For  convenience,  there are also "byname" functions that correspond to
       the "bynumber" functions, the only difference being that the second ar-
       gument is a name instead of a number.  If  PCRE2_DUPNAMES  is  set  and
       there are duplicate names, these functions scan all the groups with the
       given  name,  and  return  the  captured substring from the first named
       group that is set.

       If there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING  is
       returned.  If  all  groups  with the name have numbers that are greater
       than the number of slots in the ovector, PCRE2_ERROR_UNAVAILABLE is re-
       turned. If there is at least one group with a slot in the ovector,  but
       no group is found to be set, PCRE2_ERROR_UNSET is returned.

       Warning: If the pattern uses the (?| feature to set up multiple capture
       groups  with  the same number, as described in the section on duplicate
       group numbers in the pcre2pattern page, you cannot use names to distin-
       guish the different capture groups, because names are not  included  in
       the  compiled  code.  The  matching process uses only numbers. For this
       reason, the use of different names for  groups  with  the  same  number
       causes an error at compile time.


CREATING A NEW STRING WITH SUBSTITUTIONS

       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext, PCRE2_SPTR replacement,
         PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer,
         PCRE2_SIZE *outlengthptr);

       This  function  optionally calls pcre2_match() and then makes a copy of
       the subject string in outputbuffer, replacing parts that  were  matched
       with the replacement string, whose length is supplied in rlength, which
       can  be given as PCRE2_ZERO_TERMINATED for a zero-terminated string. As
       a special case, if replacement is NULL and rlength  is  zero,  the  re-
       placement  is assumed to be an empty string. If rlength is non-zero, an
       error occurs if replacement is NULL.

       There is an option (see PCRE2_SUBSTITUTE_REPLACEMENT_ONLY below) to re-
       turn just the replacement string(s). The default action is  to  perform
       just  one  replacement  if  the pattern matches, but there is an option
       that requests multiple replacements  (see  PCRE2_SUBSTITUTE_GLOBAL  be-
       low).

       If  successful,  pcre2_substitute() returns the number of substitutions
       that were carried out. This may be zero if no match was found,  and  is
       never  greater  than one unless PCRE2_SUBSTITUTE_GLOBAL is set. A nega-
       tive value is returned if an error is detected.

       Matches in which a \K item in a lookahead in  the  pattern  causes  the
       match  to  end  before it starts are not supported, and give rise to an
       error return. For global replacements, matches in which \K in a lookbe-
       hind causes the match to start earlier than the point that was  reached
       in the previous iteration are also not supported. (These cases are only
       possible  if  the pattern was compiled with the backwards-compatibility
       option PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK.)

       The first seven arguments of pcre2_substitute() are  the  same  as  for
       pcre2_match(), except that the partial matching options are not permit-
       ted,  and  match_data may be passed as NULL, in which case a match data
       block is obtained and freed within this function, using memory  manage-
       ment  functions from the match context, if provided, or else those that
       were used to allocate memory for the compiled code.

       If match_data is not NULL and PCRE2_SUBSTITUTE_MATCHED is not set,  the
       provided block is used for all calls to pcre2_match(), and its contents
       afterwards  are  the  result  of  the  final  call  made  internally by
       pcre2_substitute() to the matching function. For global  changes,  this
       will always be a no-match error. The contents of the ovector within the
       match data block may or may not have been changed.

       As  well as the usual options for pcre2_match(), a number of additional
       options can be set in the options argument of pcre2_substitute().   One
       such  option is PCRE2_SUBSTITUTE_MATCHED. When this is set, an external
       match_data block must be provided, and it must have already  been  used
       for  an  external call to pcre2_match() (or pcre2_jit_match()) with the
       same pattern, subject pointer, effective subject length, start  offset,
       and match option arguments (substitute-specific options can be added to
       the   options  argument).  If  any  of  these  parameters  is  changed,
       pcre2_substitute() returns an error. The data in the  match_data  block
       (return code, offset vector) is used for the first substitution instead
       of calling pcre2_match() from within pcre2_substitute(). This allows an
       application to check for a match before choosing to substitute, without
       having to repeat the match.

       If   the  contents  of  the  subject  buffer  are  mutated  in  between
       pcre2_match() and  a  call  to  pcre2_substitute()  with  PCRE2_SUBSTI-
       TUTE_MATCHED,  the  behaviour  is  unsafe; in particular, in this case,
       PCRE2 is unable to ensure that the offsets in the ovector point to  the
       start of characters (with UTF-encoded input).

       The  contents  of  the  externally  supplied  match  data block are not
       changed when PCRE2_SUBSTITUTE_MATCHED is set, and so the match block is
       permitted for use in another call  using  PCRE2_SUBSTITUTE_MATCHED.  If
       PCRE2_SUBSTITUTE_GLOBAL  is also set, pcre2_match() is called after the
       first substitution to check for furthe matches, but this is done  using
       an internally obtained match data block, thus always leaving the exter-
       nal block unchanged.

       The  code  argument is not used for matching before the first substitu-
       tion when PCRE2_SUBSTITUTE_MATCHED is set, but  it  must  be  provided,
       even  when  PCRE2_SUBSTITUTE_GLOBAL is not set, because it contains in-
       formation such as the UTF setting and the number of capturing parenthe-
       ses in the pattern.

       When using PCRE2_SUBSTITUTE_MATCHED, you should not modify the  subject
       string  in  between  the  prior call to pcre2_match() and pcre2_substi-
       tute(), as the substitution assumes that the passed-in ovector is  com-
       patible  with  the  subject string. Although PCRE2 does verify that the
       subject is a pointer to the same buffer, it cannot  in  general  verify
       whether  the  contents  of the buffer have changed. For example, if the
       subject buffer is mutated from one valid UTF-8 string to another  valid
       string,  of  the  same length in code units, the ovector offsets are no
       longer guaranteed to point to the start of  a  character.  Beware  that
       with  PCRE2_SUBSTITUTE_MATCHED  in  UTF mode, the subject string is not
       re-scanned for UTF validity when pcre2_substitute() first uses it.

       The default action of pcre2_substitute() is to return  a  copy  of  the
       subject string with matched substrings replaced. However, if PCRE2_SUB-
       STITUTE_REPLACEMENT_ONLY  is  set,  only the replacement substrings are
       returned. In the global case, multiple replacements are concatenated in
       the output buffer. Substitution callouts (see below)  can  be  used  to
       separate them if necessary.

       Partial  matching  is supported, with limitations: if matching succeeds
       but with a  partial  match,  then  pcre2_substitute  returns  PCRE2_ER-
       ROR_PARTIAL.  When  partial-matching  (either  of PCRE2_PARTIAL_HARD or
       PCRE2_PARTIAL_SOFT is passed),  then  PCRE2_SUBSTITUTE_REPLACEMENT_ONLY
       must also be set, or else PCRE2_ERROR_BADOPTION is returned. Similarly,
       certain  replacement items ($' and $_) cause PCRE2_ERROR_PARTIALSUBS to
       be returned when partial-matching, even if a complete match is found.

       The outlengthptr argument of pcre2_substitute() must point to  a  vari-
       able  that contains the length, in code units, of the output buffer. If
       the function is successful, the value is updated to contain the  length
       in  code  units  of the new string, excluding the trailing zero that is
       automatically added.

       If the function is not successful, the value set via  outlengthptr  de-
       pends  on  the  type  of  error.  For  syntax errors in the replacement
       string, the value is the offset in the replacement string where the er-
       ror was detected. For other errors, the value  is  PCRE2_UNSET  by  de-
       fault. This includes the case of the output buffer being too small, un-
       less PCRE2_SUBSTITUTE_OVERFLOW_LENGTH is set.

       PCRE2_SUBSTITUTE_OVERFLOW_LENGTH  changes  what happens when the output
       buffer is too small. The default action is to return PCRE2_ERROR_NOMEM-
       ORY immediately. If this option  is  set,  however,  pcre2_substitute()
       continues to go through the motions of matching and substituting (with-
       out,  of  course,  writing  anything)  in  order to compute the size of
       buffer that is needed, which will include the extra space for the  ter-
       minating  NUL. This value is passed back via the outlengthptr variable,
       with the result of the function still being PCRE2_ERROR_NOMEMORY.

       Passing a buffer size of zero is a permitted way  of  finding  out  how
       much  memory  is needed for given substitution. However, this does mean
       that the entire operation is carried out twice. Depending on the appli-
       cation, it may be more efficient to allocate a large  buffer  and  free
       the   excess   afterwards,   instead  of  using  PCRE2_SUBSTITUTE_OVER-
       FLOW_LENGTH.

       The replacement string, which is interpreted as a  UTF  string  in  UTF
       mode,  is checked for UTF validity unless PCRE2_NO_UTF_CHECK is set. An
       invalid UTF replacement string causes an immediate return with the rel-
       evant UTF error code.

       If PCRE2_SUBSTITUTE_LITERAL is set, the replacement string is  not  in-
       terpreted in any way. By default, however, a dollar character is an es-
       cape  character  that can specify the insertion of characters from cap-
       ture groups and names from (*MARK) or other control verbs in  the  pat-
       tern. Dollar is the only escape character (backslash is treated as lit-
       eral). The following forms are recognized:

         $$                  insert a dollar character
         $n or ${n}          insert the contents of group n
         $0 or $&            insert the entire matched substring
         $`                  insert the substring that precedes the match
         $'                  insert the substring that follows the match
         $_                  insert the entire input string
         $+                   insert  the highest-numbered capture group which
       matched
         $*MARK or ${*MARK}  insert a control verb name

       Either a group number or a group name can be given for n,  for  example
       $2  or $NAME. Curly brackets are required only if the following charac-
       ter would be interpreted as part of the number or name. The number  may
       be  zero to include the entire matched string. For example, if the pat-
       tern  a(b)c  is  matched  with  "=abc="  and  the  replacement   string
       "+$1$0$1+", the result is "=+babcb+=".

       The  JavaScript  form $<name>, where the angle brackets are part of the
       syntax, is also recognized for group names, but not for  group  numbers
       or *MARK.

       $*MARK  inserts the name from the last encountered backtracking control
       verb on the matching path that has a name. (*MARK) must always  include
       a  name,  but  the  other  verbs  need not. For example, in the case of
       (*MARK:A)(*PRUNE) the name inserted is "A", but for (*MARK:A)(*PRUNE:B)
       the relevant name is "B". This facility can be used to  perform  simple
       simultaneous substitutions, as this pcre2test example shows:

         /(*MARK:pear)apple|(*MARK:orange)lemon/g,replace=${*MARK}
             apple lemon
          2: pear orange

       PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject
       string,  replacing every matching substring. If this option is not set,
       only the first matching substring is replaced. The search  for  matches
       takes  place in the original subject string (that is, previous replace-
       ments do not affect it).  Iteration is  implemented  by  advancing  the
       startoffset  value  for  each search, which is always passed the entire
       subject string. If an offset limit is set in the match context, search-
       ing stops when that limit is reached.

       Because global substitutions apply the pattern repeatedly to  the  sub-
       ject  string, and always iterate over non-overlapping matches, the sub-
       stitutions done by pcre2_substitute() do not match and substitute  text
       inside  the replacement strings themselves (no recursive/iterative sub-
       stitution). However, applications can easily implement  other  alterna-
       tive replacement strategies, such as iteratively replacing, then match-
       ing and replacing on the result. The replacement loop inside pcre2_sub-
       stitute()  is simple and can be emulated in client code by allocating a
       buffer, searching for matches in a loop, and calling pcre2_substitute()
       with PCRE2_SUBSTITUTE_REPLACEMENT_ONLY an PCRE2_SUBSTITUTE_MATCHED, and
       without PCRE2_SUBSTITUTE_GLOBAL.

       You can restrict the effect of a global substitution to  a  portion  of
       the subject string by setting either or both of startoffset and an off-
       set limit. Here is a pcre2test example:

         /B/g,replace=!,use_offset_limit
         ABC ABC ABC ABC\=offset=3,offset_limit=12
          2: ABC A!C A!C ABC

       When  continuing  with  global substitutions after matching a substring
       with zero length, an attempt to find a non-empty match at the same off-
       set is performed.  If this is not successful, the offset is advanced by
       one character except when CRLF is a valid newline sequence and the next
       two characters are CR, LF. In this case, the offset is advanced by  two
       characters.

       PCRE2_SUBSTITUTE_UNKNOWN_UNSET causes references to capture groups that
       do not appear in the pattern to be treated as unset groups. This option
       should  be used with care, because it means that a typo in a group name
       or number no longer causes the PCRE2_ERROR_NOSUBSTRING error.

       PCRE2_SUBSTITUTE_UNSET_EMPTY causes unset capture groups (including un-
       known groups when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) to be  treated
       as  empty  strings  when inserted as described above. If this option is
       not set, an attempt to insert an unset group causes the PCRE2_ERROR_UN-
       SET error. This option does not  influence  the  extended  substitution
       syntax described below.

       PCRE2_SUBSTITUTE_EXTENDED  causes extra processing to be applied to the
       replacement string. Without this option, only the dollar  character  is
       special,  and  only  the  group insertion forms listed above are valid.
       When PCRE2_SUBSTITUTE_EXTENDED is set, several things change:

       Firstly, backslash in a replacement string is interpreted as an  escape
       character.  The usual forms such as \x{ddd} can be used to specify par-
       ticular character codes, and backslash followed by any non-alphanumeric
       character quotes that character. Extended quoting can  be  coded  using
       \Q...\E,  exactly  as in pattern strings. The escapes \b and \v are in-
       terpreted as the characters backspace and vertical tab, respectively.

       The interpretation of backslash followed by one or more digits  is  the
       same  as  in a pattern, which in Perl has some ambiguities. Details are
       given in the pcre2pattern page.

       The Python form \g<n>, where the angle brackets are part of the  syntax
       and n is either a group name or number, is recognized as an alternative
       way of inserting the contents of a group, for example \g<3>.

       There  are  also four escape sequences for forcing the case of inserted
       letters.  Case forcing applies to all  inserted  characters,  including
       those  from capture groups and letters within \Q...\E quoted sequences.
       The insertion mechanism has three states: no case forcing, force  upper
       case,  and  force  lower  case. The escape sequences change the current
       state: \U and \L change to upper or lower case  forcing,  respectively,
       and  \E  (when not terminating a \Q quoted sequence) reverts to no case
       forcing. The sequences \u and \l force the next character (if it  is  a
       letter)  to upper or lower case, respectively, and then the state auto-
       matically reverts to no case forcing.

       However, if \u is immediately followed by \L or \l is immediately  fol-
       lowed  by  \U,  the next character's case is forced by the first escape
       sequence, and subsequent characters by the second. This provides a "ti-
       tle casing" facility that can be applied to group captures.  For  exam-
       ple,  if  group 1 has captured "heLLo", the replacement string "\u\L$1"
       becomes "Hello".

       If either PCRE2_UTF or PCRE2_UCP was set when the pattern was compiled,
       Unicode properties are used for  case  forcing  characters  whose  code
       points  are greater than 127. However, only simple case folding, as de-
       termined by the Unicode file CaseFolding.txt is supported.  PCRE2  does
       not  support  language-specific special casing rules such as using dif-
       ferent lower case Greek sigmas in the middle and ends of words (as  de-
       fined in the Unicode file SpecialCasing.txt).

       Note that case forcing sequences such as \U...\E do not nest. For exam-
       ple,  the  result of processing "\Uaa\LBB\Ecc\E" is "AAbbcc"; the final
       \E has no effect. Note  also  that  the  PCRE2_ALT_BSUX  and  PCRE2_EX-
       TRA_ALT_BSUX options do not apply to replacement strings.

       The  final  effect  of setting PCRE2_SUBSTITUTE_EXTENDED is to add more
       flexibility to capture group substitution. The  syntax  is  similar  to
       that used by Bash:

         ${n:-string}
         ${n:+string1:string2}

       As  in  the  simple  case, n may be a group number or a name. The first
       form specifies a default value. If group n is set,  its  value  is  in-
       serted;  if  not,  the  string is expanded and the result inserted. The
       second form specifies strings that are expanded and inserted when group
       n is set or unset, respectively. The first form is  just  a  convenient
       shorthand for

         ${n:+${n}:string}

       Backslash  can  be  used to escape colons and closing curly brackets in
       the replacement strings. A change of the case forcing  state  within  a
       replacement  string  remains  in  force  afterwards,  as  shown in this
       pcre2test example:

         /(some)?(body)/substitute_extended,replace=${1:+\U:\L}HeLLo
             body
          1: hello
             somebody
          1: HELLO

       The PCRE2_SUBSTITUTE_UNSET_EMPTY option does not affect these  extended
       substitutions.  However,  PCRE2_SUBSTITUTE_UNKNOWN_UNSET does cause un-
       known groups in the extended syntax forms to be treated as unset.

       If  PCRE2_SUBSTITUTE_LITERAL  is  set,  PCRE2_SUBSTITUTE_UNKNOWN_UNSET,
       PCRE2_SUBSTITUTE_UNSET_EMPTY, and PCRE2_SUBSTITUTE_EXTENDED are irrele-
       vant and are ignored.

   Substitution errors

       In  the  event of an error, pcre2_substitute() returns a negative error
       code. Except for PCRE2_ERROR_NOMATCH (which is never returned),  errors
       from pcre2_match() are passed straight back.

       PCRE2_ERROR_NOSUBSTRING is returned for a non-existent substring inser-
       tion, unless PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set.

       PCRE2_ERROR_UNSET is returned for an unset substring insertion (includ-
       ing  an  unknown  substring when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set)
       when the simple (non-extended) syntax is used and  PCRE2_SUBSTITUTE_UN-
       SET_EMPTY is not set.

       PCRE2_ERROR_NOMEMORY  is  returned  if  the  output  buffer  is not big
       enough. If the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option is set, the size
       of buffer that is needed is returned via outlengthptr. Note  that  this
       does not happen by default.

       PCRE2_ERROR_NULL is returned if PCRE2_SUBSTITUTE_MATCHED is set but the
       match_data  argument is NULL or if the subject or replacement arguments
       are NULL. For backward compatibility reasons an exception is  made  for
       the replacement argument if the rlength argument is also 0.

       PCRE2_ERROR_BADREPLACEMENT  is  used for miscellaneous syntax errors in
       the replacement string, with more  particular  errors  being  PCRE2_ER-
       ROR_BADREPESCAPE (invalid escape sequence), PCRE2_ERROR_REPMISSINGBRACE
       (closing  curly bracket not found), PCRE2_ERROR_BADSUBSTITUTION (syntax
       error in extended group substitution),  and  PCRE2_ERROR_BADSUBSPATTERN
       (the pattern match ended before it started or the match started earlier
       than  the  current  position  in the subject, which can happen if \K is
       used in a lookaround assertion).

       As for all PCRE2 errors, a text message that describes the error can be
       obtained by calling the pcre2_get_error_message()  function  (see  "Ob-
       taining a textual error message" above).

   Substitution callouts

       int pcre2_set_substitute_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_substitute_callout_block *, void *),
         void *callout_data);

       The  pcre2_set_substitute_callout()  function  can be used to specify a
       callout function for pcre2_substitute(). This information is passed  in
       a match context. The callout function is called after each substitution
       has been processed, but it can cause the replacement not to happen.

       The  callout  function  is  not called for simulated substitutions that
       happen as a result of the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH  option.  In
       this  mode,  when substitution processing exceeds the buffer space pro-
       vided by the caller, processing continues by counting code  units.  The
       simulation  is unable to populate the callout block, and so the simula-
       tion is pessimistic about the required buffer size. Whichever is larger
       of accepted or rejected substitution is reported as the required  size.
       Therefore, the returned buffer length may be an overestimate (without a
       substitution callout, it is normally an exact measurement).

       The first argument of the callout function is a pointer to a substitute
       callout  block structure, which contains the following fields, not nec-
       essarily in this order:

         uint32_t    version;
         uint32_t    subscount;
         PCRE2_SPTR  input;
         PCRE2_SPTR  output;
         PCRE2_SIZE *ovector;
         uint32_t    oveccount;
         PCRE2_SIZE  output_offsets[2];

       The version field contains the version number of the block format.  The
       current  version  is  0.  The version number will increase in future if
       more fields are added, but the intention is never to remove any of  the
       existing fields.

       The subscount field is the number of the current match. It is 1 for the
       first callout, 2 for the second, and so on. The input and output point-
       ers are copies of the values passed to pcre2_substitute().

       The  ovector  field points to the ovector, which contains the result of
       the most recent match. The oveccount field contains the number of pairs
       that are set in the ovector, and is always greater than zero.

       The output_offsets vector contains the offsets of  the  replacement  in
       the  output  string. This has already been processed for dollar and (if
       requested) backslash substitutions as described above.

       The second argument of the callout function  is  the  value  passed  as
       callout_data  when  the  function was registered. The value returned by
       the callout function is interpreted as follows:

       If the value is zero, the replacement is accepted, and,  if  PCRE2_SUB-
       STITUTE_GLOBAL  is set, processing continues with a search for the next
       match. If the value is not zero, the current  replacement  is  not  ac-
       cepted.  If  the  value is greater than zero, processing continues when
       PCRE2_SUBSTITUTE_GLOBAL is set. Otherwise (the value is less than  zero
       or PCRE2_SUBSTITUTE_GLOBAL is not set), the rest of the input is copied
       to  the  output and the call to pcre2_substitute() exits, returning the
       number of matches so far.

   Substitution case callouts

       int pcre2_set_substitute_case_callout(pcre2_match_context *mcontext,
         PCRE2_SIZE (*callout_function)(PCRE2_SPTR, PCRE2_SIZE,
                                        PCRE2_UCHAR *, PCRE2_SIZE,
                                        int, void *),
         void *callout_data);

       The pcre2_set_substitute_case_callout() function can be used to specify
       a callout function for pcre2_substitute() to use when  performing  case
       transformations.  This does not affect any case insensitivity behaviour
       when performing a match, but only the user-visible transformations per-
       formed when processing a substitution such as:

           pcre2_substitute(..., "\\U$1", ...)

       The default case transformations applied by PCRE2 are  reasonably  com-
       plete,  and,  in  UTF  or UCP mode, perform the simple locale-invariant
       case transformations as specified by Unicode. This is suitable for  the
       internal  (invisible)  case-equivalence  procedures used during pattern
       matching, but an application may wish to use more sophisticated locale-
       aware processing for the user-visible substitution transformations.

       One example implementation of the callout_function using  the  ICU  li-
       brary would be:

           PCRE2_SIZE
           icu_case_callout(
             PCRE2_SPTR input, PCRE2_SIZE input_len,
             PCRE2_UCHAR *output, PCRE2_SIZE output_cap,
             int to_case, void *data_ptr)
           {
             UErrorCode err = U_ZERO_ERROR;
             int32_t r = to_case == PCRE2_SUBSTITUTE_CASE_LOWER
               ? u_strToLower(output, output_cap, input, input_len, NULL, &err)
               : to_case == PCRE2_SUBSTITUTE_CASE_UPPER
               ? u_strToUpper(output, output_cap, input, input_len, NULL, &err)
               : u_strToTitle(output, output_cap, input, input_len, &first_char_only,
                              NULL, &err);
             if (U_FAILURE(err)) return (~(PCRE2_SIZE)0);
             return r;
           }

       The  first  and  second  arguments of the case callout function are the
       Unicode string to transform.

       The third and fourth arguments are the output buffer and its capacity.

       The  fifth  is  one  of  the   constants   PCRE2_SUBSTITUTE_CASE_LOWER,
       PCRE2_SUBSTITUTE_CASE_UPPER,    or   PCRE2_SUBSTITUTE_CASE_TITLE_FIRST.
       PCRE2_SUBSTITUTE_CASE_LOWER and PCRE2_SUBSTITUTE_CASE_UPPER are  passed
       to  the  callout  to indicate that the case of the entire callout input
       should be case-transformed. PCRE2_SUBSTITUTE_CASE_TITLE_FIRST is passed
       to indicate that only the first character or  glyph  should  be  trans-
       formed  to  Unicode  titlecase  and the rest to Unicode lowercase (note
       that titlecasing sometimes uses Unicode properties  to  titlecase  each
       word  in a string; but PCRE2 is requesting that only the single leading
       character is to be titlecased).

       The sixth argument is the callout_data  supplied  to  pcre2_set_substi-
       tute_case_callout().

       The resulting string in the destination buffer may be larger or smaller
       than  the input, if the casing rules merge or split characters. The re-
       turn value is the length required for the output string. If a buffer of
       sufficient size was provided to the callout, then the  result  must  be
       written to the buffer and the number of code units returned. If the re-
       sult  does  not  fit in the provided buffer, then the required capacity
       must be returned and PCRE2 will not make  use  of  the  output  buffer.
       PCRE2  provides  input and output buffers which overlap, so the callout
       must support this by suitable internal buffering.

       Alternatively, if the callout wishes to indicate an error, then it  may
       return  (~(PCRE2_SIZE)0).  In this case pcre2_substitute() will immedi-
       ately fail with error PCRE2_ERROR_REPLACECASE.

       When  a  case  callout  is  combined  with  the  PCRE2_SUBSTITUTE_OVER-
       FLOW_LENGTH  option,  there are situations when pcre2_substitute() will
       return an underestimate of  the  required  buffer  size.  If  you  call
       pcre2_substitute()  once with PCRE2_SUBSTITUTE_OVERFLOW_LENGTH, and the
       input buffer is too small for the replacement string to be constructed,
       then instead of calling the case callout, pcre2_substitute() will  make
       an  estimate  of the required buffer size.  The second call should also
       pass PCRE2_SUBSTITUTE_OVERFLOW_LENGTH, because that second call is  not
       guaranteed  to succeed either, if the case callout requires more buffer
       space than expected. The caller must make repeated attempts in a loop.


DUPLICATE CAPTURE GROUP NAMES

       int pcre2_substring_nametable_scan(const pcre2_code *code,
         PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);

       When a pattern is compiled with the PCRE2_DUPNAMES  option,  names  for
       capture  groups  are not required to be unique. Duplicate names are al-
       ways allowed for groups with the same number, created by using the  (?|
       feature. Indeed, if such groups are named, they are required to use the
       same names.

       Normally,  patterns  that  use duplicate names are such that in any one
       match, only one of each set of identically-named  groups  participates.
       An example is shown in the pcre2pattern documentation.

       When   duplicates   are   present,   pcre2_substring_copy_byname()  and
       pcre2_substring_get_byname() return the first  substring  corresponding
       to  the given name that is set. Only if none are set is PCRE2_ERROR_UN-
       SET is returned. The  pcre2_substring_number_from_name()  function  re-
       turns  the error PCRE2_ERROR_NOUNIQUESUBSTRING when there are duplicate
       names.

       If you want to get full details of all captured substrings for a  given
       name,  you  must use the pcre2_substring_nametable_scan() function. The
       first argument is the compiled pattern, and the second is the name.  If
       the  third  and fourth arguments are NULL, the function returns a group
       number for a unique name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise.

       When the third and fourth arguments are not NULL, they must be pointers
       to variables that are updated by the function. After it has  run,  they
       point to the first and last entries in the name-to-number table for the
       given  name,  and the function returns the length of each entry in code
       units. In both cases, PCRE2_ERROR_NOSUBSTRING is returned if there  are
       no entries for the given name.

       The format of the name table is described above in the section entitled
       Information  about  a  pattern.  Given all the relevant entries for the
       name, you can extract each of their numbers,  and  hence  the  captured
       data.


FINDING ALL POSSIBLE MATCHES AT ONE POSITION

       The  traditional  matching  function  uses a similar algorithm to Perl,
       which stops when it finds the first match at a given point in the  sub-
       ject. If you want to find all possible matches, or the longest possible
       match  at  a  given  position,  consider using the alternative matching
       function (see below) instead. If you cannot use the  alternative  func-
       tion, you can kludge it up by making use of the callout facility, which
       is described in the pcre2callout documentation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.   When your callout function is called, extract and save the cur-
       rent matched substring. Then return 1, which  forces  pcre2_match()  to
       backtrack  and  try other alternatives. Ultimately, when it runs out of
       matches, pcre2_match() will yield PCRE2_ERROR_NOMATCH.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext,
         int *workspace, PCRE2_SIZE wscount);

       The function pcre2_dfa_match() is called  to  match  a  subject  string
       against  a  compiled pattern, using a matching algorithm that scans the
       subject string just once (not counting lookaround assertions), and does
       not backtrack (except when processing lookaround assertions). This  has
       different  characteristics to the normal algorithm, and is not compati-
       ble with Perl. Some of the features of  PCRE2  patterns  are  not  sup-
       ported. Nevertheless, there are times when this kind of matching can be
       useful.  For a discussion of the two matching algorithms, and a list of
       features that pcre2_dfa_match() does not support, see the pcre2matching
       documentation.

       The arguments for the pcre2_dfa_match() function are the  same  as  for
       pcre2_match(), plus two extras. The ovector within the match data block
       is used in a different way, and this is described below. The other com-
       mon  arguments  are used in the same way as for pcre2_match(), so their
       description is not repeated here.

       The two additional arguments provide workspace for  the  function.  The
       workspace  vector  should  contain at least 20 elements. It is used for
       keeping track of multiple paths through the pattern  tree.  More  work-
       space  is needed for patterns and subjects where there are a lot of po-
       tential matches.

       Here is an example of a simple call to pcre2_dfa_match():

         int wspace[20];
         pcre2_match_data *md = pcre2_match_data_create(4, NULL);
         int rc = pcre2_dfa_match(
           re,             /* result of pcre2_compile() */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           md,             /* the match data block */
           NULL,           /* a match context; NULL means use defaults */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre2_dfa_match()

       The unused bits of the options argument for pcre2_dfa_match()  must  be
       zero.   The   only   bits   that   may   be   set  are  PCRE2_ANCHORED,
       PCRE2_COPY_MATCHED_SUBJECT, PCRE2_ENDANCHORED, PCRE2_NOTBOL,  PCRE2_NO-
       TEOL,   PCRE2_NOTEMPTY,   PCRE2_NOTEMPTY_ATSTART,   PCRE2_NO_UTF_CHECK,
       PCRE2_PARTIAL_HARD,   PCRE2_PARTIAL_SOFT,    PCRE2_DFA_SHORTEST,    and
       PCRE2_DFA_RESTART.  All but the last four of these are exactly the same
       as for pcre2_match(), so their description is not repeated here.

         PCRE2_PARTIAL_HARD
         PCRE2_PARTIAL_SOFT

       These have the same general effect as they do  for  pcre2_match(),  but
       the  details are slightly different. When PCRE2_PARTIAL_HARD is set for
       pcre2_dfa_match(), it returns PCRE2_ERROR_PARTIAL if  the  end  of  the
       subject is reached and there is still at least one matching possibility
       that requires additional characters. This happens even if some complete
       matches  have  already  been found. When PCRE2_PARTIAL_SOFT is set, the
       return code PCRE2_ERROR_NOMATCH is converted  into  PCRE2_ERROR_PARTIAL
       if  the  end  of  the  subject  is reached, there have been no complete
       matches, but there is still at least one matching possibility. The por-
       tion of the string that was inspected when the  longest  partial  match
       was found is set as the first matching string in both cases. There is a
       more  detailed  discussion  of partial and multi-segment matching, with
       examples, in the pcre2partial documentation.

         PCRE2_DFA_SHORTEST

       Setting the PCRE2_DFA_SHORTEST option causes the matching algorithm  to
       stop as soon as it has found one match. Because of the way the alterna-
       tive  algorithm  works, this is necessarily the shortest possible match
       at the first possible matching point in the subject string.

         PCRE2_DFA_RESTART

       When pcre2_dfa_match() returns a partial match, it is possible to  call
       it again, with additional subject characters, and have it continue with
       the same match. The PCRE2_DFA_RESTART option requests this action; when
       it  is  set,  the workspace and wscount options must reference the same
       vector as before because data about the match so far is  left  in  them
       after a partial match. There is more discussion of this facility in the
       pcre2partial documentation.

   Successful returns from pcre2_dfa_match()

       When pcre2_dfa_match() succeeds, it may have matched more than one sub-
       string in the subject. Note, however, that all the matches from one run
       of  the  function  start  at the same point in the subject. The shorter
       matches are all initial substrings of the longer matches. For  example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something> <something else> <something further>
         <something> <something else>
         <something>

       On  success,  the  yield of the function is a number greater than zero,
       which is the number of matched substrings.  The  offsets  of  the  sub-
       strings  are returned in the ovector, and can be extracted by number in
       the same way as for pcre2_match(), but the numbers bear no relation  to
       any  capture groups that may exist in the pattern, because DFA matching
       does not support capturing.

       Calls to the convenience functions that extract substrings by name  re-
       turn the error PCRE2_ERROR_DFA_UFUNC (unsupported function) if used af-
       ter  a  DFA match. The convenience functions that extract substrings by
       number never return PCRE2_ERROR_NOSUBSTRING.

       The matched strings are stored in  the  ovector  in  reverse  order  of
       length;  that  is,  the longest matching string is first. If there were
       too many matches to fit into the ovector, the yield of the function  is
       zero, and the vector is filled with the longest matches.

       NOTE:  PCRE2's  "auto-possessification" optimization usually applies to
       character repeats at the end of a pattern (as well as internally).  For
       example,  the pattern "a\d+" is compiled as if it were "a\d++". For DFA
       matching, this means that only one possible match is found. If you  re-
       ally do want multiple matches in such cases, either use an ungreedy re-
       peat  such as "a\d+?" or set the PCRE2_NO_AUTO_POSSESS option when com-
       piling.

   Error returns from pcre2_dfa_match()

       The pcre2_dfa_match() function returns a negative number when it fails.
       Many of the errors are the same  as  for  pcre2_match(),  as  described
       above.  There are in addition the following errors that are specific to
       pcre2_dfa_match():

         PCRE2_ERROR_DFA_UITEM

       This  return  is  given  if pcre2_dfa_match() encounters an item in the
       pattern that it does not support, for instance, the use of \C in a  UTF
       mode or a backreference.

         PCRE2_ERROR_DFA_UCOND

       This  return  is given if pcre2_dfa_match() encounters a condition item
       that uses a backreference for the condition, or a test for recursion in
       a specific capture group. These are not supported.

         PCRE2_ERROR_DFA_UINVALID_UTF

       This return is given if pcre2_dfa_match() is called for a pattern  that
       was  compiled  with  PCRE2_MATCH_INVALID_UTF. This is not supported for
       DFA matching.

         PCRE2_ERROR_DFA_WSSIZE

       This return is given if pcre2_dfa_match() runs  out  of  space  in  the
       workspace vector.

         PCRE2_ERROR_DFA_RECURSE

       When a recursion or subroutine call is processed, the matching function
       calls  itself  recursively,  using  private  memory for the ovector and
       workspace.  This error is given if the internal ovector  is  not  large
       enough.  This  should  be  extremely  rare, as a vector of size 1000 is
       used.

         PCRE2_ERROR_DFA_BADRESTART

       When pcre2_dfa_match() is called  with  the  PCRE2_DFA_RESTART  option,
       some  plausibility  checks  are  made on the contents of the workspace,
       which should contain data about the previous partial match. If  any  of
       these checks fail, this error is given.


SEE ALSO

       pcre2build(3),    pcre2callout(3),    pcre2demo(3),   pcre2matching(3),
       pcre2partial(3), pcre2posix(3), pcre2sample(3), pcre2unicode(3).


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 08 December 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                    08 December 2025                    PCRE2API(3)
------------------------------------------------------------------------------


PCRE2BUILD(3)              Library Functions Manual              PCRE2BUILD(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


BUILDING PCRE2

       PCRE2  is distributed with a configure script that can be used to build
       the library in Unix-like environments using the Autotools applications.
       Also in the distribution are files to support building using CMake  in-
       stead  of  configure. The text file README contains general information
       about building with Autotools (some of which is  repeated  below),  and
       also has some comments about building on various operating systems. The
       files  in the vms directory support building under OpenVMS.  There is a
       lot more information about building PCRE2 without using Autotools  (in-
       cluding  information  about  using CMake and building "by hand") in the
       text file called NON-AUTOTOOLS-BUILD.  You should consult this file  as
       well as the README file if you are building in a non-Unix-like environ-
       ment.


PCRE2 BUILD-TIME OPTIONS

       The rest of this document describes the optional features of PCRE2 that
       can  be  selected  when  the library is compiled. It assumes use of the
       configure script, where the optional features  are  selected  or  dese-
       lected  by  providing options to configure before running the make com-
       mand. However, the same options can be selected in both  Unix-like  and
       non-Unix-like  environments if you are using CMake instead of configure
       to build PCRE2.

       If you are not using Autotools or CMake, option selection can  be  done
       by  editing  the config.h file, or by passing parameter settings to the
       compiler, as described in NON-AUTOTOOLS-BUILD.

       The complete list of options for configure (which includes the standard
       ones such as the selection of the installation directory)  can  be  ob-
       tained by running

         ./configure --help

       The  following  sections include descriptions of "on/off" options whose
       names begin with --enable or --disable. Because of the way that config-
       ure works, --enable and --disable always come in pairs, so the  comple-
       mentary  option always exists as well, but as it specifies the default,
       it is not described.  Options that specify values have names that start
       with --with. At the end of a configure run, a summary of the configura-
       tion is output.


BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES

       By default, a library called libpcre2-8 is built, containing  functions
       that  take  string  arguments contained in arrays of bytes, interpreted
       either as single-byte characters, or UTF-8 strings. You can also  build
       two  other libraries, called libpcre2-16 and libpcre2-32, which process
       strings that are contained in arrays of 16-bit and 32-bit  code  units,
       respectively. These can be interpreted either as single-unit characters
       or  UTF-16/UTF-32 strings. To build these additional libraries, add one
       or both of the following to the configure command:

         --enable-pcre2-16
         --enable-pcre2-32

       If you do not want the 8-bit library, add

         --disable-pcre2-8

       as well. At least one of the three libraries must be built.  Note  that
       the  POSIX wrapper is for the 8-bit library only, and that pcre2grep is
       an 8-bit program. Neither of these are built if  you  select  only  the
       16-bit or 32-bit libraries.


BUILDING SHARED AND STATIC LIBRARIES

       The  Autotools PCRE2 building process uses libtool to build both shared
       and static libraries by default. You can suppress an  unwanted  library
       by adding one of

         --disable-shared
         --disable-static

       to  the  configure command. Setting --disable-shared ensures that PCRE2
       libraries are built as static libraries. The  binaries  that  are  then
       created  as  part  of  the  build  process  (for example, pcre2test and
       pcre2grep) are linked statically with one or more PCRE2 libraries,  but
       may  also  be  dynamically linked with other libraries such as libc. If
       you want these binaries to be fully statically linked, you can set  LD-
       FLAGS like this:

       LDFLAGS=--static ./configure --disable-shared

       Note  the two hyphens in --static. Of course, this works only if static
       versions of all the relevant libraries are available for linking.


UNICODE AND UTF SUPPORT

       By default, PCRE2 is built with support for Unicode and  UTF  character
       strings.  To build it without Unicode support, add

         --disable-unicode

       to  the configure command. This setting applies to all three libraries.
       It is not possible to build one library with Unicode  support  and  an-
       other without in the same configuration.

       Of  itself, Unicode support does not make PCRE2 treat strings as UTF-8,
       UTF-16 or UTF-32. To do that, applications that use the library can set
       the PCRE2_UTF option when they call pcre2_compile() to compile  a  pat-
       tern.   Alternatively,  patterns  may be started with (*UTF) unless the
       application has locked this out by setting PCRE2_NEVER_UTF.

       UTF support allows the libraries to process character code points up to
       0x10ffff in the strings that they handle. Unicode  support  also  gives
       access  to  the Unicode properties of characters, using pattern escapes
       such as \P, \p, and \X. Only the general category properties such as Lu
       and Nd, script names, and some bi-directional and binary properties are
       supported.  Details are given in the pcre2pattern documentation.

       Pattern escapes such as \d and \w do not by default make use of Unicode
       properties. The application can request that they  do  by  setting  the
       PCRE2_UCP  option.  Unless  the  application has set PCRE2_NEVER_UCP, a
       pattern may also request this by starting with (*UCP).


DISABLING THE USE OF \C

       The \C escape sequence, which matches a single code unit, even in a UTF
       mode, can cause unpredictable behaviour because it may leave  the  cur-
       rent  matching  point in the middle of a multi-code-unit character. The
       application can lock it out by setting the PCRE2_NEVER_BACKSLASH_C  op-
       tion when calling pcre2_compile(). There is also a build-time option

         --enable-never-backslash-C

       (note the upper case C) which locks out the use of \C entirely.


JUST-IN-TIME COMPILER SUPPORT

       Just-in-time  (JIT) compiler support is included in the build by speci-
       fying

         --enable-jit

       This support is available only for certain hardware  architectures.  If
       this  option  is  set for an unsupported architecture, a building error
       occurs.  If in doubt, use

         --enable-jit=auto

       which enables JIT only if the current hardware is  supported.  You  can
       check  if JIT is enabled in the configuration summary that is output at
       the end of a configure run. If you are enabling JIT under  SELinux  you
       may also want to add

         --enable-jit-sealloc

       which enables the use of an execmem allocator in JIT that is compatible
       with  SELinux.  This  has  no  effect  if  JIT  is not enabled. See the
       pcre2jit documentation for a discussion of JIT usage. When JIT  support
       is enabled, pcre2grep automatically makes use of it, unless you add

         --disable-pcre2grep-jit

       to the configure command.


NEWLINE RECOGNITION

       By  default, PCRE2 interprets the linefeed (LF) character as indicating
       the end of a line. This is the normal newline  character  on  Unix-like
       systems.  You can compile PCRE2 to use carriage return (CR) instead, by
       adding

         --enable-newline-is-cr

       to the configure command. There is also an  --enable-newline-is-lf  op-
       tion, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two-character sequence CRLF (CR immediately followed by LF). If you
       want this, add

         --enable-newline-is-crlf

       to the configure command. There is a fourth option, specified by

         --enable-newline-is-anycrlf

       which  causes  PCRE2 to recognize any of the three sequences CR, LF, or
       CRLF as indicating a line ending. A fifth option, specified by

         --enable-newline-is-any

       causes PCRE2 to recognize any Unicode  newline  sequence.  The  Unicode
       newline sequences are the three just mentioned, plus the single charac-
       ters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line,
       U+0085),  LS  (line  separator,  U+2028),  and PS (paragraph separator,
       U+2029). The final option is

         --enable-newline-is-nul

       which causes NUL (binary zero) to be set  as  the  default  line-ending
       character.

       Whatever default line ending convention is selected when PCRE2 is built
       can  be  overridden by applications that use the library. At build time
       it is recommended to use the standard for your operating system.


WHAT \R MATCHES

       By default, the sequence \R in a pattern matches  any  Unicode  newline
       sequence,  independently  of  what has been selected as the line ending
       sequence. If you specify

         --enable-bsr-anycrlf

       the default is changed so that \R matches only CR, LF, or  CRLF.  What-
       ever  is selected when PCRE2 is built can be overridden by applications
       that use the library.


HANDLING VERY LARGE PATTERNS

       Within a compiled pattern, offset values are used  to  point  from  one
       part  to another (for example, from an opening parenthesis to an alter-
       nation metacharacter). By default, in the 8-bit and  16-bit  libraries,
       two-byte  values  are used for these offsets, leading to a maximum size
       for a compiled pattern of around 64 thousand code units. This is suffi-
       cient to handle all but the most gigantic patterns. Nevertheless,  some
       people do want to process truly enormous patterns, so it is possible to
       compile  PCRE2  to use three-byte or four-byte offsets by adding a set-
       ting such as

         --with-link-size=3

       to the configure command. The value given must be 2, 3, or 4.  For  the
       16-bit  library,  a  value of 3 is rounded up to 4. In these libraries,
       using longer offsets slows down the operation of PCRE2 because  it  has
       to  load additional data when handling them. For the 32-bit library the
       value is always 4 and cannot be overridden; the value  of  --with-link-
       size is ignored.


LIMITING PCRE2 RESOURCE USAGE

       The pcre2_match() function increments a counter each time it goes round
       its  main  loop. Putting a limit on this counter controls the amount of
       computing resource used by a single call to  pcre2_match().  The  limit
       can be changed at run time, as described in the pcre2api documentation.
       The  default is 10 million, but this can be changed by adding a setting
       such as

         --with-match-limit=500000

       to  the  configure  command.  This  setting   also   applies   to   the
       pcre2_dfa_match()  matching  function,  and to JIT matching (though the
       counting is done differently).

       The pcre2_match() function uses  heap  memory  to  record  backtracking
       points.  The  more  nested  backtracking points there are (that is, the
       deeper the search tree), the more memory is needed. There is  an  upper
       limit,  specified in kibibytes (units of 1024 bytes). This limit can be
       changed at run time, as described in the  pcre2api  documentation.  The
       default  limit (in effect unlimited) is 20 million. You can change this
       by a setting such as

         --with-heap-limit=500

       which limits the amount of heap to 500 KiB. This limit applies only  to
       interpretive matching in pcre2_match() and pcre2_dfa_match(), which may
       also  use  the  heap for internal workspace when processing complicated
       patterns. This limit does not apply when JIT (which has its own  memory
       arrangements) is used.

       You  can  also explicitly limit the depth of nested backtracking in the
       pcre2_match() interpreter. This limit defaults to the value that is set
       for --with-match-limit. You can set a lower default  limit  by  adding,
       for example,

         --with-match-limit-depth=10000

       to  the  configure  command.  This value can be overridden at run time.
       This depth limit indirectly limits the amount of heap  memory  that  is
       used,  but because the size of each backtracking "frame" depends on the
       number of capturing parentheses in a pattern, the amount of  heap  that
       is  used  before  the  limit is reached varies from pattern to pattern.
       This limit was more useful in versions before 10.30, where function re-
       cursion was used for backtracking.

       As well as applying to pcre2_match(), the depth limit also controls the
       depth of recursive function calls in pcre2_dfa_match(). These are  used
       for  lookaround  assertions,  atomic  groups, and recursion within pat-
       terns.  The limit does not apply to JIT matching.


LIMITING VARIABLE-LENGTH LOOKBEHIND ASSERTIONS

       Lookbehind assertions in which one or more branches can match  a  vari-
       able  number  of  characters  are  supported only if there is a maximum
       matching length for each top-level branch. There is  a  limit  to  this
       maximum  that defaults to 255 characters. You can alter this default by
       a setting such as

         --with-max-varlookbehind=100

       The limit can be changed at runtime by calling pcre2_set_max_varlookbe-
       hind(). Lookbehind assertions in which every  branch  matches  a  fixed
       number of characters (not necessarily all the same) are not constrained
       by this limit.


CREATING CHARACTER TABLES AT BUILD TIME

       PCRE2 uses fixed tables for processing characters whose code points are
       less than 256. By default, PCRE2 is built with a set of tables that are
       distributed  in  the file src/pcre2_chartables.c.dist. These tables are
       for ASCII codes only. If you add

         --enable-rebuild-chartables

       to the configure command, the distributed tables are  no  longer  used.
       Instead, a program called pcre2_dftables is compiled and run. This out-
       puts the source for new set of tables, created in the default locale of
       your  C  run-time  system. This method of replacing the tables does not
       work if you are cross compiling, because pcre2_dftables needs to be run
       on the local host and therefore not compiled with the cross compiler.

       If you need to create alternative tables when cross compiling, you will
       have to do so "by hand". There may also be other reasons  for  creating
       tables  manually.   To  cause  pcre2_dftables  to be built on the local
       host, run a normal compiling command, and then run the program with the
       output file as its argument, for example:

         cc src/pcre2_dftables.c -o pcre2_dftables
         ./pcre2_dftables src/pcre2_chartables.c

       This builds the tables in the default locale of the local host. If  you
       want to specify a locale, you must use the -L option:

         LC_ALL=fr_FR ./pcre2_dftables -L src/pcre2_chartables.c

       You can also specify -b (with or without -L). This causes the tables to
       be  written in binary instead of as source code. A set of binary tables
       can be loaded into memory by an application and  passed  to  pcre2_com-
       pile() in the same way as tables created by calling pcre2_maketables().
       The  tables are just a string of bytes, independent of hardware charac-
       teristics such as endianness. This means they can be  bundled  with  an
       application  that  runs in different environments, to ensure consistent
       behaviour.


USING EBCDIC CODE

       PCRE2 assumes by default that it will run in an environment  where  the
       character  code is ASCII or Unicode, which is a superset of ASCII. This
       is the case for most computer operating systems. PCRE2 can, however, be
       compiled to run in an 8-bit EBCDIC environment by adding

         --enable-ebcdic --disable-unicode

       to the configure command. You should only use it if you know  that  you
       are  in  an EBCDIC environment (for example, an IBM mainframe operating
       system).

       This setting implies --enable-rebuild-chartables, in  order  to  ensure
       that  you  have  the correct default character tables for your system's
       codepage. There is an exception when you set  --enable-ebcdic-ignoring-
       compiler  (see  below), which allows using a default set of EBCDIC 1047
       character tables rather than forcing  use  of  --enable-rebuild-charta-
       bles.

       It  is  not  supported  to enable both EBCDIC input and either ASCII or
       UTF-8/16/32 in the same build of the library. When PCRE2 is built  with
       EBCDIC  support,  it  always operates in EBCDIC, and consequently --en-
       able-unicode and --enable-ebcdic are mutually exclusive.

       The EBCDIC character that corresponds to an ASCII LF is assumed to have
       the value 0x15 by default. However, in some EBCDIC  environments,  0x25
       is used. In such an environment you should use

         --enable-ebcdic-nl25

       (which  implies  --enable-ebcdic).  The EBCDIC character for CR has the
       same value as in ASCII, namely, 0x0d. Whichever of 0x15 and 0x25 is not
       chosen as LF is made to correspond to the Unicode NEL character (which,
       in Unicode, is 0x85).

       The options that select newline behaviour, such as --enable-newline-is-
       cr, and equivalent run-time options, refer to these character values in
       an EBCDIC environment.

       On systems requiring an EBCDIC build of PCRE2, the compiler  should  be
       set  to  use the correct codepage, so that C character literals such as
       'z' use the correct numeric value for whichever EBCDIC  codpage  is  in
       use. (PCRE2 cannot support multiple EBCDIC codepages dynamically.) How-
       ever, if this not possible, then you can use

         --enable-ebcdic-ignoring-compiler

       in  order to disregard the compiler's codepage, and instead force PCRE2
       to use numeric constants corresponding to the EBCDIC 1047 codepage  in-
       stead.  This  can  be  used  to  build  (or  test) EBCDIC support on an
       ASCII/UTF-8 system such as Linux.


PCRE2GREP SUPPORT FOR EXTERNAL SCRIPTS

       By default pcre2grep supports the use of callouts with string arguments
       within the patterns it is matching. There are two kinds: one that  gen-
       erates output using local code, and another that calls an external pro-
       gram  or  script.   If --disable-pcre2grep-callout-fork is added to the
       configure command, only the first kind  of  callout  is  supported;  if
       --disable-pcre2grep-callout  is  used,  all callouts are completely ig-
       nored. For more details of pcre2grep callouts, see the pcre2grep  docu-
       mentation.


PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT

       By  default,  pcre2grep reads all files as plain text. You can build it
       so that it recognizes files whose names end in .gz or .bz2,  and  reads
       them with libz or libbz2, respectively, by adding one or both of

         --enable-pcre2grep-libz
         --enable-pcre2grep-libbz2

       to the configure command. These options naturally require that the rel-
       evant  libraries  are installed on your system. Configuration will fail
       if they are not.


PCRE2GREP BUFFER SIZE

       pcre2grep uses an internal buffer to hold a "window" on the file it  is
       scanning, in order to be able to output "before" and "after" lines when
       it finds a match. The default starting size of the buffer is 20KiB. The
       buffer  itself  is  three times this size, but because of the way it is
       used for holding "before" lines, the longest line that is guaranteed to
       be processable is the notional buffer size. If a longer line is encoun-
       tered, pcre2grep automatically expands the buffer, up  to  a  specified
       maximum  size, whose default is 1MiB or the starting size, whichever is
       the larger. You can change the default parameter values by adding,  for
       example,

         --with-pcre2grep-bufsize=51200
         --with-pcre2grep-max-bufsize=2097152

       to  the  configure  command. The caller of pcre2grep can override these
       values by using --buffer-size  and  --max-buffer-size  on  the  command
       line.


PCRE2TEST OPTION FOR LIBREADLINE SUPPORT

       If you add one of

         --enable-pcre2test-libreadline
         --enable-pcre2test-libedit

       to  the configure command, pcre2test is linked with the libreadline or-
       libedit library, respectively, and when its input is from  a  terminal,
       it  reads  it using the readline() function. This provides line-editing
       and history facilities. Note that libreadline is  GPL-licensed,  so  if
       you  distribute  a binary of pcre2test linked in this way, there may be
       licensing issues. These can be avoided by linking instead with libedit,
       which has a BSD licence.

       Setting --enable-pcre2test-libreadline causes the -lreadline option  to
       be  added to the pcre2test build. In many operating environments with a
       system-installed readline library this is sufficient. However, in  some
       environments (e.g. if an unmodified distribution version of readline is
       in  use),  some  extra configuration may be necessary. The INSTALL file
       for libreadline says this:

         "Readline uses the termcap functions, but does not link with
         the termcap or curses library itself, allowing applications
         which link with readline the to choose an appropriate library."

       If your environment has not been set up so that an appropriate  library
       is automatically included, you may need to add something like

         LIBS="-lncurses"

       immediately before the configure command.


INCLUDING DEBUGGING CODE

       If you add

         --enable-debug

       to  the configure command, additional debugging code is included in the
       build. This feature is intended for use by the PCRE2 maintainers.


DEBUGGING WITH VALGRIND SUPPORT

       If you add

         --enable-valgrind

       to the configure command, PCRE2 will use valgrind annotations  to  mark
       certain  memory  regions as unaddressable. This allows it to detect in-
       valid memory accesses, and is mostly useful for debugging PCRE2 itself.


CODE COVERAGE REPORTING

       If your C compiler is gcc, you can build a version of  PCRE2  that  can
       generate a code coverage report for its test suite. To enable this, you
       must install lcov version 1.6 or above. Then specify

         --enable-coverage

       to the configure command and build PCRE2 in the usual way.

       Note that using ccache (a caching C compiler) is incompatible with code
       coverage  reporting. If you have configured ccache to run automatically
       on your system, you must set the environment variable

         CCACHE_DISABLE=1

       before running make to build PCRE2, so that ccache is not used.

       When --enable-coverage is used,  the  following  addition  targets  are
       added to the Makefile:

         make coverage

       This  creates  a  fresh coverage report for the PCRE2 test suite. It is
       equivalent to running "make coverage-reset", "make  coverage-baseline",
       "make check", and then "make coverage-report".

         make coverage-reset

       This zeroes the coverage counters, but does nothing else.

         make coverage-baseline

       This captures baseline coverage information.

         make coverage-report

       This creates the coverage report.

         make coverage-clean-report

       This  removes the generated coverage report without cleaning the cover-
       age data itself.

         make coverage-clean-data

       This removes the captured coverage data without removing  the  coverage
       files created at compile time (*.gcno).

         make coverage-clean

       This  cleans all coverage data including the generated coverage report.
       For more information about code coverage, see the gcov and  lcov  docu-
       mentation.


DISABLING THE Z AND T FORMATTING MODIFIERS

       The  C99  standard  defines formatting modifiers z and t for size_t and
       ptrdiff_t values, respectively. By default, PCRE2 uses these  modifiers
       in environments other than old versions of Microsoft Visual Studio when
       __STDC_VERSION__  is  defined  and has a value greater than or equal to
       199901L (indicating support for C99).  However, there is at  least  one
       environment that claims to be C99 but does not support these modifiers.
       If

         --disable-percent-zt

       is specified, no use is made of the z or t modifiers. Instead of %td or
       %zu,  a  suitable  format is used depending in the size of long for the
       platform.


SUPPORT FOR FUZZERS

       There is a special option for use by people who  want  to  run  fuzzing
       tests on PCRE2:

         --enable-fuzz-support

       At present this applies only to the 8-bit library. If set, it causes an
       extra  library  called  libpcre2-fuzzsupport.a to be built, but not in-
       stalled. This contains a single  function  called  LLVMFuzzerTestOneIn-
       put()  whose  arguments are a pointer to a string and the length of the
       string. When called, this function tries to compile  the  string  as  a
       pattern,  and if that succeeds, to match it.  This is done both with no
       options and with some random options bits that are generated  from  the
       string.

       Setting  --enable-fuzz-support  also  causes  a binary called pcre2fuz-
       zcheck to be created. This is normally run under valgrind or used  when
       PCRE2 is compiled with address sanitizing enabled. It calls the fuzzing
       function  and  outputs  information  about  what it is doing. The input
       strings are specified by arguments: if an argument starts with "="  the
       rest  of it is a literal input string. Otherwise, it is assumed to be a
       file name, and the contents of the file are the test string.


OBSOLETE OPTION

       In versions of PCRE2 prior to 10.30, there were two  ways  of  handling
       backtracking  in the pcre2_match() function. The default was to use the
       system stack, but if

         --disable-stack-for-recursion

       was set, memory on the heap was used. From release 10.30  onwards  this
       has  changed  (the  stack  is  no longer used) and this option now does
       nothing except give a warning.


SEE ALSO

       pcre2api(3), pcre2-config(3).


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 17 October 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                     17 October 2025                  PCRE2BUILD(3)
------------------------------------------------------------------------------


PCRE2CALLOUT(3)            Library Functions Manual            PCRE2CALLOUT(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


SYNOPSIS

       #include <pcre2.h>

       int (*pcre2_callout)(pcre2_callout_block *, void *);

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);


DESCRIPTION

       PCRE2  provides  a  feature  called "callout", which is a means of tem-
       porarily passing control to the caller of PCRE2 in the middle  of  pat-
       tern  matching.  The  caller  of PCRE2 provides an external function by
       putting its entry point in a match context (see pcre2_set_callout()  in
       the pcre2api documentation).

       When  using the pcre2_substitute() function, an additional callout fea-
       ture is available. This does a callout after each change to the subject
       string and is described in the pcre2api documentation; the rest of this
       document is concerned with callouts during pattern matching.

       Within a regular expression, (?C<arg>) indicates a point at  which  the
       external  function  is  to  be  called. Different callout points can be
       identified by putting a number less than 256 after the  letter  C.  The
       default  value is zero.  Alternatively, the argument may be a delimited
       string. The starting delimiter must be one of ` ' " ^ % # $ {  and  the
       ending delimiter is the same as the start, except for {, where the end-
       ing  delimiter  is  }.  If  the  ending  delimiter is needed within the
       string, it must be doubled. For example, this pattern has  two  callout
       points:

         (?C1)abc(?C"some ""arbitrary"" text")def

       If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled,
       PCRE2  automatically inserts callouts, all with number 255, before each
       item in the pattern except for immediately before or after an  explicit
       callout. For example, if PCRE2_AUTO_CALLOUT is used with the pattern

         A(?C3)B

       it is processed as if it were

         (?C255)A(?C3)B(?C255)

       Here is a more complicated example:

         A(\d{2}|--)

       With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were

         (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice  that  there  is a callout before and after each parenthesis and
       alternation bar. If the pattern contains a conditional group whose con-
       dition is an assertion, an automatic callout  is  inserted  immediately
       before  the  condition. Such a callout may also be inserted explicitly,
       for example:

         (?(?C9)(?=a)ab|de)  (?(?C%text%)(?!=d)ab|de)

       This applies only to assertion conditions (because they are  themselves
       independent groups).

       Callouts  can  be useful for tracking the progress of pattern matching.
       The pcre2test program has a pattern qualifier (/auto_callout) that sets
       automatic callouts.  When any callouts are  present,  the  output  from
       pcre2test  indicates  how  the pattern is being matched. This is useful
       information when you are trying to optimize the performance of  a  par-
       ticular pattern.


MISSING CALLOUTS

       You  should  be  aware  that, because of optimizations in the way PCRE2
       compiles and matches patterns, callouts sometimes do not happen exactly
       as you might expect.

   Auto-possessification

       At compile time, PCRE2 "auto-possessifies" repeated items when it knows
       that what follows cannot be part of the repeat. For example, a+[bc]  is
       compiled  as if it were a++[bc]. The pcre2test output when this pattern
       is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied
       to the string "aaaa" is:

         --->aaaa
          +0 ^        a+
          +2 ^   ^    [bc]
         No match

       This indicates that when matching [bc] fails, there is no  backtracking
       into a+ (because it is being treated as a++) and therefore the callouts
       that  would  be  taken for the backtracks do not occur. You can disable
       the  auto-possessify  feature  by  passing   PCRE2_NO_AUTO_POSSESS   to
       pcre2_compile(),  or  starting  the pattern with (*NO_AUTO_POSSESS). In
       this case, the output changes to this:

         --->aaaa
          +0 ^        a+
          +2 ^   ^    [bc]
          +2 ^  ^     [bc]
          +2 ^ ^      [bc]
          +2 ^^       [bc]
         No match

       This time, when matching [bc] fails, the matcher backtracks into a+ and
       tries again, repeatedly, until a+ itself fails.

   Automatic .* anchoring

       By default, an optimization is applied when .* is the first significant
       item in a pattern. If PCRE2_DOTALL is set, so that the  dot  can  match
       any  character,  the pattern is automatically anchored. If PCRE2_DOTALL
       is not set, a match can start only after an internal newline or at  the
       beginning of the subject, and pcre2_compile() remembers this. If a pat-
       tern  has more than one top-level branch, automatic anchoring occurs if
       all branches are anchorable.

       This optimization is disabled, however, if .* is in an atomic group  or
       if  there  is a backreference to the capture group in which it appears.
       It is also disabled if the pattern contains (*PRUNE) or  (*SKIP).  How-
       ever, the presence of callouts does not affect it.

       For  example,  if  the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT
       and applied to the string "aa", the pcre2test output is:

         --->aa
          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
         No match

       This shows that all match attempts start at the beginning of  the  sub-
       ject. In other words, the pattern is anchored. You can disable this op-
       timization  by  passing  PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(), or
       starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the  out-
       put changes to:

         --->aa
          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
          +0  ^     .*
          +2  ^^    \d
          +2  ^     \d
         No match

       This  shows more match attempts, starting at the second subject charac-
       ter.  Another optimization, described in the next section,  means  that
       there is no subsequent attempt to match with an empty subject.

   Other optimizations

       Other  optimizations  that  provide fast "no match" results also affect
       callouts.  For example, if the pattern is

         ab(?C4)cd

       PCRE2 knows that any matching string must contain the  letter  "d".  If
       the  subject  string  is  "abyz",  the  lack of "d" means that matching
       doesn't ever start, and the callout is  never  reached.  However,  with
       "abyd", though the result is still no match, the callout is obeyed.

       For  most  patterns  PCRE2  also knows the minimum length of a matching
       string, and will immediately give a "no match" return without  actually
       running  a  match if the subject is not long enough, or, for unanchored
       patterns, if it has been scanned far enough.

       You can disable these optimizations by passing the PCRE2_NO_START_OPTI-
       MIZE option  to  pcre2_compile(),  or  by  starting  the  pattern  with
       (*NO_START_OPT).  This slows down the matching process, but does ensure
       that callouts such as the example above are obeyed.


THE CALLOUT INTERFACE

       During matching, when PCRE2 reaches a callout  point,  if  an  external
       function  is  provided in the match context, it is called. This applies
       to both normal, DFA, and JIT matching. The first argument to the  call-
       out function is a pointer to a pcre2_callout block. The second argument
       is  the  void * callout data that was supplied when the callout was set
       up by calling pcre2_set_callout() (see the pcre2api documentation). The
       callout block structure contains the following fields, not  necessarily
       in this order:

         uint32_t      version;
         uint32_t      callout_number;
         uint32_t      capture_top;
         uint32_t      capture_last;
         uint32_t      callout_flags;
         PCRE2_SIZE   *offset_vector;
         PCRE2_SPTR    mark;
         PCRE2_SPTR    subject;
         PCRE2_SIZE    subject_length;
         PCRE2_SIZE    start_match;
         PCRE2_SIZE    current_position;
         PCRE2_SIZE    pattern_position;
         PCRE2_SIZE    next_item_length;
         PCRE2_SIZE    callout_string_offset;
         PCRE2_SIZE    callout_string_length;
         PCRE2_SPTR    callout_string;

       The  version field contains the version number of the block format. The
       current version is 2; the three callout string fields  were  added  for
       version  1, and the callout_flags field for version 2. If you are writ-
       ing an application that might use an  earlier  release  of  PCRE2,  you
       should  check  the version number before accessing any of these fields.
       The version number will increase in future if more  fields  are  added,
       but the intention is never to remove any of the existing fields.

   Fields for numerical callouts

       For  a  numerical  callout,  callout_string is NULL, and callout_number
       contains the number of the callout, in the range  0-255.  This  is  the
       number  that  follows  (?C for callouts that part of the pattern; it is
       255 for automatically generated callouts.

   Fields for string callouts

       For callouts with string arguments, callout_number is always zero,  and
       callout_string  points  to the string that is contained within the com-
       piled pattern. Its length is given by callout_string_length. Duplicated
       ending delimiters that were present in the original pattern string have
       been turned into single characters, but there is no other processing of
       the callout string argument. An additional code unit containing  binary
       zero  is  present  after the string, but is not included in the length.
       The delimiter that was used to start the string is also  stored  within
       the  pattern, immediately before the string itself. You can access this
       delimiter as callout_string[-1] if you need it.

       The callout_string_offset field is the code unit offset to the start of
       the callout argument string within the original pattern string. This is
       provided for the benefit of applications such as script languages  that
       might need to report errors in the callout string within the pattern.

   Fields for all callouts

       The  remaining  fields in the callout block are the same for both kinds
       of callout.

       The offset_vector field is a pointer to a vector of  capturing  offsets
       (the "ovector"). You may read the elements in this vector, but you must
       not change any of them.

       For  calls  to pcre2_match(), the offset_vector field is not (since re-
       lease 10.30) a pointer to the actual ovector that  was  passed  to  the
       matching  function in the match data block. Instead it points to an in-
       ternal ovector of a size large enough to  hold  all  possible  captured
       substrings in the pattern. Note that whenever a recursion or subroutine
       call  within  a pattern completes, the capturing state is reset to what
       it was before.

       The capture_last field contains the number of the  most  recently  cap-
       tured  substring,  and the capture_top field contains one more than the
       number of the highest numbered captured substring so far.  If  no  sub-
       strings  have yet been captured, the value of capture_last is 0 and the
       value of capture_top is 1. The values of these  fields  do  not  always
       differ   by   one;  for  example,  when  the  callout  in  the  pattern
       ((a)(b))(?C2) is taken, capture_last is 1 but capture_top is 4.

       The contents of ovector[2] to  ovector[<capture_top>*2-1]  can  be  in-
       spected  in  order to extract substrings that have been matched so far,
       in the same way as extracting substrings after a match  has  completed.
       The  values in ovector[0] and ovector[1] are always PCRE2_UNSET because
       the match is by definition not complete. Substrings that have not  been
       captured  but whose numbers are less than capture_top also have both of
       their ovector slots set to PCRE2_UNSET.

       For DFA matching, the offset_vector field points to  the  ovector  that
       was  passed  to the matching function in the match data block for call-
       outs at the top level, but to an internal ovector during the processing
       of pattern recursions, lookarounds, and atomic groups.  However,  these
       ovectors  hold no useful information because pcre2_dfa_match() does not
       support substring capturing. The value of capture_top is always  1  and
       the value of capture_last is always 0 for DFA matching.

       The subject and subject_length fields contain copies of the values that
       were passed to the matching function.

       The  start_match  field normally contains the offset within the subject
       at which the current match attempt started. However, if the escape  se-
       quence  \K  has  been encountered, this value is changed to reflect the
       modified starting point. If the pattern is not  anchored,  the  callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

       The  current_position  field  contains the offset within the subject of
       the current match pointer.

       The pattern_position field contains the offset in the pattern string to
       the next item to be matched.

       The next_item_length field contains the length of the next item  to  be
       processed  in the pattern string. When the callout is at the end of the
       pattern, the length is zero.  When  the  callout  precedes  an  opening
       parenthesis, the length includes meta characters that follow the paren-
       thesis.  For  example,  in a callout before an assertion such as (?=ab)
       the length is 3. For an alternation bar or a closing  parenthesis,  the
       length  is  one,  unless a closing parenthesis is followed by a quanti-
       fier, in which case its length is included. (This  changed  in  release
       10.23.  In  earlier  releases, before an opening parenthesis the length
       was that of the entire group, and before an alternation bar or a  clos-
       ing parenthesis the length was zero.)

       The  pattern_position  and next_item_length fields are intended to help
       in distinguishing between different automatic callouts, which all  have
       the  same  callout  number. However, they are set for all callouts, and
       are used by pcre2test to show the next item to be matched when display-
       ing callout information.

       In callouts from pcre2_match() the mark field contains a pointer to the
       zero-terminated name of the most recently passed (*MARK), (*PRUNE),  or
       (*THEN)  item  in the match, or NULL if no such items have been passed.
       Instances of (*PRUNE) or (*THEN) without a name  do  not  obliterate  a
       previous (*MARK). In callouts from the DFA matching function this field
       always contains NULL.

       The   callout_flags   field   is   always   zero   in   callouts   from
       pcre2_dfa_match() or when JIT is being used. When pcre2_match() without
       JIT is used, the following bits may be set:

         PCRE2_CALLOUT_STARTMATCH

       This is set for the first callout after the start of matching for  each
       new starting position in the subject.

         PCRE2_CALLOUT_BACKTRACK

       This  is  set if there has been a matching backtrack since the previous
       callout, or since the start of matching if this is  the  first  callout
       from a pcre2_match() run.

       Both  bits  are  set when a backtrack has caused a "bumpalong" to a new
       starting position in the subject. Output from pcre2test does not  indi-
       cate  the  presence  of these bits unless the callout_extra modifier is
       set.

       The information in the callout_flags field is provided so that applica-
       tions can track and tell their users how matching with backtracking  is
       done.  This  can be useful when trying to optimize patterns, or just to
       understand how PCRE2 works. There is no  support  in  pcre2_dfa_match()
       because  there is no backtracking in DFA matching, and there is no sup-
       port in JIT because JIT is all about maximimizing matching performance.
       In both these cases the callout_flags field is always zero.


RETURN VALUES FROM CALLOUTS

       The external callout function returns an integer to PCRE2. If the value
       is zero, matching proceeds as normal. If  the  value  is  greater  than
       zero,  matching  fails  at  the current point, but the testing of other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the match is abandoned, and the
       matching function returns the negative value.

       Negative values should normally be chosen from  the  set  of  PCRE2_ER-
       ROR_xxx  values.  In  particular, PCRE2_ERROR_NOMATCH forces a standard
       "no match" failure. The error number  PCRE2_ERROR_CALLOUT  is  reserved
       for use by callout functions; it will never be used by PCRE2 itself.


CALLOUT ENUMERATION

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       A script language that supports the use of string arguments in callouts
       might  like  to  scan  all the callouts in a pattern before running the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument is a pointer to a compiled pattern, the  second  points  to  a
       callback  function,  and the third is arbitrary user data. The callback
       function is called for every callout in the pattern  in  the  order  in
       which they appear. Its first argument is a pointer to a callout enumer-
       ation  block,  and  its second argument is the user_data value that was
       passed to pcre2_callout_enumerate(). The data block contains  the  fol-
       lowing fields:

         version                Block version number
         pattern_position       Offset to next item in pattern
         next_item_length       Length of next item in pattern
         callout_number         Number for numbered callouts
         callout_string_offset  Offset to string within pattern
         callout_string_length  Length of callout string
         callout_string         Points to callout string or is NULL

       The  version  number is currently 0. It will increase if new fields are
       ever added to the block. The remaining fields are  the  same  as  their
       namesakes  in  the pcre2_callout block that is used for callouts during
       matching, as described above.

       Note that the value of pattern_position is  unique  for  each  callout.
       However,  if  a callout occurs inside a group that is quantified with a
       non-zero minimum or a fixed maximum, the group is replicated inside the
       compiled pattern. For example, a pattern such as /(a){2}/  is  compiled
       as  if it were /(a)(a)/. This means that the callout will be enumerated
       more than once, but with the same value for  pattern_position  in  each
       case.

       The callback function should normally return zero. If it returns a non-
       zero value, scanning the pattern stops, and that value is returned from
       pcre2_callout_enumerate().


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 26 February 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                    26 February 2025                PCRE2CALLOUT(3)
------------------------------------------------------------------------------


PCRE2COMPAT(3)             Library Functions Manual             PCRE2COMPAT(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


DIFFERENCES BETWEEN PCRE2 AND PERL

       This  document describes some of the known differences in the ways that
       PCRE2 and Perl handle regular expressions.  The  differences  described
       here  are  with  respect  to  Perl version 5.38.0, but as both Perl and
       PCRE2 are continually changing, the information may at times be out  of
       date.

       1.  When  PCRE2_DOTALL  (equivalent to Perl's /s qualifier) is not set,
       the behaviour of the '.' metacharacter differs from Perl. In PCRE2, '.'
       matches the next character unless it is the  start  of  a  newline  se-
       quence.  This  means  that, if the newline setting is CR, CRLF, or NUL,
       '.' will match the code point LF (0x0A) in ASCII/Unicode  environments,
       and  NL  (either  0x15 or 0x25) when using EBCDIC. In Perl, '.' appears
       never to match LF, even when 0x0A is not a newline indicator.

       2. PCRE2 has only a subset of Perl's Unicode support. Details  of  what
       it does have are given in the pcre2unicode page.

       3.  Like  Perl, PCRE2 allows repeat quantifiers on parenthesized asser-
       tions, but they do not mean what you might think. For example, (?!a){3}
       does not assert that the next three characters are not "a". It just as-
       serts that the next character is not "a"  three  times  (in  principle;
       PCRE2  optimizes this to run the assertion just once). Perl allows some
       repeat quantifiers on other assertions, for example, \b* , but these do
       not seem to have any use. PCRE2 does not allow any kind  of  quantifier
       on non-lookaround assertions.

       4.  If a braced quantifier such as {1,2} appears where there is nothing
       to repeat (for example, at the start of a branch), PCRE2 raises an  er-
       ror  whereas  Perl  treats the quantifier characters as literal. When a
       braced quantifier (...){min,max} has min > max, Perl treats  it  as  an
       item  which  fails to match any portion of the subject (as no number of
       repetitions can meet the condition), and additionally issues a  warning
       when in warning mode. PCRE2 has no warning features, so it gives an er-
       ror in this case.

       5.  Capture groups that occur inside negative lookaround assertions are
       counted, but their entries in the offsets vector are set  only  when  a
       negative  assertion is a condition that has a matching branch (that is,
       the condition is false).  Perl may set such  capture  groups  in  other
       circumstances.

       6.  The  following Perl escape sequences are not supported: \F, \l, \L,
       \u, \U, and \N when followed by a character name. \N on its own, match-
       ing a non-newline character, and \N{U+dd..}, matching  a  Unicode  code
       point,  are  supported.  The  escapes that modify the case of following
       letters are implemented by Perl's general string-handling and  are  not
       part of its pattern matching engine. If any of these are encountered by
       PCRE2,  an  error  is  generated  by default. However, if either of the
       PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \U  and  \u  are
       interpreted as ECMAScript interprets them.

       7. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
       is built with Unicode support (the default). The properties that can be
       tested  with  \p  and \P are limited to the general category properties
       such as Lu and Nd, the derived properties  Any  and  Lc  (synonym  L&),
       script  names such as Greek or Han, Bidi_Class, Bidi_Control, and a few
       binary properties. Both PCRE2 and Perl support the Cs (surrogate) prop-
       erty, but in PCRE2 its use is limited. See the pcre2pattern  documenta-
       tion  for  details. The long synonyms for property names that Perl sup-
       ports (such as \p{Letter}) are not supported by PCRE2, nor is  it  per-
       mitted to prefix any of these properties with "Is".

       8. PCRE2 supports the \Q...\E escape for quoting substrings. Characters
       in between are treated as literals. However, this is slightly different
       from  Perl  in  that  $  and  @ are also handled as literals inside the
       quotes. In Perl, they cause variable interpolation (PCRE2 does not have
       variables). Also, Perl does "double-quotish backslash interpolation" on
       any backslashes between \Q and \E which, its documentation  says,  "may
       lead  to confusing results". PCRE2 treats a backslash between \Q and \E
       just like any other character. Note the following examples:

           Pattern            PCRE2 matches     Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
           \QA\B\E            A\B               A\B
           \Q\\E              \                 \\E

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes  by  both  PCRE2 and Perl. Another difference from Perl is that
       any appearance of \Q or \E inside what might otherwise be a  quantifier
       causes PCRE2 not to recognize the sequence as a quantifier. Perl recog-
       nizes  a  quantifier  if  (redundantly) either of the numbers is inside
       \Q...\E, but not if the separating comma is. When not recognized  as  a
       quantifier  a  sequence  such  as  {\Q1\E,2}  is treated as the literal
       string "{1,2}".

       9.  Fairly  obviously,  PCRE2  does  not  support  the  (?{code})   and
       (??{code}) constructions. However, PCRE2 does have a "callout" feature,
       which allows an external function to be called during pattern matching.
       See the pcre2callout documentation for details.

       10.  Subroutine calls (whether recursive or not) were treated as atomic
       groups up to PCRE2 release 10.23, but from release 10.30 this  changed,
       and backtracking into subroutine calls is now supported, as in Perl.

       11.  In  PCRE2,  if any of the backtracking control verbs are used in a
       group that is called as a  subroutine  (whether  or  not  recursively),
       their  effect is confined to that group; it does not extend to the sur-
       rounding pattern. This is not always the case in Perl.  In  particular,
       if  (*THEN)  is  present in a group that is called as a subroutine, its
       action is limited to that group, even if the group does not contain any
       | characters. Note that such groups are processed as  anchored  at  the
       point  where  they  are  tested.  PCRE2 also confines all control verbs
       within atomic assertions, again including (*THEN)  in  assertions  with
       only one branch.

       12.  If a pattern contains more than one backtracking control verb, the
       first one that is backtracked onto acts. For example,  in  the  pattern
       A(*COMMIT)B(*PRUNE)C  a  failure in B triggers (*COMMIT), but a failure
       in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
       it is the same as PCRE2, but there are cases where it differs.

       13. There are some differences that are concerned with the settings  of
       captured  strings  when  part  of  a  pattern is repeated. For example,
       matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves  $2  un-
       set, but in PCRE2 it is set to "b".

       14.  PCRE2's  handling  of duplicate capture group numbers and names is
       not as general as Perl's. This is a consequence of the fact  the  PCRE2
       works  internally  just with numbers, using an external table to trans-
       late between numbers and  names.  In  particular,  a  pattern  such  as
       (?|(?<a>A)|(?<b>B)),  where the two capture groups have the same number
       but different names, is not supported, and causes an error  at  compile
       time. If it were allowed, it would not be possible to distinguish which
       group  matched,  because  both  names map to capture group number 1. To
       avoid this confusing situation, an error is given at compile time.

       15. Perl used to recognize comments in some places that PCRE2 does not,
       for example, between the ( and ? at the start of a  group.  If  the  /x
       modifier  is  set,  Perl allowed white space between ( and ? though the
       latest Perls give an error (for a while it was just deprecated).  There
       may still be some cases where Perl behaves differently.

       16.  Perl,  when  in warning mode, gives warnings for character classes
       such as [A-\d] or [a-[:digit:]]. It then treats the hyphens  as  liter-
       als. PCRE2 has no warning features, so it gives an error in these cases
       because they are almost certainly user mistakes.

       17. In PCRE2, until release 10.45, the upper/lower case character prop-
       erties  Lu  and Ll were not affected when case-independent matching was
       specified. Perl has changed in this respect, and PCRE2 has now  changed
       to  match.  When  caseless  matching is in force, Lu, Ll, and Lt (title
       case) are all treated as Lc (cased letter).

       18. From release 5.32.0, Perl locks out the use of \K in lookaround as-
       sertions. From release 10.38 PCRE2 does the same by  default.  However,
       there  is  an  option for re-enabling the previous behaviour. When this
       option is set, \K is acted on when it occurs  in  positive  assertions,
       but is ignored in negative assertions.

       19.  PCRE2  provides some extensions to the Perl regular expression fa-
       cilities.  Perl 5.10 included new features that  were  not  in  earlier
       versions  of  Perl,  some  of which (such as named parentheses) were in
       PCRE2 for some time before. This list is with respect to Perl 5.38:

       (a) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set,  the
       $ meta-character matches only at the very end of the string.

       (b)  A  backslash  followed  by  a  letter  with  no special meaning is
       faulted. (Perl can be made to issue a warning.)

       (c) If PCRE2_UNGREEDY is set, the greediness of the repetition  quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

       (d)  PCRE2_ANCHORED  can be used at matching time to force a pattern to
       be tried only at the first matching position in the subject string.

       (e)    The    PCRE2_NOTBOL,    PCRE2_NOTEOL,     PCRE2_NOTEMPTY     and
       PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.

       (f)  The  \R escape sequence can be restricted to match only CR, LF, or
       CRLF by the PCRE2_BSR_ANYCRLF option.

       (g) The callout facility is PCRE2-specific.  Perl  supports  codeblocks
       and variable interpolation, but not general hooks on every match.

       (h) The partial matching facility is PCRE2-specific.

       (i)  The alternative matching function (pcre2_dfa_match()) matches in a
       different way and is not Perl-compatible.

       (j) PCRE2 recognizes some special sequences such as (*CR) or  (*NO_JIT)
       at  the  start  of  a pattern. These set overall options that cannot be
       changed within the pattern.

       (k) PCRE2 supports non-atomic positive lookaround assertions.  This  is
       an extension to the lookaround facilities. The default, Perl-compatible
       lookarounds are atomic.

       (l)  There  are three syntactical items in patterns that can refer to a
       capturing group by number: back references such  as  \g{2},  subroutine
       calls  such  as (?3), and condition references such as (?(4)...). PCRE2
       supports relative group numbers such as +2 and -4 in all  three  cases.
       Perl  supports both plus and minus for subroutine calls, but only minus
       for back references, and no relative numbering at all for conditions.

       (m) The scan substring assertion (syntax (*scs:(n)...)) is a PCRE2  ex-
       tension that is not available in Perl.

       20.  Perl has different limits than PCRE2. See the pcre2limits documen-
       tation for details. Perl went with 5.10  from  recursion  to  iteration
       keeping  the intermediate matches on the heap, which is ~10% slower but
       does not fall into any  stack-overflow  limit.  PCRE2  made  a  similar
       change at release 10.30, and also has many build-time and run-time cus-
       tomizable limits.

       21.  Unlike  Perl,  PCRE2 doesn't have character set modifiers and spe-
       cially no way to set characters by context just  like  Perl's  "/d".  A
       regular expression using PCRE2_UTF and PCRE2_UCP will use similar rules
       to  Perl's  "/u";  something closer to "/a" could be selected by adding
       other PCRE2_EXTRA_ASCII* options on top.

       22. Some recursive patterns that Perl diagnoses as infinite  recursions
       can be handled by PCRE2, either by the interpreter or the JIT. An exam-
       ple is /(?:|(?0)abcd)(?(R)|\z)/, which matches a sequence of any number
       of repeated "abcd" substrings at the end of the subject.

       23.  Both  PCRE2  and Perl error when \x{ escapes are invalid, but Perl
       tries to recover and prints a warning if the problem was  that  an  in-
       valid hexadecimal digit was found. Since PCRE2 doesn't have warnings it
       returns  an  error instead.  Additionally, Perl accepts \x{} and gener-
       ates NUL unlike PCRE2.

       24. From release 10.45, PCRE2 gives an error if \x is not followed by a
       hexadecimal digit or a curly bracket. It used to interpret this as  the
       NUL character. Perl still generates NUL, but warns when in warning mode
       in most cases.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 02 June 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                      02 June 2025                   PCRE2COMPAT(3)
------------------------------------------------------------------------------


PCRE2JIT(3)                Library Functions Manual                PCRE2JIT(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 JUST-IN-TIME COMPILER SUPPORT

       Just-in-time  compiling  is a heavyweight optimization that can greatly
       speed up pattern matching. However, it comes at the cost of extra  pro-
       cessing  before  the  match is performed, so it is of most benefit when
       the same pattern is going to be matched many times. This does not  nec-
       essarily  mean many calls of a matching function; if the pattern is not
       anchored, matching attempts may take place many times at various  posi-
       tions in the subject, even for a single call. Therefore, if the subject
       string  is  very  long,  it  may  still pay to use JIT even for one-off
       matches. JIT support is available for all  of  the  8-bit,  16-bit  and
       32-bit PCRE2 libraries.

       JIT  support  applies  only to the traditional Perl-compatible matching
       function.  It does not apply when the DFA matching  function  is  being
       used. The code for JIT support was written by Zoltan Herczeg.


AVAILABILITY OF JIT SUPPORT

       JIT  support  is  an  optional feature of PCRE2. The "configure" option
       --enable-jit (or equivalent CMake option) must be  set  when  PCRE2  is
       built  if  you want to use JIT. The support is limited to the following
       hardware platforms:

         ARM 32-bit (v7, and Thumb2)
         ARM 64-bit
         IBM s390x 64 bit
         Intel x86 32-bit and 64-bit
         LoongArch 64 bit
         MIPS 32-bit and 64-bit
         Power PC 32-bit and 64-bit
         RISC-V 32-bit and 64-bit

       If --enable-jit is set on an unsupported platform, compilation fails.

       A client program can tell if JIT support has been compiled  by  calling
       pcre2_config()  with  the PCRE2_CONFIG_JIT option. The result is one if
       PCRE2 was built with JIT support, and zero otherwise.  However,  having
       the  JIT code available does not guarantee that it will be used for any
       particular match. One reason for this is that there are a number of op-
       tions and pattern items that are not supported by JIT (see below).  An-
       other  reason  is  that  in some environments JIT is unable to get exe-
       cutable memory in which to build its compiled code. The only  guarantee
       from pcre2_config() is that if it returns zero, JIT will definitely not
       be used.

       As  of  release  10.45  there is a more informative way to test for JIT
       support. If  pcre2_compile_jit()  is  called  with  the  single  option
       PCRE2_JIT_TEST_ALLOC  it  returns  zero  if  JIT is available and has a
       working allocator. Otherwise it returns PCRE2_ERROR_NOMEMORY if JIT  is
       available but cannot allocate executable memory, or PCRE2_ERROR_JIT_UN-
       SUPPORTED if JIT support is not compiled. The code argument is ignored,
       so it can be a NULL value.

       A  simple  program  does not need to check availability in order to use
       JIT when possible. The API is implemented in a way that falls  back  to
       the  interpretive  code if JIT is not available or cannot be used for a
       given match. For programs that  need  the  best  possible  performance,
       there is a "fast path" API that is JIT-specific.


SIMPLE USE OF JIT

       To  make use of the JIT support in the simplest way, all you have to do
       is to call pcre2_jit_compile() after successfully compiling  a  pattern
       with pcre2_compile(). This function has two arguments: the first is the
       compiled  pattern pointer that was returned by pcre2_compile(), and the
       second is zero or more of the  following  option  bits:  PCRE2_JIT_COM-
       PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.

       If  JIT  support  is  not available, a call to pcre2_jit_compile() does
       nothing and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the  compiled
       pattern is passed to the JIT compiler, which turns it into machine code
       that executes much faster than the normal interpretive code, but yields
       exactly  the  same results. The returned value from pcre2_jit_compile()
       is zero on success, or a negative error code.

       There is a limit to the size of pattern that JIT supports,  imposed  by
       the  size  of machine stack that it uses. The exact rules are not docu-
       mented because they may change at any time, in particular, when new op-
       timizations are introduced.  If  a  pattern  is  too  big,  a  call  to
       pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY.

       PCRE2_JIT_COMPLETE  requests the JIT compiler to generate code for com-
       plete matches. If you want to run partial matches using the  PCRE2_PAR-
       TIAL_HARD  or  PCRE2_PARTIAL_SOFT  options of pcre2_match(), you should
       set one or both of  the  other  options  as  well  as,  or  instead  of
       PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code
       for  each of the three modes (normal, soft partial, hard partial). When
       pcre2_match() is called, the appropriate code is run if  it  is  avail-
       able. Otherwise, the pattern is matched using interpretive code.

       You  can  call pcre2_jit_compile() multiple times for the same compiled
       pattern. It does nothing if it has previously compiled code for any  of
       the  option bits. For example, you can call it once with PCRE2_JIT_COM-
       PLETE and (perhaps later, when you  find  you  need  partial  matching)
       again  with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time it
       will ignore PCRE2_JIT_COMPLETE and just compile code for partial match-
       ing. If pcre2_jit_compile() is called with no option bits set, it imme-
       diately returns zero. This is an alternative way of testing whether JIT
       support has been compiled.

       At present, it is not possible to free JIT compiled  code  except  when
       the entire compiled pattern is freed by calling pcre2_code_free().

       In  some circumstances you may need to call additional functions. These
       are described in the section entitled "Controlling the JIT  stack"  be-
       low.

       There are some pcre2_match() options that are not supported by JIT, and
       there  are  also some pattern items that JIT cannot handle. Details are
       given below.  In both cases, matching automatically falls back  to  the
       interpretive  code.  If  you want to know whether JIT was actually used
       for a particular match, you should arrange for a JIT callback  function
       to  be set up as described in the section entitled "Controlling the JIT
       stack" below, even if you do not  need  to  supply  a  non-default  JIT
       stack. Such a callback function is called whenever JIT code is about to
       be  obeyed.  If the match-time options are not right for JIT execution,
       the callback function is not obeyed.

       If the JIT compiler finds an unsupported item, no JIT  data  is  gener-
       ated. You can find out if JIT compilation was successful for a compiled
       pattern by calling pcre2_pattern_info() with the PCRE2_INFO_JITSIZE op-
       tion.  A  non-zero  result means that JIT compilation was successful. A
       result of 0 means that JIT support is not available, or the pattern was
       not processed by pcre2_jit_compile(), or the JIT compiler was not  able
       to  handle  the  pattern. Successful JIT compilation does not, however,
       guarantee the use of JIT at match time because  there  are  some  match
       time options that are not supported by JIT.


MATCHING SUBJECTS CONTAINING INVALID UTF

       When  a  pattern is compiled with the PCRE2_UTF option, subject strings
       are normally expected to be a valid sequence of UTF code units. By  de-
       fault,  this is checked at the start of matching and an error is gener-
       ated if invalid UTF is detected. The PCRE2_NO_UTF_CHECK option  can  be
       passed to pcre2_match() to skip the check (for improved performance) if
       you  are  sure  that  a subject string is valid. If this option is used
       with an invalid string, the result is undefined.  The  calling  program
       may crash or loop or otherwise misbehave.

       However,  a  way of running matches on strings that may contain invalid
       UTF  sequences  is  available.   Calling   pcre2_compile()   with   the
       PCRE2_MATCH_INVALID_UTF  option  has  two  effects: it tells the inter-
       preter in pcre2_match() to support invalid UTF, and, if  pcre2_jit_com-
       pile()  is subsequently called, the compiled JIT code also supports in-
       valid UTF.  Details of how this support works, in both the JIT and  the
       interpretive cases, is given in the pcre2unicode documentation.

       There  is  also  an  obsolete  option  for  pcre2_jit_compile()  called
       PCRE2_JIT_INVALID_UTF, which currently exists only for backward compat-
       ibility.    It   is   superseded   by   the   pcre2_compile()    option
       PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be removed
       in future.


UNSUPPORTED OPTIONS AND PATTERN ITEMS

       The  pcre2_match()  options  that  are  supported  for JIT matching are
       PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY,
       PCRE2_NOTEMPTY_ATSTART,  PCRE2_NO_UTF_CHECK,  PCRE2_PARTIAL_HARD,   and
       PCRE2_PARTIAL_SOFT.  The  PCRE2_ANCHORED  and PCRE2_ENDANCHORED options
       are not supported at match time.

       If the PCRE2_NO_JIT option is passed to pcre2_match() it  disables  the
       use of JIT, forcing matching by the interpreter code.

       The  only  unsupported  pattern items are \C (match a single data unit)
       when running in a UTF mode, and a callout immediately before an  asser-
       tion condition in a conditional group.


RETURN VALUES FROM JIT MATCHING

       When  a pattern is matched using JIT, the return values are the same as
       those given by the interpretive pcre2_match() code, with  the  addition
       of  one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means that the
       memory used for the JIT stack was insufficient.  See  "Controlling  the
       JIT stack" below for a discussion of JIT stack usage.

       The  error  code  PCRE2_ERROR_MATCHLIMIT is returned by the JIT code if
       searching a very large pattern tree goes on for too long, as it  is  in
       the  same circumstance when JIT is not used, but the details of exactly
       what is counted are not the same. The PCRE2_ERROR_DEPTHLIMIT error code
       is never returned when JIT matching is used.


CONTROLLING THE JIT STACK

       When the compiled JIT code runs, it needs a block of memory to use as a
       stack.  By default, it uses 32KiB on the machine stack.  However,  some
       large  or complicated patterns need more than this. The error PCRE2_ER-
       ROR_JIT_STACKLIMIT is given when there is not enough stack. Three func-
       tions are provided for managing blocks of memory for use as JIT stacks.
       There is further discussion about the use of JIT stacks in the  section
       entitled "JIT stack FAQ" below.

       The  pcre2_jit_stack_create()  function  creates a JIT stack. Its argu-
       ments are a starting size, a maximum size, and a general  context  (for
       memory  allocation  functions, or NULL for standard memory allocation).
       It returns a pointer to an opaque structure of type pcre2_jit_stack, or
       NULL if there is an error. The pcre2_jit_stack_free() function is  used
       to free a stack that is no longer needed. If its argument is NULL, this
       function  returns immediately, without doing anything. (For the techni-
       cally minded: the address space is allocated by mmap or  VirtualAlloc.)
       A  maximum  stack size of 512KiB to 1MiB should be more than enough for
       any pattern.

       The pcre2_jit_stack_assign() function specifies which  stack  JIT  code
       should use. Its arguments are as follows:

         pcre2_match_context  *mcontext
         pcre2_jit_callback    callback
         void                 *data

       The first argument is a pointer to a match context. When this is subse-
       quently passed to a matching function, its information determines which
       JIT stack is used. If this argument is NULL, the function returns imme-
       diately,  without  doing anything. There are three cases for the values
       of the other two options:

         (1) If callback is NULL and data is NULL, an internal 32KiB block
             on the machine stack is used. This is the default when a match
             context is created.

         (2) If callback is NULL and data is not NULL, data must be
             a pointer to a valid JIT stack, the result of calling
             pcre2_jit_stack_create().

         (3) If callback is not NULL, it must point to a function that is
             called with data as an argument at the start of matching, in
             order to set up a JIT stack. If the return from the callback
             function is NULL, the internal 32KiB stack is used; otherwise the
             return value must be a valid JIT stack, the result of calling
             pcre2_jit_stack_create().

       A callback function is obeyed whenever JIT code is about to be run;  it
       is not obeyed when pcre2_match() is called with options that are incom-
       patible  for JIT matching. A callback function can therefore be used to
       determine whether a match operation was executed by JIT or by  the  in-
       terpreter.

       You may safely use the same JIT stack for more than one pattern (either
       by  assigning  directly  or  by  callback), as long as the patterns are
       matched sequentially in the same thread. Currently, the only way to set
       up non-sequential matches in one thread is to use callouts: if a  call-
       out  function starts another match, that match must use a different JIT
       stack to the one used for currently suspended match(es).

       In a multithread application, if you do not specify a JIT stack, or  if
       you  assign or pass back NULL from a callback, that is thread-safe, be-
       cause each thread has its own machine stack. However, if you assign  or
       pass back a non-NULL JIT stack, this must be a different stack for each
       thread so that the application is thread-safe.

       Strictly  speaking,  even more is allowed. You can assign the same non-
       NULL stack to a match context that is used by any number  of  patterns,
       as  long  as  they are not used for matching by multiple threads at the
       same time. For example, you could use the same stack  in  all  compiled
       patterns,  with  a global mutex in the callback to wait until the stack
       is available for use. However, this is an inefficient solution, and not
       recommended.

       This is a suggestion for how a multithreaded program that needs to  set
       up non-default JIT stacks might operate:

         During thread initialization
           thread_local_var = pcre2_jit_stack_create(...)

         During thread exit
           pcre2_jit_stack_free(thread_local_var)

         Use a one-line callback function
           return thread_local_var

       All  the  functions  described in this section do nothing if JIT is not
       available.


JIT STACK FAQ

       (1) Why do we need JIT stacks?

       PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack
       where the local data of the current node is pushed before checking  its
       child nodes.  Allocating real machine stack on some platforms is diffi-
       cult. For example, the stack chain needs to be updated every time if we
       extend  the  stack  on  PowerPC.  Although it is possible, its updating
       time overhead decreases performance. So we do the recursion in memory.

       (2) Why don't we simply allocate blocks of memory with malloc()?

       Modern operating systems have a nice feature: they can reserve  an  ad-
       dress space instead of allocating memory. We can safely allocate memory
       pages inside this address space, so the stack could grow without moving
       memory  data (this is important because of pointers). Thus we can allo-
       cate 1MiB address space, and use only a  single  memory  page  (usually
       4KiB)  if that is enough. However, we can still grow up to 1MiB anytime
       if needed.

       (3) Who "owns" a JIT stack?

       The owner of the stack is the user program, not the JIT studied pattern
       or anything else. The user program must ensure that if a stack is being
       used by pcre2_match(), (that is, it is assigned to a match context that
       is passed to the pattern currently running), that  stack  must  not  be
       used  by any other threads (to avoid overwriting the same memory area).
       The best practice for multithreaded programs is to allocate a stack for
       each thread, and return this stack through the JIT callback function.

       (4) When should a JIT stack be freed?

       You can free a JIT stack at any time, as long as it will not be used by
       pcre2_match() again. When you assign the stack to a match context, only
       a pointer is set. There is no reference counting or  any  other  magic.
       You can free compiled patterns, contexts, and stacks in any order, any-
       time.   Just do not call pcre2_match() with a match context pointing to
       an already freed stack, as that will cause SEGFAULT. (Also, do not free
       a stack currently used by pcre2_match() in  another  thread).  You  can
       also  replace the stack in a context at any time when it is not in use.
       You should free the previous stack before assigning a replacement.

       (5) Should I allocate/free a  stack  every  time  before/after  calling
       pcre2_match()?

       No,  because  this  is  too  costly in terms of resources. However, you
       could implement some clever idea which release the stack if it  is  not
       used  in  let's  say  two minutes. The JIT callback can help to achieve
       this without keeping a list of patterns.

       (6) OK, the stack is for long term memory allocation. But what  happens
       if  a  pattern causes stack overflow with a stack of 1MiB? Is that 1MiB
       kept until the stack is freed?

       Especially on embedded systems, it might be a good idea to release mem-
       ory sometimes without freeing the stack. There is no API  for  this  at
       the  moment.  Probably a function call which returns with the currently
       allocated memory for any stack and another which allows releasing  mem-
       ory (shrinking the stack) would be a good idea if someone needs this.

       (7) This is too much of a headache. Isn't there any better solution for
       JIT stack handling?

       No,  thanks to Windows. If POSIX threads were used everywhere, we could
       throw out this complicated API.


FREEING JIT SPECULATIVE MEMORY

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       The JIT executable allocator does not free all memory when it is possi-
       ble. It expects new allocations, and keeps some free memory  around  to
       improve  allocation  speed. However, in low memory conditions, it might
       be better to free all possible memory. You can cause this to happen  by
       calling  pcre2_jit_free_unused_memory(). Its argument is a general con-
       text, for custom memory management, or NULL for standard memory manage-
       ment.


EXAMPLE CODE

       This is a single-threaded example that specifies a  JIT  stack  without
       using  a  callback.  A real program should include error checking after
       all the function calls.

         int rc;
         pcre2_code *re;
         pcre2_match_data *match_data;
         pcre2_match_context *mcontext;
         pcre2_jit_stack *jit_stack;

         re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
           &errornumber, &erroffset, NULL);
         rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
         mcontext = pcre2_match_context_create(NULL);
         jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL);
         pcre2_jit_stack_assign(mcontext, NULL, jit_stack);
         match_data = pcre2_match_data_create(re, 10);
         rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext);
         /* Process result */

         pcre2_code_free(re);
         pcre2_match_data_free(match_data);
         pcre2_match_context_free(mcontext);
         pcre2_jit_stack_free(jit_stack);


JIT FAST PATH API

       Because the API described above falls back to interpreted matching when
       JIT is not available, it is convenient for programs  that  are  written
       for  general  use  in  many  environments.  However,  calling  JIT  via
       pcre2_match() does have a performance impact. Programs that are written
       for use where JIT is known to be available, and  which  need  the  best
       possible  performance,  can  instead  use a "fast path" API to call JIT
       matching directly instead of calling pcre2_match() (obviously only  for
       patterns that have been successfully processed by pcre2_jit_compile()).

       The  fast  path  function is called pcre2_jit_match(), and it takes ex-
       actly the same arguments as pcre2_match(). However, the subject  string
       must  be  specified  with  a  length; PCRE2_ZERO_TERMINATED is not sup-
       ported. Unsupported option bits (for example, PCRE2_ANCHORED, PCRE2_EN-
       DANCHORED, and  PCRE2_COPY_MATCHED_SUBJECT)  are  ignored,  as  is  the
       PCRE2_NO_JIT  option.  The  return  values  are  also  the  same as for
       pcre2_match(), plus PCRE2_ERROR_JIT_BADOPTION if a matching mode  (par-
       tial or complete) is requested that was not compiled.

       When  you call pcre2_match(), as well as testing for invalid options, a
       number of other sanity checks are performed on the arguments. For exam-
       ple, if the subject pointer is NULL but the length is non-zero, an  im-
       mediate  error  is given. Also, unless PCRE2_NO_UTF_CHECK is set, a UTF
       subject string is tested for validity. In the interests of speed, these
       checks do not happen on the JIT fast  path.  If  invalid  UTF  data  is
       passed  when  PCRE2_MATCH_INVALID_UTF  was not set for pcre2_compile(),
       the result is undefined. The program may crash or loop  or  give  wrong
       results.  In  the  absence  of  PCRE2_MATCH_INVALID_UTF you should call
       pcre2_jit_match() in UTF mode only if  you  are  sure  the  subject  is
       valid.

       Bypassing  the  sanity  checks  and the pcre2_match() wrapping can give
       speedups of more than 10%.


SEE ALSO

       pcre2api(3), pcre2unicode(3)


AUTHOR

       Philip Hazel (FAQ by Zoltan Herczeg)
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 08 August 2026
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                     08 August 2026                     PCRE2JIT(3)
------------------------------------------------------------------------------


PCRE2LIMITS(3)             Library Functions Manual             PCRE2LIMITS(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


SIZE AND OTHER LIMITATIONS

       There are some size limitations in PCRE2 but it is hoped that they will
       never in practice be relevant.

       The  maximum  size  of  a compiled pattern is approximately 64 thousand
       code units for the 8-bit and 16-bit libraries if PCRE2 is compiled with
       the default internal linkage size, which  is  2  bytes  for  these  li-
       braries.  If  you  want  to  process regular expressions that are truly
       enormous, you can compile PCRE2 with an internal linkage size of 3 or 4
       (when building the 16-bit library, 3 is  rounded  up  to  4).  See  the
       README file in the source distribution and the pcre2build documentation
       for  details.  In  these cases the limit is substantially larger.  How-
       ever, the speed of execution is slower. In the 32-bit library, the  in-
       ternal linkage size is always 4.

       The maximum length of a source pattern string is essentially unlimited;
       it  is  the largest number a PCRE2_SIZE variable can hold. However, the
       program that calls pcre2_compile() can specify a smaller limit.

       The maximum length (in code units) of a subject string is one less than
       the largest number a PCRE2_SIZE variable can hold. PCRE2_SIZE is an un-
       signed integer type, usually defined as size_t. Its maximum value (that
       is ~(PCRE2_SIZE)0) is reserved as a special indicator  for  zero-termi-
       nated strings and unset offsets.

       All values in repeating quantifiers must be less than 65536.

       There are two different limits that apply to branches of lookbehind as-
       sertions.   If every branch in such an assertion matches a fixed number
       of characters, the maximum length of any branch is 65535 characters. If
       any branch matches a variable number of characters,  then  the  maximum
       matching  length  for every branch is limited. The default limit is set
       at compile time, defaulting to 255, but can be changed by  the  calling
       program.

       There  is no limit to the number of parenthesized groups, but there can
       be no more than 65535 capture groups, and there is a limit to the depth
       of nesting of parenthesized subpatterns of all kinds. This  is  imposed
       in  order to limit the amount of system stack used at compile time. The
       default limit can be specified when PCRE2 is built; if not, the default
       is set to  250.  An  application  can  change  this  limit  by  calling
       pcre2_set_parens_nest_limit() to set the limit in a compile context.

       The maximum length of the name for a named capture group as well as the
       number of such groups is configurable at build time. The maximum length
       for the name defaults to 128 code units, and the maximum number of such
       groups to 10000.

       The  maximum  length  of  a  name  in  a (*MARK), (*PRUNE), (*SKIP), or
       (*THEN) verb is 255 code units for the 8-bit  library  and  65535  code
       units for the 16-bit and 32-bit libraries.

       The  maximum  length  of  a string argument to a callout is the largest
       number a 32-bit unsigned integer can hold.

       The maximum amount of heap memory used for matching  is  controlled  by
       the  heap  limit,  which can be set in a pattern or in a match context.
       The default is a very large number, effectively unlimited.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 03 September 2025
       Copyright (c) 1997-2023 University of Cambridge.


PCRE2 10.48                    03 September 2025                PCRE2LIMITS(3)
------------------------------------------------------------------------------


PCRE2MATCHING(3)           Library Functions Manual           PCRE2MATCHING(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 MATCHING ALGORITHMS

       This document describes the two different algorithms that are available
       in  PCRE2  for  matching  a compiled regular expression against a given
       subject string. The "standard" algorithm is the  one  provided  by  the
       pcre2_match()  function.  This works in the same way as Perl's matching
       function, and provides a Perl-compatible matching operation. The  just-
       in-time (JIT) optimization that is described in the pcre2jit documenta-
       tion is compatible with this function.

       An alternative algorithm is provided by the pcre2_dfa_match() function;
       it operates in a different way, and is not Perl-compatible. This alter-
       native  has advantages and disadvantages compared with the standard al-
       gorithm, and these are described below.

       When there is only one possible way in which a given subject string can
       match a pattern, the two algorithms give the same answer. A  difference
       arises, however, when there are multiple possibilities. For example, if
       the anchored pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.


REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented  as  a  tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree.  Matching  the
       pattern  to a given subject string (from a given starting point) can be
       thought of as a search of the tree.  There are two  ways  to  search  a
       tree:  depth-first  and  breadth-first, and these correspond to the two
       matching algorithms provided by PCRE2.


THE STANDARD MATCHING ALGORITHM

       In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
       sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
       depth-first search of the pattern tree. That is, it  proceeds  along  a
       single path through the tree, checking that the subject matches what is
       required.  When  there  is a mismatch, the algorithm tries any alterna-
       tives at the current point, and if they all fail, it backs  up  to  the
       previous  branch  point  in  the  tree,  and tries the next alternative
       branch at that level. This often involves backing  up  (moving  to  the
       left)  in  the  subject  string  as well. The order in which repetition
       branches are tried is controlled by the greedy or  ungreedy  nature  of
       the quantifier.

       If  a  leaf  node  is reached, a matching string has been found, and at
       that point the algorithm stops. Thus, if there is more than one  possi-
       ble  match, this algorithm returns the first one that it finds. Whether
       this is the shortest, the longest, or some intermediate length  depends
       on the way the alternations and the greedy or ungreedy repetition quan-
       tifiers are specified in the pattern.

       Because  it  ends  up  with a single path through the tree, it is rela-
       tively straightforward for this algorithm to keep  track  of  the  sub-
       strings  that  are  matched  by portions of the pattern in parentheses.
       This provides support for capturing parentheses and backreferences.


THE ALTERNATIVE MATCHING ALGORITHM

       This algorithm conducts a breadth-first search of  the  tree.  Starting
       from  the  first  matching  point  in the subject, it scans the subject
       string from left to right, once, character by character, and as it does
       this, it remembers all the paths through the tree that represent  valid
       matches.  In  Friedl's  terminology, this is a kind of "DFA algorithm",
       though it is not implemented as a traditional finite state machine  (it
       keeps multiple states active simultaneously).

       Although  the  general  principle of this matching algorithm is that it
       scans the subject string only once, without backtracking, there is  one
       exception:  when  a lookaround assertion is encountered, the characters
       following or preceding the current point have to be  independently  in-
       spected.

       The  scan  continues until either the end of the subject is reached, or
       there are no more unterminated paths. At this point,  terminated  paths
       represent  the different matching possibilities (if there are none, the
       match has failed).  Thus, if there is more  than  one  possible  match,
       this  algorithm  finds  all  of  them,  and in particular, it finds the
       longest. The matches are returned in the output  vector  in  decreasing
       order  of  length.  There  is an option to stop the algorithm after the
       first match (which is necessarily the shortest) is found.

       Note that the size of vector needed to contain all the results  depends
       on  the  number of simultaneous matches, not on the number of capturing
       parentheses in  the  pattern.  Using  pcre2_match_data_create_from_pat-
       tern()  to  create the match data block is therefore not advisable when
       doing DFA matching.

       Note also that all the matches that are found start at the  same  point
       in the subject. If the pattern

         cat(er(pillar)?)?

       is  matched  against the string "the caterpillar catchment", the result
       is the three strings "caterpillar", "cater", and "cat"  that  start  at
       the  fifth  character  of the subject. The algorithm does not automati-
       cally move on to find matches that start at later positions.

       PCRE2's "auto-possessification" optimization usually applies to charac-
       ter repeats at the end of a pattern (as well as internally). For  exam-
       ple, the pattern "a\d+" is compiled as if it were "a\d++" because there
       is  no  point even considering the possibility of backtracking into the
       repeated digits. For DFA matching, this means that  only  one  possible
       match  is  found. If you really do want multiple matches in such cases,
       either use an ungreedy repeat ("a\d+?") or set  the  PCRE2_NO_AUTO_POS-
       SESS option when compiling.

       There  are  a  number of features of PCRE2 regular expressions that are
       not supported or behave differently in the alternative  matching  func-
       tion. Those that are not supported cause an error if encountered.

       1.  Because the algorithm finds all possible matches, the greedy or un-
       greedy nature of repetition quantifiers is not relevant (though it  may
       affect  auto-possessification,  as  just  described).  During matching,
       greedy and ungreedy quantifiers are treated in exactly  the  same  way.
       However, possessive quantifiers can make a difference when what follows
       could  also  match  what  is  quantified, for example in a pattern like
       this:

         ^a++\w!

       This pattern matches "aaab!" but not "aaa!", which would be matched  by
       a  non-possessive quantifier. Similarly, if an atomic group is present,
       it is matched as if it were a standalone pattern at the current  point,
       and  the  longest match is then "locked in" for the rest of the overall
       pattern.

       2. When dealing with multiple paths through the tree simultaneously, it
       is not straightforward to keep track of  captured  substrings  for  the
       different  matching  possibilities,  and PCRE2's implementation of this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

       3. Because no substrings are captured, a number of related features are
       not available:

       (a) Backreferences;

       (b) Conditional expressions that use a backreference as  the  condition
       or test for a specific group recursion;

       (c) Script runs;

       (d) Scan substring assertions.

       4. Because many paths through the tree may be active, the \K escape se-
       quence,  which  resets the start of the match when encountered (but may
       be on some paths and not on others), is not supported.

       5. Callouts are supported, but the value of the  capture_top  field  is
       always 1, and the value of the capture_last field is always 0.

       6.  The  \C  escape  sequence, which (in the standard algorithm) always
       matches a single code unit, even in a UTF mode, is not supported in UTF
       modes because the  alternative  algorithm  moves  through  the  subject
       string  one  character  (not code unit) at a time, for all active paths
       through the tree.

       7. Except for (*FAIL), the backtracking control verbs such as  (*PRUNE)
       are  not  supported.  (*FAIL)  is supported, and behaves like a failing
       negative assertion.

       8. The PCRE2_MATCH_INVALID_UTF option for pcre2_compile() is  not  sup-
       ported by pcre2_dfa_match().


ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The  main  advantage  of the alternative algorithm is that all possible
       matches (at a single point in the subject) are automatically found, and
       in particular, the longest match is found. To find more than one  match
       at  the  same point using the standard algorithm, you have to do kludgy
       things with callouts.

       Partial matching is possible with this algorithm, though  it  has  some
       limitations.  The  pcre2partial  documentation gives details of partial
       matching and discusses multi-segment matching.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative algorithm suffers from a number of disadvantages:

       1. It is substantially slower than  the  standard  algorithm.  This  is
       partly  because  it has to search for all possible matches, but is also
       because it is less susceptible to optimization.

       2. Capturing parentheses and other features such as backreferences that
       rely on them are not supported.

       3. Matching within invalid UTF strings is not supported.

       4. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.

       5. JIT optimization is not supported.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 22 February 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                    22 February 2025               PCRE2MATCHING(3)
------------------------------------------------------------------------------


PCRE2PARTIAL(3)            Library Functions Manual            PCRE2PARTIAL(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PARTIAL MATCHING IN PCRE2

       In  normal use of PCRE2, if there is a match up to the end of a subject
       string, but more characters are needed to  match  the  entire  pattern,
       PCRE2_ERROR_NOMATCH  is  returned,  just  like any other failing match.
       There are circumstances where it might be helpful to  distinguish  this
       "partial match" case.

       One  example  is  an application where the subject string is very long,
       and not all available at once. The requirement here is to be able to do
       the matching segment by segment, but special action is  needed  when  a
       matched substring spans the boundary between two segments.

       Another  example is checking a user input string as it is typed, to en-
       sure that it conforms to a required format. Invalid characters  can  be
       immediately diagnosed and rejected, giving instant feedback.

       Partial  matching  is a PCRE2-specific feature; it is not Perl-compati-
       ble. It is requested  by  setting  one  of  the  PCRE2_PARTIAL_HARD  or
       PCRE2_PARTIAL_SOFT  options  when calling a matching function. The dif-
       ference between the two options is whether or not a  partial  match  is
       preferred  to  an alternative complete match, though the details differ
       between the two types of matching function. If both  options  are  set,
       PCRE2_PARTIAL_HARD takes precedence.

       If  you  want to use partial matching with just-in-time optimized code,
       as well as setting a partial match option for  the  matching  function,
       you  must  also  call pcre2_jit_compile() with one or both of these op-
       tions:

         PCRE2_JIT_PARTIAL_HARD
         PCRE2_JIT_PARTIAL_SOFT

       PCRE2_JIT_COMPLETE should also be set if you are going to run  non-par-
       tial  matches  on  the same pattern. Separate code is compiled for each
       mode. If the appropriate JIT mode has not been  compiled,  interpretive
       matching code is used.

       Setting  a partial matching option disables two of PCRE2's standard op-
       timization hints. PCRE2 remembers the last literal code unit in a  pat-
       tern,  and  abandons  matching  immediately if it is not present in the
       subject string.  This optimization cannot be used for a subject  string
       that  might match only partially. PCRE2 also remembers a minimum length
       of a matching string, and does not bother to run the matching  function
       on  shorter  strings.  This  optimization  is also disabled for partial
       matching.


REQUIREMENTS FOR A PARTIAL MATCH

       A possible partial match occurs during matching when  the  end  of  the
       subject  string is reached successfully, but either more characters are
       needed to complete the match, or the addition of more characters  might
       change what is matched.

       Example  1: if the pattern is /abc/ and the subject is "ab", more char-
       acters are definitely needed to complete a match.  In  this  case  both
       hard and soft matching options yield a partial match.

       Example  2: if the pattern is /ab+/ and the subject is "ab", a complete
       match can be found, but the addition of more  characters  might  change
       what  is  matched. In this case, only PCRE2_PARTIAL_HARD returns a par-
       tial match; PCRE2_PARTIAL_SOFT returns the complete match.

       On reaching the end of the subject, when PCRE2_PARTIAL_HARD is set,  if
       the next pattern item is \z, \Z, \b, \B, or $ there is always a partial
       match.   Otherwise, for both options, the next pattern item must be one
       that inspects a character, and at least one of the  following  must  be
       true:

       (1)  At  least  one  character has already been inspected. An inspected
       character need not form part of the final  matched  string;  lookbehind
       assertions  and the \K escape sequence provide ways of inspecting char-
       acters before the start of a matched string.

       (2) The pattern contains one or more lookbehind assertions. This condi-
       tion exists in case there is a lookbehind that inspects characters  be-
       fore the start of the match.

       (3)  There  is a special case when the whole pattern can match an empty
       string.  When the starting point is at the  end  of  the  subject,  the
       empty  string  match is a possibility, and if PCRE2_PARTIAL_SOFT is set
       and neither of the above conditions is true, it is  returned.  However,
       because  adding  more  characters  might  result  in a non-empty match,
       PCRE2_PARTIAL_HARD returns a partial match, which in  this  case  means
       "there  is going to be a match at this point, but until some more char-
       acters are added, we do not know if it will be an empty string or some-
       thing longer".


PARTIAL MATCHING USING pcre2_match()

       When  a  partial  matching  option  is  set,  the  result  of   calling
       pcre2_match() can be one of the following:

       A successful match
         A complete match has been found, starting and ending within this sub-
         ject.

       PCRE2_ERROR_NOMATCH
         No match can start anywhere in this subject.

       PCRE2_ERROR_PARTIAL
         Adding  more  characters may result in a complete match that uses one
         or more characters from the end of this subject.

       When a partial match is returned, the first two elements in the ovector
       point to the portion of the subject that was matched, but the values in
       the rest of the ovector are undefined. The appearance of \K in the pat-
       tern has no effect for a partial match. Consider this pattern:

         /abc\K123/

       If it is matched against "456abc123xyz" the result is a complete match,
       and the ovector defines the matched string as "123", because \K  resets
       the  "start  of  match" point. However, if a partial match is requested
       and the subject string is "456abc12", a partial match is found for  the
       string  "abc12",  because  all these characters are needed for a subse-
       quent re-match with additional characters.

       If there is more than one partial match, the first one that  was  found
       provides the data that is returned. Consider this pattern:

         /123\w+X|dogY/

       If  this is matched against the subject string "abc123dog", both alter-
       natives fail to match, but the end of the  subject  is  reached  during
       matching,  so PCRE2_ERROR_PARTIAL is returned. The offsets are set to 3
       and 9, identifying "123dog" as the first partial match. (In this  exam-
       ple,  there are two partial matches, because "dog" on its own partially
       matches the second alternative.)

   How a partial match is processed by pcre2_match()

       What happens when a partial match is identified depends on which of the
       two partial matching options is set.

       If PCRE2_PARTIAL_HARD is set, PCRE2_ERROR_PARTIAL is returned  as  soon
       as  a partial match is found, without continuing to search for possible
       complete matches. This option is "hard" because it prefers  an  earlier
       partial match over a later complete match. For this reason, the assump-
       tion  is  made  that  the end of the supplied subject string is not the
       true end of the available data, which is why \z, \Z, \b, \B, and $  al-
       ways give a partial match.

       If  PCRE2_PARTIAL_SOFT  is  set,  the  partial match is remembered, but
       matching continues as normal, and other alternatives in the pattern are
       tried. If no complete match can be found,  PCRE2_ERROR_PARTIAL  is  re-
       turned instead of PCRE2_ERROR_NOMATCH. This option is "soft" because it
       prefers a complete match over a partial match. All the various matching
       items  in a pattern behave as if the subject string is potentially com-
       plete; \z, \Z, and $ match at the end of the subject,  as  normal,  and
       for \b and \B the end of the subject is treated as a non-alphanumeric.

       The  difference  between the two partial matching options can be illus-
       trated by a pattern such as:

         /dog(sbody)?/

       This matches either "dog" or "dogsbody", greedily (that is, it  prefers
       the  longer  string  if  possible). If it is matched against the string
       "dog" with PCRE2_PARTIAL_SOFT, it yields a complete  match  for  "dog".
       However,  if  PCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PAR-
       TIAL. On the other hand, if the pattern is made ungreedy the result  is
       different:

         /dog(sbody)??/

       In  this  case  the  result  is always a complete match because that is
       found first, and matching never  continues  after  finding  a  complete
       match. It might be easier to follow this explanation by thinking of the
       two patterns like this:

         /dog(sbody)?/    is the same as  /dogsbody|dog/
         /dog(sbody)??/   is the same as  /dog|dogsbody/

       The  second pattern will never match "dogsbody", because it will always
       find the shorter match first.

   Example of partial matching using pcre2test

       The pcre2test data modifiers partial_hard (or ph) and partial_soft  (or
       ps)  set  PCRE2_PARTIAL_HARD and PCRE2_PARTIAL_SOFT, respectively, when
       calling pcre2_match(). Here is a run of pcre2test using a pattern  that
       matches the whole subject in the form of a date:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25dec3\=ph
         Partial match: 23dec3
         data> 3ju\=ph
         Partial match: 3ju
         data> 3juj\=ph
         No match

       This  example  gives  the  same  results for both hard and soft partial
       matching options. Here is an example where there is a difference:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\=ps
          0: 25jun04
          1: jun
         data> 25jun04\=ph
         Partial match: 25jun04

       With  PCRE2_PARTIAL_SOFT,  the  subject  is  matched  completely.   For
       PCRE2_PARTIAL_HARD, however, the subject is assumed not to be complete,
       so there is only a partial match.


MULTI-SEGMENT MATCHING WITH pcre2_match()

       PCRE  was  not originally designed with multi-segment matching in mind.
       However, over time, features (including  partial  matching)  that  make
       multi-segment matching possible have been added. A very long string can
       be  searched  segment  by  segment by calling pcre2_match() repeatedly,
       with the aim of achieving the same results that would happen if the en-
       tire string was available for searching all  the  time.  Normally,  the
       strings  that  are  being  sought are much shorter than each individual
       segment, and are in the middle of very long strings, so the pattern  is
       normally not anchored.

       Special  logic  must  be implemented to handle a matched substring that
       spans a segment boundary. PCRE2_PARTIAL_HARD should be used, because it
       returns a partial match at the end of a segment whenever there  is  the
       possibility  of  changing  the  match  by  adding  more characters. The
       PCRE2_NOTBOL option should also be set for all but the first segment.

       When a partial match occurs, the next segment must be added to the cur-
       rent subject and the match re-run, using the  startoffset  argument  of
       pcre2_match()  to  begin  at the point where the partial match started.
       For example:

           re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
         data> ...the date is 23ja\=ph
         Partial match: 23ja
         data> ...the date is 23jan19 and on that day...\=offset=15
          0: 23jan19
          1: jan

       Note the use of the offset modifier to start the new  match  where  the
       partial match was found. In this example, the next segment was added to
       the  one  in  which  the  partial  match  was  found.  This is the most
       straightforward approach, typically using a memory buffer that is twice
       the size of each segment. After a partial match, the first half of  the
       buffer  is  discarded,  the  second  half  is moved to the start of the
       buffer, and a new segment is added before repeating the match as in the
       example above. After a no match, the entire buffer can be discarded.

       If there are memory constraints, you may want to discard text that pre-
       cedes a partial match before adding the  next  segment.  Unfortunately,
       this  is  not  at  present straightforward. In cases such as the above,
       where the pattern does not contain any lookbehinds, it is sufficient to
       retain only the partially matched substring. However,  if  the  pattern
       contains  a  lookbehind assertion, characters that precede the start of
       the partial match may have been inspected during the matching  process.
       When  pcre2test displays a partial match, it indicates these characters
       with '<' if the allusedtext modifier is set:

           re> "(?<=123)abc"
         data> xx123ab\=ph,allusedtext
         Partial match: 123ab
                        <<<

       However, the allusedtext modifier is not available  for  JIT  matching,
       because  JIT  matching  does  not  record the first (or last) consulted
       characters.  For this reason, this information is not available via the
       API. It is therefore not possible in general to obtain the exact number
       of characters that must be retained in order to get the right match re-
       sult. If you cannot retain the  entire  segment,  you  must  find  some
       heuristic way of choosing.

       If  you know the approximate length of the matching substrings, you can
       use that to decide how much text to retain. The only lookbehind  infor-
       mation  that  is  currently  available via the API is the length of the
       longest individual lookbehind in a pattern, but this can be  misleading
       if  there  are  nested  lookbehinds.  The  value  returned  by  calling
       pcre2_pattern_info() with the PCRE2_INFO_MAXLOOKBEHIND  option  is  the
       maximum number of characters (not code units) that any individual look-
       behind   moves   back   when   it  is  processed.  A  pattern  such  as
       "(?<=(?<!b)a)" has a maximum lookbehind value of one, but inspects  two
       characters before its starting point.

       In  a  non-UTF or a 32-bit case, moving back is just a subtraction, but
       in UTF-8 or UTF-16 you have  to  count  characters  while  moving  back
       through the code units.


PARTIAL MATCHING USING pcre2_dfa_match()

       The DFA function moves along the subject string character by character,
       without  backtracking,  searching  for  all possible matches simultane-
       ously. If the end of the subject is reached before the end of the  pat-
       tern, there is the possibility of a partial match.

       When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if
       there  have  been  no complete matches. Otherwise, the complete matches
       are returned.  If PCRE2_PARTIAL_HARD is  set,  a  partial  match  takes
       precedence  over  any  complete matches. The portion of the string that
       was matched when the longest partial match was  found  is  set  as  the
       first matching string.

       Because  the DFA function always searches for all possible matches, and
       there is no difference between greedy and ungreedy repetition, its  be-
       haviour  is different from the pcre2_match(). Consider the string "dog"
       matched against this ungreedy pattern:

         /dog(sbody)??/

       Whereas the standard function stops as soon as it  finds  the  complete
       match  for  "dog",  the  DFA  function also finds the partial match for
       "dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set.


MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()

       When a partial match has been found using the DFA matching function, it
       is possible to continue the match by providing additional subject  data
       and  calling  the function again with the same compiled regular expres-
       sion, this time setting the PCRE2_DFA_RESTART option. You must pass the
       same working space as before, because this is where details of the pre-
       vious partial match are stored. You can set the  PCRE2_PARTIAL_SOFT  or
       PCRE2_PARTIAL_HARD  options  with PCRE2_DFA_RESTART to continue partial
       matching over multiple segments. Here is an example using pcre2test:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\=dfa,ps
         Partial match: 23ja
         data> n05\=dfa,dfa_restart
          0: n05

       The first call has "23ja" as the subject, and requests  partial  match-
       ing;  the  second  call  has  "n05"  as  the  subject for the continued
       (restarted) match.  Notice that when the match is  complete,  only  the
       last  part  is  shown;  PCRE2 does not retain the previously partially-
       matched string. It is up to the calling program to do that if it  needs
       to.  This  means  that, for an unanchored pattern, if a continued match
       fails, it is not possible to try again at a  new  starting  point.  All
       this facility is capable of doing is continuing with the previous match
       attempt. For example, consider this pattern:

         1234|3789

       If  the  first  part of the subject is "ABC123", a partial match of the
       first alternative is found at offset 3. There is no partial  match  for
       the second alternative, because such a match does not start at the same
       point  in  the  subject  string. Attempting to continue with the string
       "7890" does not yield a match  because  only  those  alternatives  that
       match  at one point in the subject are remembered. Depending on the ap-
       plication, this may or may not be what you want.

       If you do want to allow for starting again at the next  character,  one
       way  of  doing it is to retain some or all of the segment and try a new
       complete match, as described for pcre2_match() above. Another possibil-
       ity is to work with two buffers. If a partial match at offset n in  the
       first  buffer  is followed by "no match" when PCRE2_DFA_RESTART is used
       on the second buffer, you can then try a new match starting  at  offset
       n+1 in the first buffer.

       Therefore,  PCRE2_DFA_RESTART  does not remove the need to keep earlier
       subject data. In order to perform a bumpalong and continue matching  at
       the  next character, all subject data from the current match start must
       be retained, together with enough preceding data  to  satisfy  the  re-
       quirements   of   lookbehind  assertions  (remembering  as  above  that
       PCRE2_INFO_MAXLOOKBEHIND only describes the  buffer  requirement  of  a
       single  lookbehind,  and  nested lookbehinds can read back further from
       the  match  start).  This  makes  streaming  match   processing   using
       PCRE2_DFA_RESTART  challenging, and the application cannot easily set a
       bound on the number of chunks to buffer.  For  multi-segment  searching
       that is intended to produce the same result as searching the whole sub-
       ject,  it is usually simpler to avoid PCRE2_DFA_RESTART and run a fresh
       match, using either matching function, over the retained text.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 16 August 2026
       Copyright (c) 1997-2019 University of Cambridge.


PCRE2 10.48                     16 August 2026                 PCRE2PARTIAL(3)
------------------------------------------------------------------------------


PCRE2PATTERN(3)            Library Functions Manual            PCRE2PATTERN(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 REGULAR EXPRESSION DETAILS

       The  syntax and semantics of the regular expressions that are supported
       by PCRE2 are described in detail below. There is a quick-reference syn-
       tax summary in the pcre2syntax page. PCRE2 tries to match  Perl  syntax
       and  semantics as closely as it can.  PCRE2 also supports some alterna-
       tive regular expression syntax that does not  conflict  with  the  Perl
       syntax  in order to provide some compatibility with regular expressions
       in Python, .NET, and Oniguruma. There are in addition some options that
       enable alternative syntax and semantics that are not  the  same  as  in
       Perl.

       Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of which have copious examples. Jeffrey Friedl's "Mastering Regular Ex-
       pressions",  published by O'Reilly, covers regular expressions in great
       detail. This description of PCRE2's regular expressions is intended  as
       reference material.

       This  document  discusses the regular expression patterns that are sup-
       ported by PCRE2 when its  main  matching  function,  pcre2_match(),  is
       used.    PCRE2    also    has   an   alternative   matching   function,
       pcre2_dfa_match(), which matches using a different  algorithm  that  is
       not  Perl-compatible.  Some  of  the  features  discussed below are not
       available when DFA matching is used. The advantages  and  disadvantages
       of  the  alternative function, and how it differs from the normal func-
       tion, are discussed in the pcre2matching page.


EBCDIC CHARACTER CODES

       Most computers use ASCII or Unicode for encoding characters, and  PCRE2
       assumes this by default. However, it can be compiled to run in an envi-
       ronment that uses the EBCDIC code, which is the case for some IBM main-
       frame  operating  systems. In the sections below, character code values
       are ASCII or Unicode; in an EBCDIC  environment  these  characters  may
       have  different  code values, and there are no code points greater than
       255. Differences in behaviour when PCRE2 is running in an EBCDIC  envi-
       ronment are described in the section "EBCDIC environments" below, which
       you can ignore unless you really are in an EBCDIC environment.


SPECIAL START-OF-PATTERN ITEMS

       A  number  of options that can be passed to pcre2_compile() can also be
       set by special items at the start of a pattern. These are not Perl-com-
       patible, but are provided to make these options accessible  to  pattern
       writers  who are not able to change the program that processes the pat-
       tern. Any number of these items may appear, but they must  all  be  to-
       gether  right  at the start of the pattern string, and the letters must
       be in upper case.

   UTF support

       In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either
       as single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32
       can be specified for the 32-bit library, in which  case  it  constrains
       the  character  values  to  valid  Unicode  code points. To process UTF
       strings, PCRE2 must be built to include Unicode support (which  is  the
       default).  When  using  UTF  strings you must either call the compiling
       function with one or both of the PCRE2_UTF  or  PCRE2_MATCH_INVALID_UTF
       options,  or  the  pattern must start with the special sequence (*UTF),
       which is equivalent to setting the relevant PCRE2_UTF.  How  setting  a
       UTF mode affects pattern matching is mentioned in several places below.
       There is also a summary of features in the pcre2unicode page.

       Some applications that allow their users to supply patterns may wish to
       restrict   them   to   non-UTF   data  for  security  reasons.  If  the
       PCRE2_NEVER_UTF option is passed to pcre2_compile(), (*UTF) is not  al-
       lowed, and its appearance in a pattern causes an error.

   Unicode property support

       Another  special  sequence that may appear at the start of a pattern is
       (*UCP).  This has the same effect as setting the PCRE2_UCP  option:  it
       causes  sequences such as \d and \w to use Unicode properties to deter-
       mine character types, instead of recognizing only characters with codes
       less than 256 via a lookup table. If also causes upper/lower casing op-
       erations to use Unicode properties  for  characters  with  code  points
       greater  than  127,  even when UTF is not set.  These behaviours can be
       changed within the pattern; see the section entitled  "Internal  Option
       Setting" below.

       Some applications that allow their users to supply patterns may wish to
       restrict  them  for  security reasons. If the PCRE2_NEVER_UCP option is
       passed to pcre2_compile(), (*UCP) is not allowed, and its appearance in
       a pattern causes an error.

   Locking out empty string matching

       Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same
       effect as passing the PCRE2_NOTEMPTY or  PCRE2_NOTEMPTY_ATSTART  option
       to whichever matching function is subsequently called to match the pat-
       tern.  These options lock out the matching of empty strings, either en-
       tirely, or only at the start of the subject.

   Disabling auto-possessification

       If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect  as
       setting  the  PCRE2_NO_AUTO_POSSESS  option, or calling pcre2_set_opti-
       mize() with a PCRE2_AUTO_POSSESS_OFF directive. This stops  PCRE2  from
       making  quantifiers  possessive  when what follows cannot match the re-
       peated item. For example, by default a+b is treated as a++b.  For  more
       details, see the pcre2api documentation.

   Disabling start-up optimizations

       If  a  pattern  starts  with (*NO_START_OPT), it has the same effect as
       setting the PCRE2_NO_START_OPTIMIZE option, or calling  pcre2_set_opti-
       mize() with a PCRE2_START_OPTIMIZE_OFF directive. This disables several
       optimizations  for  quickly  reaching  "no match" results. For more de-
       tails, see the pcre2api documentation.

   Disabling automatic anchoring

       If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the  same  effect
       as setting the PCRE2_NO_DOTSTAR_ANCHOR option, or calling pcre2_set_op-
       timize()  with a PCRE2_DOTSTAR_ANCHOR_OFF directive.  This disables op-
       timizations that apply to patterns whose top-level branches  all  start
       with  .*  (match any number of arbitrary characters). For more details,
       see the pcre2api documentation.

   Disabling JIT compilation

       If a pattern that starts with (*NO_JIT) is  successfully  compiled,  an
       attempt  by  the  application  to apply the JIT optimization by calling
       pcre2_jit_compile() is ignored.

   Setting match resource limits

       The pcre2_match() function contains a counter that is incremented every
       time it goes round its main loop. The caller of pcre2_match() can set a
       limit on this counter, which therefore limits the amount  of  computing
       resource used for a match. The maximum depth of nested backtracking can
       also  be  limited;  this indirectly restricts the amount of heap memory
       that is used, but there is also an explicit memory limit  that  can  be
       set.

       These  facilities  are  provided to catch runaway matches that are pro-
       voked by patterns with huge matching trees. A common example is a  pat-
       tern  with  nested unlimited repeats applied to a long string that does
       not match. When one of these limits is reached, pcre2_match() gives  an
       error  return.  The limits can also be set by items at the start of the
       pattern of the form

         (*LIMIT_HEAP=d)
         (*LIMIT_MATCH=d)
         (*LIMIT_DEPTH=d)

       where d is any number of decimal digits. However, the value of the set-
       ting must be less than the value set (or defaulted) by  the  caller  of
       pcre2_match()  for  it  to have any effect. In other words, the pattern
       writer can lower the limits set by the programmer, but not raise  them.
       If  there  is  more  than one setting of one of these limits, the lower
       value is used. The heap limit is specified in kibibytes (units of  1024
       bytes).

       Prior  to  release  10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This
       name is still recognized for backwards compatibility.

       The heap limit applies only when the pcre2_match() or pcre2_dfa_match()
       interpreters are used for matching. It does not apply to JIT. The match
       limit is used (but in a different way) when JIT is being used, or  when
       pcre2_dfa_match() is called, to limit computing resource usage by those
       matching  functions.  The depth limit is ignored by JIT but is relevant
       for DFA matching, which uses function recursion for  recursions  within
       the  pattern  and  for lookaround assertions and atomic groups. In this
       case, the depth limit controls the depth of such recursion.

   Newline conventions

       PCRE2 supports six different conventions for indicating line breaks  in
       strings:  a  single  CR (carriage return) character, a single LF (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, any Unicode newline sequence,  or  the  NUL  character  (binary
       zero).  The  pcre2api  page  has further discussion about newlines, and
       shows how to set the newline convention when calling pcre2_compile().

       It is also possible to specify a newline convention by starting a  pat-
       tern string with one of the following sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences
         (*NUL)       the NUL character (binary zero)

       These override the default and the options given to the compiling func-
       tion. For example, on a Unix system where LF is the default newline se-
       quence, the pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. If more than one of these settings is present, the
       last one is used.

       The  newline  convention affects where the circumflex and dollar asser-
       tions are true. It also affects the interpretation of the dot metachar-
       acter when PCRE2_DOTALL is not set, and the behaviour of  \N  when  not
       followed  by  an opening brace. However, it does not affect what the \R
       escape sequence matches. By default, this is any  Unicode  newline  se-
       quence,  for  Perl compatibility. However, this can be changed; see the
       next section and the description of \R in the section entitled "Newline
       sequences" below. A change of \R setting can be combined with a  change
       of newline convention.

   Specifying what \R matches

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the  complete  set  of  Unicode  line  endings)  by  setting the option
       PCRE2_BSR_ANYCRLF at compile time. This effect can also be achieved  by
       starting  a  pattern  with (*BSR_ANYCRLF). For completeness, (*BSR_UNI-
       CODE) is also recognized, corresponding to PCRE2_BSR_UNICODE.


CHARACTERS AND METACHARACTERS

       A regular expression is a pattern that is  matched  against  a  subject
       string  from  left  to right. Most characters stand for themselves in a
       pattern, and match the corresponding characters in the  subject.  As  a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless  matching  is  specified  (the  PCRE2_CASELESS  option or (?i)
       within the pattern), letters are matched independently  of  case.  Note
       that  there  are  two  ASCII  characters, K and S, that, in addition to
       their lower case ASCII equivalents, are  case-equivalent  with  Unicode
       U+212A  (Kelvin  sign)  and  U+017F  (long  S) respectively when either
       PCRE2_UTF or PCRE2_UCP is set, unless the PCRE2_EXTRA_CASELESS_RESTRICT
       option is in force (either passed to pcre2_compile() or set by  (*CASE-
       LESS_RESTRICT)  or  (?r)  within the pattern). If the PCRE2_EXTRA_TURK-
       ISH_CASING option is in force (either passed to pcre2_compile() or  set
       by  (*TURKISH_CASING)  within  the  pattern),  then the 'i' letters are
       matched according to Turkish and Azeri languages.

       The power of regular expressions comes from the ability to include wild
       cards, character classes, alternatives, and repetitions in the pattern.
       These are encoded in the pattern by the use of metacharacters, which do
       not stand for themselves but instead are interpreted  in  some  special
       way.

       There  are  two different sets of metacharacters: those that are recog-
       nized anywhere in the pattern except within square brackets, and  those
       that  are  recognized  within square brackets. Outside square brackets,
       the metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start group or control verb
         )      end group or control verb
         *      0 or more quantifier
         +      1 or more quantifier; also "possessive quantifier"
         ?      0 or 1 quantifier; also quantifier minimizer
         {      potential start of min/max quantifier

       Brace characters { and } are also used to enclose  data  for  construc-
       tions  such  as  \g{2} or \k{name}. In almost all uses of braces, space
       and/or horizontal tab characters that follow { or precede } are allowed
       and are ignored. In the case of quantifiers, they may also  appear  be-
       fore  or  after the comma. The exception to this is \u{...} which is an
       ECMAScript compatibility feature  that  is  recognized  only  when  the
       PCRE2_EXTRA_ALT_BSUX  option  is  set.  ECMAScript does not ignore such
       white space; it causes the item to be interpreted as literal.

       Part of a pattern that is in square brackets  is  called  a  "character
       class". In a character class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (if followed by POSIX syntax)
         ]      terminates the character class

       If  a  pattern  is  compiled with the PCRE2_EXTENDED option, most white
       space in the pattern, other than in a character class, within a \Q...\E
       sequence, or between a # outside a character class and  the  next  new-
       line,  inclusive,  is ignored. An escaping backslash can be used to in-
       clude a white space or a # character as part of  the  pattern.  If  the
       PCRE2_EXTENDED_MORE  option  is  set, the same applies, but in addition
       unescaped space and horizontal tab  characters  are  ignored  inside  a
       character  class.  Note: only these two characters are ignored, not the
       full set of pattern white space characters that are ignored  outside  a
       character  class.  Option settings can be changed within a pattern; see
       the section entitled "Internal Option Setting" below.

       The following sections describe the use of each of the metacharacters.


BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a character that is not a digit or a letter, it takes away any  special
       meaning  that  character  may  have. This use of backslash as an escape
       character applies both inside and outside character classes.

       For example, if you want to match a * character, you must write  \*  in
       the  pattern. This escaping action applies whether or not the following
       character would otherwise be interpreted as a metacharacter, so  it  is
       always  safe  to  precede  a non-alphanumeric with backslash to specify
       that it stands for itself.  In particular, if you want to match a back-
       slash, you write \\.

       Only ASCII digits and letters have any special meaning  after  a  back-
       slash. All other characters (in particular, those whose code points are
       greater than 127) are treated as literals.

       If  you want to treat all characters in a sequence as literals, you can
       do so by putting them between \Q and \E. Note that this includes  white
       space  even  when  the  PCRE2_EXTENDED option is set so that most other
       white space is ignored. The behaviour is different from Perl in that  $
       and @ are handled as literals in \Q...\E sequences in PCRE2, whereas in
       Perl,  $  and  @ cause variable interpolation. Also, Perl does "double-
       quotish backslash interpolation" on any backslashes between \Q  and  \E
       which,  its  documentation says, "may lead to confusing results". PCRE2
       treats a backslash between \Q and \E just  like  any  other  character.
       Note the following examples:

         Pattern            PCRE2 matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
         \QA\B\E            A\B            A\B
         \Q\\E              \              \\E

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.  An isolated \E that is not preceded by \Q is ignored.  If  \Q
       is  not followed by \E later in the pattern, the literal interpretation
       continues to the end of the pattern (that is,  \E  is  assumed  at  the
       end).  If  the  isolated \Q is inside a character class, this causes an
       error, because the character class is then not terminated by a  closing
       square bracket.

       Another  difference from Perl is that any appearance of \Q or \E inside
       what might otherwise be a quantifier causes PCRE2 not to recognize  the
       sequence as a quantifier. Perl recognizes a quantifier if (redundantly)
       either  of  the  numbers  is  inside \Q...\E, but not if the separating
       comma is. When not recognized  as  a  quantifier  a  sequence  such  as
       {\Q1\E,2} is treated as the literal string "{1,2}".

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters  in patterns in a visible manner. There is no restriction on the
       appearance of non-printing characters in a pattern, but when a  pattern
       is being prepared by text editing, it is often easier to use one of the
       following  escape  sequences  instead of the binary character it repre-
       sents. In an ASCII or Unicode environment, these escapes  are  as  fol-
       lows:

         \a          alarm, that is, the BEL character (hex 07)
         \cx         "control-x", where x is a non-control ASCII character
         \e          escape (hex 1B)
         \f          form feed (hex 0C)
         \n          linefeed (hex 0A)
         \r          carriage return (hex 0D) (but see below)
         \t          tab (hex 09)
         \0dd        character with octal code 0dd
         \ddd        character with octal code ddd, or back reference
         \o{ddd..}   character with octal code ddd..
         \xhh        character with hex code hh
         \x{hhh..}   character with hex code hhh..
         \N{U+hhh..} character with Unicode hex code point hhh..

       A description of how back references work is given later, following the
       discussion of parenthesized groups.

       By  default, after \x that is not followed by {, one or two hexadecimal
       digits are read (letters can be in upper or lower case). If the charac-
       ter that follows \x is neither { nor a hexadecimal digit, an error  oc-
       curs.  This is different from Perl's default behaviour, which generates
       a NUL character, but is in line with the behaviour of  Perl's  'strict'
       mode in re.

       Any  number  of  hexadecimal  digits may appear between \x{ and }. If a
       character other than a hexadecimal digit appears between \x{ and },  or
       if there is no terminating }, an error occurs.

       Characters whose code points are less than 256 can be defined by either
       of the two syntaxes for \x or by an octal sequence. There is no differ-
       ence in the way they are handled. For example, \xdc is exactly the same
       as  \x{dc}  or \334.  However, using the braced versions does make such
       sequences easier to read.

       Support is available for some ECMAScript (aka  JavaScript)  escape  se-
       quences via two compile-time options. If PCRE2_ALT_BSUX is set, the se-
       quence  \x  followed  by { is not recognized. Only if \x is followed by
       two hexadecimal digits is it recognized as a character  escape.  Other-
       wise  it  is interpreted as a literal "x" character. In this mode, sup-
       port for code points greater than 256 is provided by \u, which must  be
       followed  by  four hexadecimal digits; otherwise it is interpreted as a
       literal "u" character.

       PCRE2_EXTRA_ALT_BSUX has the same effect as PCRE2_ALT_BSUX and, in  ad-
       dition, \u{hhh..} is recognized as the character specified by hexadeci-
       mal code point.  There may be any number of hexadecimal digits, but un-
       like  other places that also use curly brackets, spaces are not allowed
       and would result in the string being interpreted  as  a  literal.  This
       syntax is from ECMAScript 6.

       The  \N{U+hhh..} escape sequence is recognized only when PCRE2 is oper-
       ating in UTF mode. Perl also uses \N{name}  to  specify  characters  by
       Unicode  name;  PCRE2  does  not support this. Note that when \N is not
       followed by an opening brace (curly bracket) it has an entirely differ-
       ent meaning, matching any character that is not a newline.

       There are some legacy applications where the escape sequence \r is  ex-
       pected  to  match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option
       is set, \r in a pattern is converted to \n so  that  it  matches  a  LF
       (linefeed) instead of a CR (carriage return) character.

       An  error  occurs if \c is not followed by a character whose ASCII code
       point is in the range 32 to 126. The precise effect of \cx is  as  fol-
       lows:  if x is a lower case letter, it is converted to upper case. Then
       bit 6 of the character (hex 40) is inverted. Thus \cA to \cZ become hex
       01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B),  and
       \c;  becomes hex 7B (; is 3B). If the code unit following \c has a code
       point less than 32 or greater than 126, a compile-time error occurs.

       For differences in the way some escapes behave in EBCDIC  environments,
       see section "EBCDIC environments" below.

   Octal escapes and back references

       The  escape \o must be followed by a sequence of octal digits, enclosed
       in braces. An error occurs if this is not the case.  This  escape  pro-
       vides  a  way  of  specifying  character  code  points as octal numbers
       greater than 0777, and it also allows octal numbers and  backreferences
       to be unambiguously distinguished.

       If  braces  are  not  used, after \0 up to two further octal digits are
       read.  However, if the PCRE2_EXTRA_NO_BS0 option is set, at  least  one
       more  octal digit must follow \0 (use \00 to generate a NUL character).
       Make sure you supply two digits after the initial zero if  the  pattern
       character that follows is itself an octal digit.

       Inside  a  character  class,  when a backslash is followed by any octal
       digit, up to three octal digits are read to generate a code point.  Any
       subsequent  digits  stand  for  themselves. The sequences \8 and \9 are
       treated as the literal characters "8" and "9".

       Outside a character class, Perl's handling of a backslash followed by a
       digit other than 0 is complicated by ambiguity, and  Perl  has  changed
       over time, causing PCRE2 also to change. From PCRE2 release 10.45 there
       is  an  option called PCRE2_EXTRA_PYTHON_OCTAL that causes PCRE2 to use
       Python's unambiguous rules. The next two subsections describe  the  two
       sets of rules.

       For greater clarity and unambiguity, it is best to avoid following \ by
       a  digit  greater than zero. Instead, use \o{...} or \x{...} to specify
       numerical character code points, and \g{...} to specify backreferences.

   Perl rules for non-class backslash 1-9

       All the digits that follow the backslash are read as a decimal  number.
       If  the  number  is  less  than 10, begins with the digit 8 or 9, or if
       there are at least that many previous capture groups in the expression,
       the entire sequence is taken as a  back  reference.  Otherwise,  up  to
       three octal digits are read to form a character code. For example:

         \040   is another way of writing an ASCII space
         \40    is the same, provided there are fewer than 40
                   previous capture groups
         \7     is always a backreference
         \11    might be a backreference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a backreference, otherwise the
                   character with octal code 113
         \377   might be a backreference, otherwise
                   the value 255 (decimal)
         \81    is always a backreference

       Note  that octal values of 100 or greater that are specified using this
       syntax must not be introduced by a leading zero, because no  more  than
       three octal digits are ever read.

   Python rules for non_class backslash 1-9

       If  there  are at least three octal digits after the backslash, exactly
       three are read as an octal code point number, but the value must be  no
       greater  than  \377,  even  in modes where higher code point values are
       supported. Any subsequent digits stand for  themselves.  If  there  are
       fewer  than three octal digits, the sequence is taken as a decimal back
       reference. Thus, for example, \12 is always a back reference,  indepen-
       dent  of how many captures there are in the pattern. An error is gener-
       ated for a reference to a non-existent capturing group.

   Constraints on character values

       Characters that are specified using octal or  hexadecimal  numbers  are
       limited to certain values, as follows:

         8-bit non-UTF mode    no greater than 0xff
         16-bit non-UTF mode   no greater than 0xffff
         32-bit non-UTF mode   no greater than 0xffffffff
         All UTF modes         no greater than 0x10ffff and a valid code point

       Invalid Unicode code points are all those in the range 0xd800 to 0xdfff
       (the  so-called  "surrogate"  code  points). The check for these can be
       disabled by  the  caller  of  pcre2_compile()  by  setting  the  option
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES.  However, this is possible only in
       UTF-8 and UTF-32 modes, because these values are not  representable  in
       UTF-16.

   Escape sequences in character classes

       All the sequences that define a single character value can be used both
       inside  and  outside character classes. In addition, inside a character
       class, \b is interpreted as the backspace character (hex 08).

       When not followed by an opening brace, \N is not allowed in a character
       class.  \B, \R, and \X are not special inside a character  class.  Like
       other  unrecognized  alphabetic  escape sequences, they cause an error.
       Outside a character class, these sequences have different meanings.

   Unsupported escape sequences

       In Perl, the sequences \F, \l, \L, \u, and \U  are  recognized  by  its
       string  handler and used to modify the case of following characters. By
       default, PCRE2 does not support these  escape  sequences  in  patterns.
       However,  if  either  of the PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX op-
       tions is set, \U matches a "U" character, and \u can be used to  define
       a character by code point, as described above.

   Absolute and relative backreferences

       The sequence \g followed by a signed or unsigned number, optionally en-
       closed  in  braces,  is  an absolute or relative backreference. A named
       backreference can be coded as \g{name}.  Backreferences  are  discussed
       later, following the discussion of parenthesized groups.

   Absolute and relative subroutine calls

       For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a capture group as a  subroutine.
       Details  are  discussed  later.   Note  that  \g{...} (Perl syntax) and
       \g<...> (Oniguruma syntax) are not synonymous. The former is a backref-
       erence; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal white space character
         \H     any character that is not a horizontal white space character
         \N     any character that is not a newline
         \s     any white space character
         \S     any character that is not a white space character
         \v     any vertical white space character
         \V     any character that is not a vertical white space character
         \w     any "word" character
         \W     any "non-word" character

       The \N escape sequence has the same meaning as  the  "."  metacharacter
       when  PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change
       the meaning of \N. Note that when \N is followed by an opening brace it
       has a different meaning. See the section entitled "Non-printing charac-
       ters" above for details. Perl also uses \N{name} to specify  characters
       by Unicode name; PCRE2 does not support this.

       Each  pair of lower and upper case escape sequences partitions the com-
       plete set of characters into two disjoint  sets.  Any  given  character
       matches  one, and only one, of each pair. The sequences can appear both
       inside and outside character classes. They each match one character  of
       the  appropriate  type.  If the current matching point is at the end of
       the subject string, all of them fail, because there is no character  to
       match.

       The  default  \s  characters  are HT (9), LF (10), VT (11), FF (12), CR
       (13), and space (32), which are defined as white space in the  "C"  lo-
       cale.  This  list may vary if locale-specific matching is taking place.
       For example, in some locales the "non-breaking space" character  (\xA0)
       is recognized as white space, and in others the VT character is not.

       A  "word"  character is an underscore or any character that is a letter
       or digit.  By default, the definition of letters  and  digits  is  con-
       trolled by PCRE2's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the pcre2api
       page).  For  example,  in  a French locale such as "fr_FR" in Unix-like
       systems, or "french" in Windows, some character codes greater than  127
       are  used  for  accented letters, and these are then matched by \w. The
       use of locales with Unicode is discouraged.

       By default, characters whose code points are  greater  than  127  never
       match \d, \s, or \w, and always match \D, \S, and \W, although this may
       be  different  for characters in the range 128-255 when locale-specific
       matching is happening.  These escape sequences  retain  their  original
       meanings  from  before  Unicode support was available, mainly for effi-
       ciency reasons. If the  PCRE2_UCP  option  is  set,  the  behaviour  is
       changed  so  that  Unicode  properties  are used to determine character
       types, as follows:

         \d  any character that matches \p{Nd} (decimal digit)
         \s  any character that matches \p{Z} or \h or \v
         \w  any character that matches \p{L}, \p{N}, \p{Mn}, or \p{Pc}

       The addition of \p{Mn} (non-spacing mark) and the replacement of an ex-
       plicit test for underscore with a test for \p{Pc}  (connector  punctua-
       tion) happened in PCRE2 release 10.43. This brings PCRE2 into line with
       Perl.

       The  upper case escapes match the inverse sets of characters. Note that
       \d matches only decimal digits, whereas \w matches any  Unicode  digit,
       as well as other character categories. Note also that PCRE2_UCP affects
       \b,  and  \B  because  they are defined in terms of \w and \W. Matching
       these sequences is noticeably slower when PCRE2_UCP is set.

       The effect of PCRE2_UCP on any one of these  escape  sequences  can  be
       negated  by  the  options PCRE2_EXTRA_ASCII_BSD, PCRE2_EXTRA_ASCII_BSS,
       and PCRE2_EXTRA_ASCII_BSW, respectively. These options can be  set  and
       reset  within a pattern by means of an internal option setting (see be-
       low).

       The sequences \h, \H, \v, and \V, in contrast to the  other  sequences,
       which  match  only ASCII characters by default, always match a specific
       list of code points, whether or not PCRE2_UCP is  set.  The  horizontal
       space characters are:

         U+0009     Horizontal tab (HT)
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed (LF)
         U+000B     Vertical tab (VT)
         U+000C     Form feed (FF)
         U+000D     Carriage return (CR)
         U+0085     Next line (NEL)
         U+2028     Line separator
         U+2029     Paragraph separator

       In  8-bit,  non-UTF-8  mode,  only the characters with code points less
       than 256 are relevant.

   Newline sequences

       Outside a character class, by default, the escape sequence  \R  matches
       any  Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
       to the following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This is an example of an "atomic group", details of which are given be-
       low.  This particular group matches either the  two-character  sequence
       CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (form feed,  U+000C),  CR  (car-
       riage  return,  U+000D), or NEL (next line, U+0085). Because this is an
       atomic group, the two-character sequence is treated as  a  single  unit
       that cannot be split.

       In other modes, two additional characters whose code points are greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator,  U+2029).  Unicode support is not needed for these characters to
       be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE2_BSR_ANYCRLF  at  compile time. (BSR is an abbreviation for "back-
       slash R".) This can be made the default when PCRE2 is built; if this is
       the case, the other behaviour can be requested via  the  PCRE2_BSR_UNI-
       CODE  option. It is also possible to specify these settings by starting
       a pattern string with one of the following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to the compiling func-
       tion.  Note that these special settings, which are not Perl-compatible,
       are recognized only at the very start of a pattern, and that they  must
       be  in upper case. If more than one of them is present, the last one is
       used. They can be combined with a change of newline convention; for ex-
       ample, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF) or (*UCP) special  sequences.
       Inside  a  character class, \R is treated as an unrecognized escape se-
       quence, and causes an error.

   Unicode character properties

       When PCRE2 is built with Unicode support  (the  default),  three  addi-
       tional  escape sequences that match characters with specific properties
       are available. They can be used in any mode, though in 8-bit and 16-bit
       non-UTF modes these sequences are of course limited to testing  charac-
       ters  whose  code points are less than U+0100 or U+10000, respectively.
       In 32-bit non-UTF mode, code points greater than 0x10ffff (the  Unicode
       limit)  may  be  encountered. These are all treated as being in the Un-
       known script and with an unassigned type.

       Matching characters by Unicode property is not fast, because PCRE2  has
       to  do  a  multistage table lookup in order to find a character's prop-
       erty. That is why the traditional escape sequences such as \d and \w do
       not use Unicode properties in PCRE2 by default,  though  you  can  make
       them  do  so by setting the PCRE2_UCP option or by starting the pattern
       with (*UCP).

       The extra escape sequences that provide property support are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       a Unicode extended grapheme cluster

       For compatibility with Perl, negation can be specified by  including  a
       circumflex  between  the  opening  brace and the property. For example,
       \p{^Lu} is the same as \P{Lu}.

       In accordance with Unicode's "loose matching" rules, ASCII white  space
       characters, hyphens, and underscores are ignored in the properties rep-
       resented by xx above. As well as the space character, ASCII white space
       can be tab, linefeed, vertical tab, formfeed, or carriage return.

       Some  properties  are  specified as a name only; others as a name and a
       value, separated by a colon or an equals sign.  The  names  and  values
       consist  of ASCII letters and digits (with one Perl-specific exception,
       see below). They are not case sensitive. Note, however,  that  the  es-
       capes  themselves,  \p  and \P, are case sensitive. There are abbrevia-
       tions for many names. The following examples are all equivalent:

         \p{bidiclass=al}
         \p{BC=al}
         \p{ Bidi_Class : AL }
         \p{ Bi-di class = Al }
         \P{ ^ Bi-di class = Al }

       There is support for Unicode script  names,  Unicode  general  category
       properties,  "Any",  which  matches  any character (including newline),
       Bidi_Class, a number of binary (yes/no) properties,  and  some  special
       PCRE2 properties (described below).  Certain other Perl properties such
       as  "InMusicalSymbols"  are  not  supported by PCRE2. Note that \P{Any}
       does not match any characters, so always causes a match failure.

   Script properties for \p and \P

       There are three different syntax forms for matching a script. Each Uni-
       code character has a basic script and,  optionally,  a  list  of  other
       scripts ("Script Extensions") with which it is commonly used. Using the
       Adlam script as an example, \p{sc:Adlam} matches characters whose basic
       script is Adlam, whereas \p{scx:Adlam} matches, in addition, characters
       that  have  Adlam in their extensions list. The full names "script" and
       "script extensions" for the property types are recognized and,  as  for
       all  property  specifications,  an equals sign is an alternative to the
       colon. If a script name is given without a property type, for  example,
       \p{Adlam},  it is treated as \p{scx:Adlam}. Perl changed to this inter-
       pretation at release 5.26 and PCRE2 changed at release 10.40.

       Unassigned characters (and in non-UTF 32-bit mode, characters with code
       points greater than 0x10FFFF) are assigned the "Unknown" script. Others
       that are not part of an identified script are lumped together as  "Com-
       mon". The current list of recognized script names and their 4-character
       abbreviations can be obtained by running this command:

         pcre2test -LS


   The general category property for \p and \P

       Each character has exactly one Unicode general category property, spec-
       ified  by  a  two-letter  abbreviation. If only one letter is specified
       with \p or \P, it includes all the  general  category  properties  that
       start  with  that letter. In this case, in the absence of negation, the
       curly brackets in the escape sequence are optional; these two  examples
       have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Lc    Cased letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       Perl  originally  used  the  name L& for the Lc property. This is still
       supported by Perl, but discouraged. PCRE2 also still supports it.  This
       property  matches any character that has the Lu, Ll, or Lt property, in
       other words, any letter  that  is  not  classified  as  a  modifier  or
       "other".  From release 10.45 of PCRE2 the properties Lu, Ll, and Lt are
       all treated  as  Lc  when  case-independent  matching  is  set  by  the
       PCRE2_CASELESS  option or (?i) within the pattern. The other properties
       are not affected by caseless matching.

       The Cs (Surrogate) property  applies  only  to  characters  whose  code
       points  are in the range U+D800 to U+DFFF. These characters are no dif-
       ferent to any other character when PCRE2 is not in UTF mode (using  the
       16-bit  or  32-bit  library).   However,  they are not valid in Unicode
       strings and so cannot be tested by PCRE2 in UTF mode, unless UTF valid-
       ity  checking  has   been   turned   off   (see   the   discussion   of
       PCRE2_NO_UTF_CHECK in the pcre2api page).

       The  long  synonyms  for  property  names  that  Perl supports (such as
       \p{Letter}) are not supported by PCRE2, nor is it permitted  to  prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

   Binary (yes/no) properties for \p and \P

       Unicode  defines  a  number  of  binary properties, that is, properties
       whose only values are true or false. You can obtain  a  list  of  those
       that  are  recognized  by \p and \P, along with their abbreviations, by
       running this command:

         pcre2test -LP


   The Bidi_Class property for \p and \P

         \p{Bidi_Class:<class>}   matches a character with the given class
         \p{BC:<class>}           matches a character with the given class

       The recognized classes are:

         AL          Arabic letter
         AN          Arabic number
         B           paragraph separator
         BN          boundary neutral
         CS          common separator
         EN          European number
         ES          European separator
         ET          European terminator
         FSI         first strong isolate
         L           left-to-right
         LRE         left-to-right embedding
         LRI         left-to-right isolate
         LRO         left-to-right override
         NSM         non-spacing mark
         ON          other neutral
         PDF         pop directional format
         PDI         pop directional isolate
         R           right-to-left
         RLE         right-to-left embedding
         RLI         right-to-left isolate
         RLO         right-to-left override
         S           segment separator
         WS          white space

       As in all property specifications, an equals sign may be  used  instead
       of  a  colon  and  the class names are case-insensitive. Only the short
       names listed above are recognized; PCRE2 does not  at  present  support
       any long alternatives.

   Extended grapheme clusters

       The  \X  escape  matches  any number of Unicode characters that form an
       "extended grapheme cluster", and treats the sequence as an atomic group
       (see below).  Unicode supports various kinds of composite character  by
       giving  each  character  a grapheme breaking property, and having rules
       that use these properties to define the boundaries of extended grapheme
       clusters. The rules are defined in Unicode Standard Annex 29,  "Unicode
       Text  Segmentation".  Unicode 11.0.0 abandoned the use of some previous
       properties that had been used for emojis.  Instead it introduced  vari-
       ous  emoji-specific  properties.  PCRE2  uses  only the Extended Picto-
       graphic property.

       \X always matches at least one character. Then it  decides  whether  to
       add additional characters according to the following rules for ending a
       cluster:

       1. End at the end of the subject string.

       2.  Do not end between CR and LF; otherwise end after any control char-
       acter.

       3. Do not break Hangul (a Korean  script)  syllable  sequences.  Hangul
       characters  are of five types: L, V, T, LV, and LVT. An L character may
       be followed by an L, V, LV, or LVT character; an LV or V character  may
       be  followed  by  a V or T character; an LVT or T character may be fol-
       lowed only by a T character.

       4. Do not end before extending characters or spacing marks or the zero-
       width joiner (ZWJ) character. Characters with the "mark"  property  al-
       ways have the "extend" grapheme breaking property.

       5. Do not end after prepend characters.

       6.  Do not end within emoji modifier sequences or emoji ZWJ (zero-width
       joiner) sequences. An emoji ZWJ sequence consists of a  character  with
       the  Extended_Pictographic property, optionally followed by one or more
       characters with the Extend property, followed  by  the  ZWJ  character,
       followed by another Extended_Pictographic character.

       7.  Do not break within emoji flag sequences. That is, do not break be-
       tween regional indicator (RI) characters if there are an odd number  of
       RI characters before the break point.

       8. Otherwise, end the cluster.

   PCRE2's additional properties

       As  well as the standard Unicode properties described above, PCRE2 sup-
       ports four more that make it possible to convert traditional escape se-
       quences such as \w and \s to use Unicode properties. PCRE2  uses  these
       non-standard,  non-Perl  properties  internally  when PCRE2_UCP is set.
       However, they may also be used explicitly. These properties are:

         Xan   Any alphanumeric character
         Xps   Any POSIX space character
         Xsp   Any Perl space character
         Xwd   Any Perl "word" character

       Xan matches characters that have either the L (letter) or the  N  (num-
       ber)  property. Xps matches the characters tab, linefeed, vertical tab,
       form feed, or carriage return, and any other character that has  the  Z
       (separator)  property  (this  includes the space character). Xsp is the
       same as Xps; in PCRE1 it used to exclude vertical tab, for Perl compat-
       ibility, but Perl changed. Xwd matches the same characters as Xan, plus
       those that match Mn (non-spacing mark) or  Pc  (connector  punctuation,
       which includes underscore).

       There  is another non-standard property, Xuc, which matches any charac-
       ter that can be represented by a Universal Character Name  in  C++  and
       other  programming  languages.  These are the characters $, @, ` (grave
       accent), and all characters with Unicode code points  greater  than  or
       equal  to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that
       most base (ASCII) characters are excluded. (Universal  Character  Names
       are  of  the  form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit.
       Note that the Xuc property does not match these sequences but the char-
       acters that they represent.)

   Resetting the match start

       In normal use, the escape sequence \K  causes  any  previously  matched
       characters not to be included in the final matched sequence that is re-
       turned. For example, the pattern:

         foo\Kbar

       matches  "foobar",  but  reports that it has matched "bar". \K does not
       interact with anchoring in any way. The pattern:

         ^foo\Kbar

       matches only when the subject begins  with  "foobar"  (in  single  line
       mode),  though  it again reports the matched string as "bar". This fea-
       ture is similar to a lookbehind assertion (described  below),  but  the
       part of the pattern that precedes \K is not constrained to match a lim-
       ited  number  of characters, as is required for a lookbehind assertion.
       The use of \K does not interfere with  the  setting  of  captured  sub-
       strings.  For example, when the pattern

         (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       From  version  5.32.0  Perl  forbids the use of \K in lookaround asser-
       tions. From release 10.38 PCRE2 also forbids this by default.  However,
       the  PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK  option  can be used when calling
       pcre2_compile() to re-enable the previous behaviour. When  this  option
       is set, \K is acted upon when it occurs inside positive assertions, but
       is  ignored  in  negative  assertions. Note that when a pattern such as
       (?=ab\K) matches, the reported start of the match can be  greater  than
       the  end  of the match. Using \K in a lookbehind assertion at the start
       of a pattern can also lead to odd effects. For example,  consider  this
       pattern:

         (?<=\Kfoo)bar

       If  the  subject  is  "foobar", a call to pcre2_match() with a starting
       offset of 3 succeeds and reports the matching string as "foobar",  that
       is,  the  start  of  the reported match is earlier than where the match
       started.

   Simple assertions

       The final use of backslash is for certain simple assertions. An  asser-
       tion  specifies a condition that has to be met at a particular point in
       a match, without consuming any characters from the subject string.  The
       use  of groups for more complicated assertions is described below.  The
       backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

       Inside a character class, \b has a different meaning;  it  matches  the
       backspace  character.  If  any  other  of these assertions appears in a
       character class, an "invalid escape sequence" error is generated.

       A word boundary is a position in the subject string where  the  current
       character  and  the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or  end  of  the
       string  if  the  first or last character matches \w, respectively. When
       PCRE2 is built with Unicode support, the meanings of \w and \W  can  be
       changed by setting the PCRE2_UCP option. When this is done, it also af-
       fects  \b and \B. Neither PCRE2 nor Perl has a separate "start of word"
       or "end of word" metasequence. However, whatever  follows  \b  normally
       determines  which  it  is. For example, the fragment \ba matches "a" at
       the start of a word.

       The \A, \Z, and \z assertions differ from  the  traditional  circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions  are  not  affected  by the PCRE2_NOTBOL or PCRE2_NOTEOL options,
       which affect only the behaviour of the circumflex and dollar  metachar-
       acters.  However,  if the startoffset argument of pcre2_match() is non-
       zero, indicating that matching is to start at a point  other  than  the
       beginning  of  the subject, \A can never match.  The difference between
       \Z and \z is that \Z matches before a newline at the end of the  string
       as well as at the very end, whereas \z matches only at the end.

       The  \G assertion is true only when the current matching position is at
       the start point of the matching process, as specified by the  startoff-
       set  argument  of  pcre2_match().  It differs from \A when the value of
       startoffset is non-zero. By calling pcre2_match() multiple  times  with
       appropriate  arguments,  you  can  mimic Perl's /g option, and it is in
       this kind of implementation where \G can be useful.

       Note, however, that PCRE2's implementation of \G,  being  true  at  the
       starting  character  of  the matching process, is subtly different from
       Perl's, which defines it as true at the end of the previous  match.  In
       Perl,  these  can  be  different when the previously matched string was
       empty. Because PCRE2 does just one match at a time, it cannot reproduce
       this behaviour.

       If all the alternatives of a pattern begin with \G, the  expression  is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       The  circumflex  and  dollar  metacharacters are zero-width assertions.
       That is, they test for a particular condition being true  without  con-
       suming any characters from the subject string. These two metacharacters
       are  concerned  with matching the starts and ends of lines. If the new-
       line convention is set so that only the two-character sequence CRLF  is
       recognized  as  a newline, isolated CR and LF characters are treated as
       ordinary data characters, and are not recognized as newlines.

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only  if  the  current  matching
       point  is  at the start of the subject string. If the startoffset argu-
       ment of pcre2_match() is non-zero, or if PCRE2_NOTBOL is  set,  circum-
       flex  can  never match if the PCRE2_MULTILINE option is unset. Inside a
       character class, circumflex has an entirely different meaning (see  be-
       low).

       Circumflex  need  not be the first character of the pattern if a number
       of alternatives are involved, but it should be the first thing in  each
       alternative  in  which  it appears if the pattern is ever to match that
       branch. If all possible alternatives start with a circumflex, that  is,
       if  the  pattern  is constrained to match only at the start of the sub-
       ject, it is said to be an "anchored" pattern.  (There  are  also  other
       constructs that can cause a pattern to be anchored.)

       The  dollar  character is an assertion that is true only if the current
       matching point is at the end of the subject string, or immediately  be-
       fore  a newline at the end of the string (by default), unless PCRE2_NO-
       TEOL is set. Note, however, that it does not actually  match  the  new-
       line.  Dollar need not be the last character of the pattern if a number
       of alternatives are involved, but it should be the  last  item  in  any
       branch  in which it appears. Dollar has no special meaning in a charac-
       ter class.

       The meaning of dollar can be changed so that it  matches  only  at  the
       very  end  of the string, by setting the PCRE2_DOLLAR_ENDONLY option at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar metacharacters are changed if
       the PCRE2_MULTILINE option is set. When this  is  the  case,  a  dollar
       character  matches before any newlines in the string, as well as at the
       very end, and a circumflex matches immediately after internal  newlines
       as  well as at the start of the subject string. It does not match after
       a newline that ends the string, for compatibility with  Perl.  However,
       this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option.

       For  example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) in multiline mode, but  not  otherwise.
       Consequently,  patterns  that  are anchored in single line mode because
       all branches start with ^ are not anchored in  multiline  mode,  and  a
       match  for  circumflex  is  possible  when  the startoffset argument of
       pcre2_match() is non-zero. The PCRE2_DOLLAR_ENDONLY option  is  ignored
       if PCRE2_MULTILINE is set.

       When  the  newline  convention (see "Newline conventions" below) recog-
       nizes the two-character sequence CRLF as a newline, this is  preferred,
       even  if  the  single  characters CR and LF are also recognized as new-
       lines. For example, if the newline convention  is  "any",  a  multiline
       mode  circumflex matches before "xyz" in the string "abc\r\nxyz" rather
       than after CR, even though CR on its own is a valid newline.  (It  also
       matches at the very start of the string, of course.)

       Note  that  the sequences \A, \Z, and \z can be used to match the start
       and end of the subject in both modes, and if all branches of a  pattern
       start  with \A it is always anchored, whether or not PCRE2_MULTILINE is
       set.


FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one charac-
       ter in the subject string except (by default) a character  that  signi-
       fies the end of a line. One or more characters may be specified as line
       terminators (see "Newline conventions" above).

       Dot  never matches a single line-ending character. When the two-charac-
       ter sequence CRLF is the only line ending, dot does not match CR if  it
       is  immediately followed by LF, but otherwise it matches all characters
       (including isolated CRs and LFs). When ANYCRLF  is  selected  for  line
       endings,  no  occurrences  of CR of LF match dot. When all Unicode line
       endings are being recognized, dot does not match CR or LF or any of the
       other line ending characters.

       The behaviour of dot with regard to newlines can  be  changed.  If  the
       PCRE2_DOTALL  option  is  set, a dot matches any one character, without
       exception.  If the two-character sequence CRLF is present in  the  sub-
       ject string, it takes two dots to match it.

       The  handling of dot is entirely independent of the handling of circum-
       flex and dollar, the only relationship being  that  they  both  involve
       newlines. Dot has no special meaning in a character class.

       The  escape  sequence  \N when not followed by an opening brace behaves
       like a dot, except that it is not affected by the PCRE2_DOTALL  option.
       In  other words, it matches any character except one that signifies the
       end of a line.

       When \N is followed by an opening brace it has a different meaning. See
       the section entitled "Non-printing characters" above for details.  Perl
       also  uses  \N{name}  to specify characters by Unicode name; PCRE2 does
       not support this.


MATCHING A SINGLE CODE UNIT

       Outside a character class, the escape sequence \C matches any one  code
       unit,  whether or not a UTF mode is set. In the 8-bit library, one code
       unit is one byte; in the 16-bit library it is a  16-bit  unit;  in  the
       32-bit  library  it  is  a 32-bit unit. Unlike a dot, \C always matches
       line-ending characters. The feature is provided in  Perl  in  order  to
       match individual bytes in UTF-8 mode, but it is unclear how it can use-
       fully be used.

       Because  \C  breaks  up characters into individual code units, matching
       one unit with \C in UTF-8 or UTF-16 mode means that  the  rest  of  the
       string may start with a malformed UTF character. This has undefined re-
       sults, because PCRE2 assumes that it is matching character by character
       in a valid UTF string (by default it checks the subject string's valid-
       ity  at  the  start  of  processing  unless  the  PCRE2_NO_UTF_CHECK or
       PCRE2_MATCH_INVALID_UTF option is used).

       An  application  can  lock  out  the  use  of   \C   by   setting   the
       PCRE2_NEVER_BACKSLASH_C  option  when  compiling  a pattern. It is also
       possible to build PCRE2 with the use of \C permanently disabled.

       PCRE2 does not allow \C to appear in lookbehind  assertions  (described
       below)  in UTF-8 or UTF-16 modes, because this would make it impossible
       to calculate the length of  the  lookbehind.  Neither  the  alternative
       matching function pcre2_dfa_match() nor the JIT optimizer support \C in
       these UTF modes.  The former gives a match-time error; the latter fails
       to optimize and so the match is always run using the interpreter.

       In  the  32-bit  library, however, \C is always supported (when not ex-
       plicitly locked out) because it always  matches  a  single  code  unit,
       whether or not UTF-32 is specified.

       In general, the \C escape sequence is best avoided. However, one way of
       using  it  that avoids the problem of malformed UTF-8 or UTF-16 charac-
       ters is to use a lookahead to check the length of the  next  character,
       as  in  this  pattern,  which could be used with a UTF-8 string (ignore
       white space and line breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       In this example, a group that starts  with  (?|  resets  the  capturing
       parentheses  numbers in each alternative (see "Duplicate Group Numbers"
       below). The assertions at the start of each branch check the next UTF-8
       character for values whose encoding uses 1, 2, 3, or 4  bytes,  respec-
       tively.  The  character's individual bytes are then captured by the ap-
       propriate number of \C groups.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial by default.  If a closing square bracket is required as  a  member
       of the class, it should be the first data character in the class (after
       an  initial  circumflex,  if present) or escaped with a backslash. This
       means that, by default, an empty class cannot be defined.  However,  if
       the  PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at
       the start does end the (empty) class.

       A character class matches a single character in the subject. A  matched
       character must be in the set of characters defined by the class, unless
       the  first  character in the class definition is a circumflex, in which
       case the subject character must not be in the set defined by the class.
       If a circumflex is actually required as a member of the  class,  ensure
       it is not the first character, or escape it with a backslash.

       For example, the character class [aeiou] matches any lower case English
       vowel,  whereas [^aeiou] matches all other characters. Note that a cir-
       cumflex is just a convenient notation  for  specifying  the  characters
       that  are  in the class by enumerating those that are not. A class that
       starts with a circumflex is not an assertion; it still consumes a char-
       acter from the subject string, and therefore it fails to match  if  the
       current pointer is at the end of the string.

       Characters  in  a class may be specified by their code points using \o,
       \x, or \N{U+hh..} in the usual way. When caseless matching is set,  any
       letters  in a class represent both their upper case and lower case ver-
       sions, so for example, a caseless [aeiou] matches "A" as well  as  "a",
       and  a  caseless [^aeiou] does not match "A", whereas a caseful version
       would. Note that there are two ASCII characters, K and S, that, in  ad-
       dition  to their lower case ASCII equivalents, are case-equivalent with
       Unicode U+212A (Kelvin sign) and U+017F (long S) respectively when  ei-
       ther PCRE2_UTF or PCRE2_UCP is set. If you do not want these ASCII/non-
       ASCII  case  equivalences,  you  can suppress them by setting PCRE2_EX-
       TRA_CASELESS_RESTRICT, either as an option in a compile context, or  by
       including (*CASELESS_RESTRICT) or (?r) within a pattern.

       Characters  that  might  indicate  line breaks are never treated in any
       special way when matching character classes, whatever  line-ending  se-
       quence  is  in  use,  and  whatever  setting  of  the  PCRE2_DOTALL and
       PCRE2_MULTILINE options is used. A class such as  [^a]  always  matches
       one of these characters.

       The generic character type escape sequences \d, \D, \h, \H, \p, \P, \s,
       \S,  \v,  \V,  \w,  and \W may appear in a character class, and add the
       characters that they  match  to  the  class.  For  example,  [\dABCDEF]
       matches  any  hexadecimal digit. In UTF modes, the PCRE2_UCP option af-
       fects the meanings of \d, \s, \w and their upper case partners, just as
       it does when they appear outside a character class, as described in the
       section entitled "Generic character types" above. The  escape  sequence
       \b  has  a  different  meaning inside a character class; it matches the
       backspace character. The sequences \B, \R, and \X are not  special  in-
       side  a  character class. Like any other unrecognized escape sequences,
       they cause an error. The same is true for \N when not  followed  by  an
       opening brace.

       The  minus (hyphen) character can be used to specify a range of charac-
       ters in a character class. For example, [d-m] matches  any  letter  be-
       tween  d and m, inclusive. If a minus character is required in a class,
       it must be escaped with a backslash or appear in a  position  where  it
       cannot  be interpreted as indicating a range, typically as the first or
       last character in the class, or immediately after a range. For example,
       [b-d-z] matches letters in the range b to d, a hyphen character, or z.

       There is some special treatment for alphabetic ranges in  EBCDIC  envi-
       ronments; see the section "EBCDIC environments" below.

       Perl treats a hyphen as a literal if it appears before or after a POSIX
       class (see below) or before or after a character type escape such as \d
       or  \H.  However, unless the hyphen is the last character in the class,
       Perl outputs a warning in its warning mode, as this is  most  likely  a
       user  error. As PCRE2 has no facility for warning, an error is given in
       these cases.

       It is not possible to have the literal character "]" as the end charac-
       ter of a range. A pattern such as [W-]46] is interpreted as a class  of
       two  characters ("W" and "-") followed by a literal string "46]", so it
       would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
       backslash  it  is interpreted as the end of a range, so [W-\]46] is in-
       terpreted as a class containing a range and two other  characters.  The
       octal  or  hexadecimal  representation of "]" can also be used to end a
       range.

       Ranges normally include all code points between the start and end char-
       acters, inclusive. They can also be used for code points specified  nu-
       merically,  for  example [\000-\037]. Ranges can include any characters
       that are valid for the current mode. In any  UTF  mode,  the  so-called
       "surrogate"  characters (those whose code points lie between 0xd800 and
       0xdfff inclusive) may not  be  specified  explicitly  by  default  (the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES  option  disables this check). How-
       ever, ranges such as [\x{d7ff}-\x{e000}], which include the surrogates,
       are always permitted.

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_`wxyzabc], matched caselessly, and  in  a  non-UTF  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases.

       A circumflex can conveniently be used with  the  upper  case  character
       types  to specify a more restricted set of characters than the matching
       lower case type.  For example, the class [^\W_] matches any  letter  or
       digit, but not underscore, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       The  metacharacters  that are recognized in character classes are back-
       slash, hyphen (when it can be interpreted as specifying a range),  cir-
       cumflex  (only  at  the  start),  and  the  terminating  closing square
       bracket. An opening square bracket is also special when it can  be  in-
       terpreted  as  introducing a POSIX class (see "Posix character classes"
       below), or a special compatibility feature (see "Compatibility  feature
       for  word boundaries" below. Escaping any non-alphanumeric character in
       a class turns it into a literal, whether or not it would otherwise be a
       metacharacter.


PERL EXTENDED CHARACTER CLASSES

       From release 10.45 PCRE2 supports Perl's  (?[...])  extended  character
       class syntax. This can be used to perform set operations such as inter-
       section on character classes.

       The  syntax  permitted  within  (?[...]) is quite different to ordinary
       character classes. Inside the extended class, there  is  an  expression
       syntax  consisting of "atoms", operators, and ordinary parentheses "()"
       used for grouping. Such classes  always  have  the  Perl  /xx  modifier
       (PCRE2  option  PCRE2_EXTENDED_MORE)  turned on within them. This means
       that literal space and tab characters are  ignored  everywhere  in  the
       class.

       The  allowed  atoms  are  individual characters specified by escape se-
       quences such as \n or  \x{123},  character  types  such  as  \d,  POSIX
       classes such as [:alpha:], and nested ordinary (non-extended) character
       classes. For example, in (?[\d & [...]]) the nested class [...] follows
       the  usual  rules  for ordinary character classes, in which parentheses
       are not metacharacters, and character literals and ranges  are  permit-
       ted.

       Character  literals and ranges may not appear outside a nested ordinary
       character class because they are not atoms in the extended syntax.  The
       extended  syntax does not introduce any additional escape sequences, so
       (?[\y]) is an unknown escape, as it would be in [\y].

       In the extended syntax, ^ does not negate a class (except within an or-
       dinary class nested inside an extended class); it is instead  a  binary
       operator.

       The  binary  operators  are "&" (intersection), "|" or "+" (union), "-"
       (subtraction) and "^" (symmetric difference). These  are  left-associa-
       tive  and  "&"  has  higher (tighter) precedence, while the others have
       equal lower precedence. The one prefix unary operator is  "!"  (comple-
       ment), with highest precedence.


UTS#18 EXTENDED CHARACTER CLASSES

       The  PCRE2_ALT_EXTENDED_CLASS  option  enables an alternative to Perl's
       (?[...])  syntax, allowing instead extended class behaviour inside  or-
       dinary  [...]  character classes. This altered syntax for [...] classes
       is loosely described by the Unicode standard UTS#18. The  PCRE2_ALT_EX-
       TENDED_CLASS  option  does not prevent use of (?[...]) classes; it just
       changes the meaning of all [...] classes that are not nested  inside  a
       Perl (?[...]) class.

       Firstly, in ordinary Perl [...] syntax, an expression such as "[a[]" is
       a  character  class  with  two  literal  characters "a" and "[", but in
       UTS#18  extended  classes  the  "["  character  becomes  an  additional
       metacharacter  within classes, denoting the start of a nested class, so
       a literal "[" must be escaped as "\[".

       Secondly, within the UTS#18 extended syntax, there are operators  "||",
       "&&",  "--"  and "~~" which denote character class union, intersection,
       subtraction, and symmetric difference respectively.  In  standard  Perl
       syntax,  these would simply be needlessly-repeated literals (except for
       "--" which could be the start or end of a range).  In  UTS#18  extended
       classes these operators can be used in constructs such as [\p{L}--[QW]]
       for  "Unicode letters, other than Q and W".  A literal "-" at the start
       or end of a range must be escaped, so while "[--1]" in Perl  syntax  is
       the  range from hyphen to "1", it must be escaped as "[\--1]" in UTS#18
       extended classes.

       Unlike Perl's (?[...]) extended classes, the PCRE2_EXTENDED_MORE option
       to ignore space and tab characters is  not  automatically  enabled  for
       UTS#18 extended classes, but it is honoured if set.

       Extended  UTS#18  classes  can  be nested, and nested classes are them-
       selves extended classes (unlike Perl, where nested classes must be sim-
       ple classes).  For example, [\p{L}&&[\p{Thai}||\p{Greek}]] matches  any
       letter  that is in the Thai or Greek scripts. Note that this means that
       no special grouping characters (such as the parentheses used in  Perl's
       (?[...]) class syntax) are needed.

       Individual  class items (literal characters, literal ranges, properties
       such as \d or \p{...}, and nested classes) can be combined by  juxtapo-
       sition or by an operator. Juxtaposition is the implicit union operator,
       and  binds  more tightly than any explicit operator. Thus a sequence of
       literals and/or ranges behaves as if it is enclosed in square brackets.
       For example, [A-Z0-9&&[^E8]] is the same  as  [[A-Z0-9]&&[^E8]],  which
       matches any upper case alphanumeric character except "E" or "8".

       Precedence between the explicit operators is not defined, so mixing op-
       erators  is  a  syntax  error.  For example, [A&&B--C] is an error, but
       [A&&[B--C]] is valid.

       This is an emerging syntax which is being adopted gradually across  the
       regex  ecosystem:  for  example JavaScript adopted the "/v" flag in EC-
       MAScript 2024; Python's "re" module reserves the syntax for future  use
       with a FutureWarning for unescaped use of "[" as a literal within char-
       acter  classes.  Due to UTS#18 providing insufficient guidance, engines
       interpret the syntax differently.  Rust's "regex"  crate  and  Python's
       "regex"  PyPi  module  both implement UTS#18 extended classes, but with
       slight  incompatibilities  ([A||B&&C]  is  parsed  as  [A||[B&&C]]   in
       Python's "regex" but as [[A||B]&&C] in Rust's "regex").

       PCRE2's  syntax  adds  syntax  restrictions  similar to ECMASCript's /v
       flag, so that all the UTS#18 extended  classes  accepted  as  valid  by
       PCRE2  have the property that they are interpreted either with the same
       behaviour, or as invalid, by all other major engines.  Please  file  an
       issue if you are aware of cross-engine differences in behaviour between
       PCRE2 and another major engine.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed  by [: and :] within the enclosing square brackets. PCRE2 also
       supports this notation, in both ordinary and extended classes. For  ex-
       ample,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (the same as \s from PCRE2 8.34)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The  default  "space" characters are HT (9), LF (10), VT (11), FF (12),
       CR (13), and space (32). If locale-specific matching is  taking  place,
       the  list  of  space characters may be different; there may be fewer or
       more of them. "Space" and \s match the same set of  characters,  as  do
       "word" and \w.

       The  name  "word"  is  a Perl extension, and "blank" is a GNU extension
       from Perl 5.8. Another Perl extension is negation, which  is  indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By default, characters with values greater than 127 do not match any of
       the POSIX character classes, although this may be different for charac-
       ters  in  the range 128-255 when locale-specific matching is happening.
       However, in UCP mode, unless certain options are set (see below),  some
       of  the  classes  are  changed so that Unicode character properties are
       used. This is achieved by replacing POSIX classes with other sequences,
       as follows:

         [:alnum:]  becomes  \p{Xan}
         [:alpha:]  becomes  \p{L}
         [:blank:]  becomes  \h
         [:cntrl:]  becomes  \p{Cc}
         [:digit:]  becomes  \p{Nd}
         [:lower:]  becomes  \p{Ll}
         [:space:]  becomes  \p{Xps}
         [:upper:]  becomes  \p{Lu}
         [:word:]   becomes  \p{Xwd}

       Negated versions, such as [:^alpha:] use \P instead of \p.  Four  other
       POSIX classes are handled specially in UCP mode:

       [:graph:] This  matches  characters that have glyphs that mark the page
                 when printed. In Unicode property terms, it matches all char-
                 acters with the L, M, N, P, S, or Cf properties, except for:

                   U+061C           Arabic Letter Mark
                   U+180E           Mongolian Vowel Separator
                   U+2066 - U+2069  Various "isolate"s


       [:print:] This matches the same  characters  as  [:graph:]  plus  space
                 characters  that  are  not controls, that is, characters with
                 the Zs property.

       [:punct:] This matches all characters that have the Unicode P (punctua-
                 tion) property, plus those characters with code  points  less
                 than 256 that have the S (Symbol) property.

       [:xdigit:]
                 In  addition  to  the  ASCII  hexadecimal  digits,  this also
                 matches the "fullwidth" versions of those  characters,  whose
                 Unicode  code  points  start at U+FF10. This is a change that
                 was made in PCRE2 release 10.43 for Perl compatibility.

       The other POSIX classes are unchanged  by  PCRE2_UCP,  and  match  only
       characters with code points less than 256.

       There are two options that can be used to restrict the POSIX classes to
       ASCII   characters   when   PCRE2_UCP  is  set.  The  option  PCRE2_EX-
       TRA_ASCII_DIGIT affects just [:digit:] and [:xdigit:].  Within  a  pat-
       tern,  this  can  be  set  and unset by (?aT) and (?-aT). The PCRE2_EX-
       TRA_ASCII_POSIX option disables UCP processing for all  POSIX  classes,
       including  [:digit:] and [:xdigit:]. Within a pattern, (?aP) and (?-aP)
       set and unset both these options for consistency.


COMPATIBILITY FEATURE FOR WORD BOUNDARIES

       In the POSIX.2 compliant library that was included in 4.4BSD Unix,  the
       ugly  syntax  [[:<:]]  and [[:>:]] is used for matching "start of word"
       and "end of word". PCRE2 treats these items as follows:

         [[:<:]]  is converted to  \b(?=\w)
         [[:>:]]  is converted to  \b(?<=\w)

       Only these exact character sequences are recognized. A sequence such as
       [a[:<:]b] provokes error for an unrecognized  POSIX  class  name.  This
       support  is not compatible with Perl. It is provided to help migrations
       from other environments, and is best not used in any new patterns. Note
       that \b matches at the start and the end of a word (see "Simple  asser-
       tions"  above),  and in a Perl-style pattern the preceding or following
       character normally shows which is wanted, without the need for the  as-
       sertions  that are used above in order to give exactly the POSIX behav-
       iour. Note also that the PCRE2_UCP option changes  the  meaning  of  \w
       (and  therefore  \b)  by  default,  so  it also affects these POSIX se-
       quences.


VERTICAL BAR

       Vertical bar characters are used to separate alternative patterns.  For
       example, the pattern

         gilbert|sullivan

       matches  either "gilbert" or "sullivan". Any number of alternatives may
       appear, and an empty  alternative  is  permitted  (matching  the  empty
       string). The matching process tries each alternative in turn, from left
       to  right, and the first one that succeeds is used. If the alternatives
       are within a group (defined below), "succeeds" means matching the  rest
       of the main pattern as well as the alternative in the group.


INTERNAL OPTION SETTING

       The  settings  of  several options can be changed within a pattern by a
       sequence of letters enclosed between "(?" and ")".  The  following  are
       Perl-compatible, and are described in detail in the pcre2api documenta-
       tion. The option letters are:

         i  for PCRE2_CASELESS
         m  for PCRE2_MULTILINE
         n  for PCRE2_NO_AUTO_CAPTURE
         s  for PCRE2_DOTALL
         x  for PCRE2_EXTENDED
         xx for PCRE2_EXTENDED_MORE

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the relevant letters with a hy-
       phen,  for  example (?-im). The two "extended" options are not indepen-
       dent; unsetting either one cancels the effects of both of them.

       A  combined  setting  and  unsetting  such  as  (?im-sx),  which   sets
       PCRE2_CASELESS  and  PCRE2_MULTILINE  while  unsetting PCRE2_DOTALL and
       PCRE2_EXTENDED, is also permitted. Only one hyphen may  appear  in  the
       options  string.  If a letter appears both before and after the hyphen,
       the option is unset. An empty options setting "(?)" is  allowed.  Need-
       less to say, it has no effect.

       If  the  first character following (? is a circumflex, it causes all of
       the above options to be unset. Letters may  follow  the  circumflex  to
       cause some options to be re-instated, but a hyphen may not appear.

       Some  PCRE2-specific options can be changed by the same mechanism using
       these pairs or individual letters:

         aD for PCRE2_EXTRA_ASCII_BSD
         aS for PCRE2_EXTRA_ASCII_BSS
         aW for PCRE2_EXTRA_ASCII_BSW
         aP for PCRE2_EXTRA_ASCII_POSIX and PCRE2_EXTRA_ASCII_DIGIT
         aT for PCRE2_EXTRA_ASCII_DIGIT
         r  for PCRE2_EXTRA_CASELESS_RESTRICT
         J  for PCRE2_DUPNAMES
         U  for PCRE2_UNGREEDY

       However, except for 'r', these are not unset by (?^), which is  equiva-
       lent  to  (?-imnrsx).  If  'a' is not followed by any of the upper case
       letters shown above, it sets (or unsets) all the ASCII options.

       PCRE2_EXTRA_ASCII_DIGIT  has  no  additional  effect   when   PCRE2_EX-
       TRA_ASCII_POSIX  is  set,  but  including it in (?aP) means that (?-aP)
       suppresses all ASCII restrictions for POSIX classes.

       When one of these option changes occurs at top level (that is, not  in-
       side  group parentheses), the change applies until a subsequent change,
       or the end of the pattern. An option change within a group  (see  below
       for  a  description of groups) affects only that part of the group that
       follows it. At the end of the group these  options  are  reset  to  the
       state they were before the group. For example,

         (a(?i)b)c

       matches  abc  and  aBc and no other strings (assuming PCRE2_CASELESS is
       not set externally). Any changes made in one alternative  do  carry  on
       into subsequent branches within the same group. For example,

         (a(?i)b|c)

       matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the
       first branch is abandoned before the option setting.  This  is  because
       the  effects  of option settings happen at compile time. There would be
       some very weird behaviour otherwise.

       As a convenient shorthand, if any option settings are required  at  the
       start  of a non-capturing group (see the next section), the option let-
       ters may appear between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings.

       Note: There are other PCRE2-specific options,  applying  to  the  whole
       pattern,  which  can be set by the application when the compiling func-
       tion is called. In addition, the pattern can  contain  special  leading
       sequences  such  as (*CRLF) to override what the application has set or
       what has been defaulted.  Details are given  in  the  section  entitled
       "Newline sequences" above. There are also the (*UTF) and (*UCP) leading
       sequences  that can be used to set UTF and Unicode property modes; they
       are equivalent to setting the PCRE2_UTF and PCRE2_UCP options,  respec-
       tively.  However,  the  application  can  set  the  PCRE2_NEVER_UTF  or
       PCRE2_NEVER_UCP options, which lock out  the  use  of  the  (*UTF)  and
       (*UCP) sequences.


GROUPS

       Groups  are  delimited  by  parentheses  (round brackets), which can be
       nested.  Turning part of a pattern into a group does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches "cataract", "caterpillar", or "cat". Without  the  parentheses,
       it would match "cataract", "erpillar" or an empty string.

       2.  It  creates a "capture group". This means that, when the whole pat-
       tern matches, the portion of the subject string that matched the  group
       is  passed back to the caller, separately from the portion that matched
       the whole pattern.  (This applies  only  to  the  traditional  matching
       function; the DFA matching function does not support capturing.)

       Opening parentheses are counted from left to right (starting from 1) to
       obtain  numbers for capture groups. For example, if the string "the red
       king" is matched against the pattern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The fact that plain parentheses fulfil  two  functions  is  not  always
       helpful.   There are often times when grouping is required without cap-
       turing. If an opening parenthesis is followed by a question mark and  a
       colon,  the  group  does  not do any capturing, and is not counted when
       computing the number of any subsequent capture groups. For example,  if
       the string "the white queen" is matched against the pattern

         the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capture groups is 65535.

       As  a  convenient shorthand, if any option settings are required at the
       start of a non-capturing group, the option letters may  appear  between
       the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried  from  left  to right, and options are not reset until the end of
       the group is reached, an option setting in one branch does affect  sub-
       sequent branches, so the above patterns match "SUNDAY" as well as "Sat-
       urday".


DUPLICATE GROUP NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a group uses
       the  same  numbers  for  its capturing parentheses. Such a group starts
       with (?| and is itself a non-capturing  group.  For  example,  consider
       this pattern:

         (?|(Sat)ur|(Sun))day

       Because  the two alternatives are inside a (?| group, both sets of cap-
       turing parentheses are numbered one. Thus, when  the  pattern  matches,
       you  can  look  at captured substring number one, whichever alternative
       matched. This construct is useful when you want to  capture  part,  but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses  are  numbered as usual, but the number is reset at the start of
       each branch. The numbers of any capturing parentheses that  follow  the
       whole group start after the highest number used in any branch. The fol-
       lowing example is taken from the Perl documentation. The numbers under-
       neath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A  backreference  to a capture group uses the most recent value that is
       set for the group. The following pattern matches "abcabc" or "defdef":

         /(?|(abc)|(def))\1/

       In contrast, a subroutine call to a capture group always refers to  the
       first  one  in the pattern with the given number. The following pattern
       matches "abcabc" or "defabc":

         /(?|(abc)|(def))(?1)/

       A relative reference such as (?-1) is no different: it is just a conve-
       nient way of computing an absolute group number.

       If a condition test for a group's having matched refers to a non-unique
       number, the test is true if any group with that number has matched.

       An alternative approach to using this "branch reset" feature is to  use
       duplicate named groups, as described in the next section.


NAMED CAPTURE GROUPS

       Identifying capture groups by number is simple, but it can be very hard
       to  keep  track of the numbers in complicated patterns. Furthermore, if
       an expression is modified, the numbers may change. To  help  with  this
       difficulty,  PCRE2  supports the naming of capture groups. This feature
       was not added to Perl until release 5.10. Python had the  feature  ear-
       lier,  and PCRE1 introduced it at release 4.0, using the Python syntax.
       PCRE2 supports both the Perl and the Python syntax.

       In PCRE2,  a  capture  group  can  be  named  in  one  of  three  ways:
       (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python.
       Names may be up to 128 code units long. When PCRE2_UTF is not set, they
       may  contain  only  ASCII  alphanumeric characters and underscores, but
       must start with a non-digit. When PCRE2_UTF is set, the syntax of group
       names is extended to allow any Unicode letter or Unicode decimal digit.
       In other words, group names must match one of these patterns:

         ^[_A-Za-z][_A-Za-z0-9]*\z   when PCRE2_UTF is not set
         ^[_\p{L}][_\p{L}\p{Nd}]*\z  when PCRE2_UTF is set

       References to capture groups from other parts of the pattern,  such  as
       backreferences,  recursion,  and conditions, can all be made by name as
       well as by number.

       Named capture groups are allocated numbers as well as names, exactly as
       if the names were not present. In both PCRE2 and Perl,  capture  groups
       are  primarily  identified  by  numbers; any names are just aliases for
       these numbers. The PCRE2 API provides function calls for extracting the
       complete name-to-number translation table from a compiled  pattern,  as
       well  as  convenience  functions  for extracting captured substrings by
       name.

       Warning: When more than one capture group has the same number,  as  de-
       scribed in the previous section, a name given to one of them applies to
       all  of them. Perl allows identically numbered groups to have different
       names.  Consider this pattern, where there are two capture groups, both
       numbered 1:

         (?|(?<AA>aa)|(?<BB>bb))

       Perl allows this, with both names AA and BB  as  aliases  of  group  1.
       Thus, after a successful match, both names yield the same value (either
       "aa" or "bb").

       In  an attempt to reduce confusion, PCRE2 does not allow the same group
       number to be associated with more than one name. The example above pro-
       vokes a compile-time error. However, there is still  scope  for  confu-
       sion. Consider this pattern:

         (?|(?<AA>aa)|(bb))

       Although the second group number 1 is not explicitly named, the name AA
       is  still an alias for any group 1. Whether the pattern matches "aa" or
       "bb", a reference by name to group AA yields the matched string.

       By default, a name must be unique within a pattern, except that  dupli-
       cate names are permitted for groups with the same number, for example:

         (?|(?<AA>aa)|(?<AA>bb))

       The duplicate name constraint can be disabled by setting the PCRE2_DUP-
       NAMES option at compile time, or by the use of (?J) within the pattern,
       as described in the section entitled "Internal Option Setting" above.

       Duplicate  names  can be useful for patterns where only one instance of
       the named capture group can match. Suppose you want to match  the  name
       of  a  weekday,  either as a 3-letter abbreviation or as the full name,
       and in both cases you want to extract the  abbreviation.  This  pattern
       (ignoring the line breaks) does the job:

         (?J)
         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There  are five capture groups, but only one is ever set after a match.
       The convenience functions for extracting the data by name  returns  the
       substring  for  the first (and in this example, the only) group of that
       name that matched. This saves searching to find which numbered group it
       was. (An alternative way of solving this problem is to  use  a  "branch
       reset" group, as described in the previous section.)

       If  you make a backreference to a non-unique named group from elsewhere
       in the pattern, the groups to which the name refers are checked in  the
       order  in  which they appear in the overall pattern. The first one that
       is set is used for the reference. For  example,  this  pattern  matches
       both "foofoo" and "barbar" but not "foobar" or "barfoo":

         (?J)(?:(?<n>foo)|(?<n>bar))\k<n>


       If you make a subroutine call to a non-unique named group, the one that
       corresponds to the first occurrence of the name is used. In the absence
       of duplicate numbers this is the one with the lowest number.

       If you use a named reference in a condition test (see the section about
       conditions below), either to check whether a capture group has matched,
       or to check for recursion, all groups with the same name are tested. If
       the  condition  is  true  for any one of them, the overall condition is
       true. This is the same behaviour as testing by number. For further  de-
       tails  of  the  interfaces  for  handling named capture groups, see the
       pcre2api documentation.


REPETITION

       Repetition is specified by quantifiers, which may  follow  any  one  of
       these items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \R escape sequence
         the \X escape sequence
         any escape sequence that matches a single character
         a character class
         a backreference
         a parenthesized group (including lookaround assertions)
         a subroutine call (recursive or otherwise)

       If a quantifier does not follow a repeatable item, an error occurs. The
       general repetition quantifier specifies a minimum and maximum number of
       permitted  matches  by  giving  two numbers in curly brackets (braces),
       separated by a comma. The numbers must be  less  than  65536,  and  the
       first must be less than or equal to the second. For example,

         z{2,4}

       matches  "zz",  "zzz",  or  "zzzz". A closing brace on its own is not a
       special character. If the second number is omitted, but  the  comma  is
       present,  there  is  no upper limit; if the second number and the comma
       are both omitted, the quantifier specifies an exact number of  required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, whereas

         \d{8}

       matches  exactly  8  digits.  If the first number is omitted, the lower
       limit is taken as zero; in this case the upper limit must be present.

         X{,4} is interpreted as X{0,4}

       This is a change in behaviour that happened in Perl  5.34.0  and  PCRE2
       10.43.  In  earlier  versions  such a sequence was not interpreted as a
       quantifier. Other regular expression engines may behave either way.

       If the characters that follow an opening brace do not match the  syntax
       of a quantifier, the brace is taken as a literal character. In particu-
       lar, this means that {,} is a literal string of three characters.

       Note that not every opening brace is potentially the start of a quanti-
       fier  because  braces  are  used  in  other  items such as \N{U+345} or
       \k{name}.

       In UTF modes, quantifiers apply to characters rather than to individual
       code units. Thus, for example, \x{100}{2} matches two characters,  each
       of which is represented by a two-byte sequence in a UTF-8 string. Simi-
       larly,  \X{3} matches three Unicode extended grapheme clusters, each of
       which may be several code units long (and  they  may  be  of  different
       lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful  for  capture  groups that are referenced as subroutines from else-
       where in the pattern (but see also the section entitled "Defining  cap-
       ture groups for use by reference only" below). Except for parenthesized
       groups,  items that have a {0} quantifier are omitted from the compiled
       pattern.

       For convenience, the three most common quantifiers have  single-charac-
       ter abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It  is  possible  to construct infinite loops by following a group that
       can match no characters with a quantifier that has no upper limit,  for
       example:

         (a?)*

       Earlier  versions  of  Perl  and PCRE1 used to give an error at compile
       time for such patterns. However, because there are cases where this can
       be useful, such patterns are now accepted, but whenever an iteration of
       such a group matches no characters, matching moves on to the next  item
       in  the  pattern  instead  of repeatedly matching an empty string. This
       does not prevent backtracking into any of the iterations  if  a  subse-
       quent item fails to match.

       By  default,  quantifiers  are "greedy", that is, they match as much as
       possible (up to the maximum number of permitted  repetitions),  without
       causing  the  rest of the pattern to fail. The classic example of where
       this gives problems is in trying to match comments in C programs. These
       appear between /* and */ and within the comment,  individual  *  and  /
       characters  may  appear. An attempt to match C comments by applying the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness  of
       the  .*  item. However, if a quantifier is followed by a question mark,
       it ceases to be greedy, and instead matches the minimum number of times
       possible, so the pattern

         /\*.*?\*/

       does the right thing with C comments. The meaning of the various  quan-
       tifiers is not otherwise changed, just the preferred number of matches.
       Do  not  confuse this use of question mark with its use as a quantifier
       in its own right.  Because it has two uses,  it  can  sometimes  appear
       doubled, as in

         \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the PCRE2_UNGREEDY option is set (an option that is not available in
       Perl),  the  quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a  question  mark.  In  other
       words, it inverts the default behaviour.

       When  a  parenthesized  group is quantified with a minimum repeat count
       that is greater than 1 or with a limited maximum, more  memory  is  re-
       quired for the compiled pattern, in proportion to the size of the mini-
       mum or maximum.

       If  a  pattern  starts  with  .*  or  .{0,} and the PCRE2_DOTALL option
       (equivalent to Perl's /s) is set, thus allowing the dot to  match  new-
       lines,  the  pattern  is  implicitly anchored, because whatever follows
       will be tried against every character position in the  subject  string,
       so  there is no point in retrying the overall match at any position af-
       ter the first. PCRE2 normally treats such a pattern as though  it  were
       preceded by \A.

       In  cases  where  it  is known that the subject string contains no new-
       lines, it is worth setting PCRE2_DOTALL in order to obtain  this  opti-
       mization, or alternatively, using ^ to indicate anchoring explicitly.

       However,  there  are  some cases where the optimization cannot be used.
       When .*  is inside capturing parentheses that  are  the  subject  of  a
       backreference  elsewhere  in the pattern, a match at the start may fail
       where a later one succeeds. Consider, for example:

         (.*)abc\1

       If the subject is "xyz123abc123" the match point is the fourth  charac-
       ter. For this reason, such a pattern is not implicitly anchored.

       Another  case where implicit anchoring is not applied is when the lead-
       ing .* is inside an atomic group. Once again, a match at the start  may
       fail where a later one succeeds. Consider this pattern:

         (?>.*?a)b

       It  matches "ab" in the subject "aab". The use of the backtracking con-
       trol verbs (*PRUNE) and (*SKIP) also disable this optimization.  To  do
       so  explicitly, either pass the compile option PCRE2_NO_DOTSTAR_ANCHOR,
       or call pcre2_set_optimize() with a PCRE2_DOTSTAR_ANCHOR_OFF directive.

       When a capture group is repeated, the value captured is  the  substring
       that matched the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is  "tweedledee". However, if there are nested capture groups, the cor-
       responding captured values may have been set  in  previous  iterations.
       For example, after

         (a|(b))+

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       With  both  maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
       repetition, failure of what follows normally causes the  repeated  item
       to  be  re-evaluated to see if a different number of repeats allows the
       rest of the pattern to match. Sometimes it is useful to  prevent  this,
       either  to  change the nature of the match, or to cause it fail earlier
       than it otherwise might, when the author of the pattern knows there  is
       no point in carrying on.

       Consider,  for  example, the pattern \d+foo when applied to the subject
       line

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the means for specifying that once a group has matched, it is not to be
       re-evaluated in this way.

       If  we  use atomic grouping for the previous example, the matcher gives
       up immediately on failing to match "foo" the first time.  The  notation
       is a kind of special parenthesis, starting with (?> as in this example:

         (?>\d+)foo

       Perl  5.28  introduced an experimental alphabetic form starting with (*
       which may be easier to remember:

         (*atomic:\d+)foo

       This kind of parenthesized group "locks up" the part of the pattern  it
       contains once it has matched, and a failure further into the pattern is
       prevented  from  backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a group of this type matches exactly
       the string of characters that an  identical  standalone  pattern  would
       match, if anchored at the current point in the subject string.

       Atomic  groups  are  not capture groups. Simple cases such as the above
       example can be thought of as a  maximizing  repeat  that  must  swallow
       everything  it can.  So, while both \d+ and \d+? are prepared to adjust
       the number of digits they match in order to make the rest of  the  pat-
       tern match, (?>\d+) can only match an entire sequence of digits.

       Atomic  groups in general can of course contain arbitrarily complicated
       expressions, and can be nested. However, when the contents of an atomic
       group is just a single repeated item, as in the example above,  a  sim-
       pler  notation, called a "possessive quantifier" can be used. This con-
       sists of an additional + character following a quantifier.  Using  this
       notation, the previous example can be rewritten as

         \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

         (abc|xyz){2,3}+

       Possessive  quantifiers are always greedy; the setting of the PCRE2_UN-
       GREEDY option is ignored. They are a convenient notation for  the  sim-
       pler  forms  of  atomic  group.  However, there is no difference in the
       meaning of a possessive quantifier and  the  equivalent  atomic  group,
       though  there  may  be a performance difference; possessive quantifiers
       should be slightly faster.

       The possessive quantifier syntax is an extension to the Perl  5.8  syn-
       tax.   Jeffrey  Friedl  originated the idea (and the name) in the first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built Sun's Java package, and PCRE1 copied it from there. It found  its
       way into Perl at release 5.10.

       PCRE2  has  an  optimization  that automatically "possessifies" certain
       simple pattern constructs. For example, the sequence A+B is treated  as
       A++B  because  there is no point in backtracking into a sequence of A's
       when  B  must  follow.   This  feature   can   be   disabled   by   the
       PCRE2_NO_AUTO_POSSESS  option,  by  calling pcre2_set_optimize() with a
       PCRE2_AUTO_POSSESS_OFF directive,  or  by  starting  the  pattern  with
       (*NO_AUTO_POSSESS).

       When a pattern contains an unlimited repeat inside a group that can it-
       self  be  repeated  an  unlimited number of times, the use of an atomic
       group is the only way to avoid some failing matches taking a very  long
       time indeed. The pattern

         (\D+|<\d+>)*[!?]

       matches  an  unlimited number of substrings that either consist of non-
       digits, or digits enclosed in <>, followed by either ! or  ?.  When  it
       matches, it runs quickly. However, if it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it  takes  a  long  time  before reporting failure. This is because the
       string can be divided between the internal \D+ repeat and the  external
       *  repeat in a large number of ways, and all have to be tried. (The ex-
       ample uses [!?] rather than a single character at the end, because both
       PCRE2 and Perl have an optimization that allows for fast failure when a
       single character is used. They remember the last single character  that
       is  required  for  a  match, and fail early if it is not present in the
       string.) If the pattern is changed so that it  uses  an  atomic  group,
       like this:

         ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.


BACKREFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0  (and  possibly further digits) is a backreference to a capture group
       earlier (that is, to its left) in the pattern, provided there have been
       that many previous capture groups.

       However, if the decimal number following the backslash is less than  8,
       it  is  always  taken  as  a backreference, and causes an error only if
       there are not that many capture groups in the entire pattern. In  other
       words, the group that is referenced need not be to the left of the ref-
       erence  for numbers less than 8. A "forward backreference" of this type
       can make sense when a repetition is involved and the group to the right
       has participated in an earlier iteration.

       It is not possible to have a numerical  "forward  backreference"  to  a
       group  whose  number  is 8 or more using this syntax because a sequence
       such as \50 is interpreted as a character defined  in  octal.  See  the
       subsection entitled "Non-printing characters" above for further details
       of  the  handling of digits following a backslash. Other forms of back-
       referencing do not suffer from this restriction. In  particular,  there
       is no problem when named capture groups are used (see below).

       Another  way  of  avoiding  the ambiguity inherent in the use of digits
       following a backslash is to use the \g  escape  sequence.  This  escape
       must be followed by a signed or unsigned number, optionally enclosed in
       braces. These examples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       An  unsigned number specifies an absolute reference without the ambigu-
       ity that is present in the older syntax. It is also useful when literal
       digits follow the reference. A signed number is a  relative  reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the capture group whose number is
       one  less  than  the number of the next group to be started, so in this
       example (where the next group would be numbered 3) is it equivalent  to
       \2,  and  \g{-2} would be equivalent to \1. Note that if this construct
       is inside a capture group, that group is included in the count,  so  in
       this example \g{-2} also refers to group 1:

         (A)(\g{-2}B)

       The  use  of  relative  references can be helpful in long patterns, and
       also in patterns that are created by joining  together  fragments  that
       contain references within themselves.

       The  sequence  \g{+1}  is a reference to the next capture group that is
       started after this item, and \g{+2} refers to the one after  that,  and
       so  on.  This  kind of forward reference can be useful in patterns that
       repeat. Perl does not support the use of + in this way.

       A backreference matches whatever actually  most  recently  matched  the
       capture  group  in  the current subject string, rather than anything at
       all that matches the group (see "Groups as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not  "sense and responsibility". If caseful matching is in force at the
       time of the backreference, the case of letters is relevant.  For  exam-
       ple,

         ((?i)rah)\s+\1

       matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
       original capture group is matched caselessly.

       There are several different ways of  writing  backreferences  to  named
       capture  groups.  The  .NET  syntax  is  \k{name}, the Python syntax is
       (?=name), and the original Perl syntax is \k<name> or \k'name'. All  of
       these  are  now  supported  by both Perl and PCRE2. Perl 5.10's unified
       backreference syntax, in which \g can be  used  for  both  numeric  and
       named  references,  is  also  supported by PCRE2.  We could rewrite the
       above example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

       A capture group that is referenced by name may appear  in  the  pattern
       before or after the reference.

       There  may be more than one backreference to the same group. If a group
       has not actually been used in a particular match, backreferences to  it
       always fail by default. For example, the pattern

         (a|(bc))\2

       always  fails  if  it starts to match "a" rather than "bc". However, if
       the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref-
       erence to an unset value matches an empty string.

       Because there may be many capture groups in a pattern, all digits  fol-
       lowing  a backslash are taken as part of a potential backreference num-
       ber. If the pattern continues with a digit  character,  some  delimiter
       must  be  used to terminate the backreference. If the PCRE2_EXTENDED or
       PCRE2_EXTENDED_MORE option is set, this can be white space.  Otherwise,
       the \g{} syntax or an empty comment (see "Comments" below) can be used.

   Recursive backreferences

       A  backreference  that occurs inside the group to which it refers fails
       when the group is first used, so, for  example,  (a\1)  never  matches.
       However,  such references can be useful inside repeated groups. For ex-
       ample, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation of the group, the backreference matches the character string cor-
       responding to the previous iteration. In order for this  to  work,  the
       pattern  must  be  such that the first iteration does not need to match
       the backreference. This can be done using alternation, as in the  exam-
       ple above, or by a quantifier with a minimum of zero.

       For versions of PCRE2 less than 10.25, backreferences of this type used
       to  cause  the  group  that  they  reference to be treated as an atomic
       group.  This restriction no longer applies, and backtracking into  such
       groups can occur as normal.


ASSERTIONS

       An  assertion  is a test that does not consume any characters. The test
       must succeed for the match to continue. The simple assertions coded  as
       \b, \B, \A, \G, \Z, \z, ^ and $ are described above.

       More  complicated  assertions  are  coded  as  parenthesized groups. If
       matching such a group succeeds, matching continues after it,  but  with
       the matching position in the subject string reset to what it was before
       the assertion was processed.

       A  special  kind  of  assertion,  called  a "scan substring" assertion,
       matches a subpattern against a previously captured substring.  This  is
       described in the section entitled "Scan substring assertions" below. It
       is a PCRE2 extension, not compatible with Perl.

       The other goup-based assertions are of two kinds: those that look ahead
       of  the current position in the subject string, and those that look be-
       hind it, and in each case an assertion may be positive (must match  for
       the assertion to be true) or negative (must not match for the assertion
       to be true).

       The  Perl-compatible  lookaround assertions are atomic. If an assertion
       is true, but there is a subsequent matching failure, there is no  back-
       tracking  into  the assertion. However, there are some cases where non-
       atomic assertions can be useful. PCRE2 has some support for these,  de-
       scribed in the section entitled "Non-atomic assertions" below, but they
       are not Perl-compatible.

       A  lookaround  assertion  may  appear as the condition in a conditional
       group (see below). In this case, the result of matching  the  assertion
       determines which branch of the condition is followed.

       Assertion  groups are not capture groups. If an assertion contains cap-
       ture groups within it, these are counted for the purposes of  numbering
       the  capture  groups in the whole pattern. Within each branch of an as-
       sertion, locally captured substrings may be  referenced  in  the  usual
       way.  For  example,  a  sequence such as (.)\g{-1} can be used to check
       that two adjacent characters are the same.

       When a branch within an assertion fails to match, any  substrings  that
       were  captured  are  discarded (as happens with any pattern branch that
       fails to match). A  negative  assertion  is  true  only  when  all  its
       branches fail to match; this means that no captured substrings are ever
       retained  after a successful negative assertion. When an assertion con-
       tains a matching branch, what happens depends on the type of assertion.

       For a positive assertion, internally captured substrings  in  the  suc-
       cessful  branch are retained, and matching continues with the next pat-
       tern item after the assertion. For a  negative  assertion,  a  matching
       branch  means  that  the assertion is not true. If such an assertion is
       being used as a condition in a conditional group (see below),  captured
       substrings  are  retained,  because  matching  continues  with the "no"
       branch of the condition. For other failing negative assertions, control
       passes to the previous backtracking point, thus discarding any captured
       strings within the assertion.

       Most assertion groups may be repeated; though it makes no sense to  as-
       sert the same thing several times, the side effect of capturing in pos-
       itive assertions may occasionally be useful. However, an assertion that
       forms  the  condition  for  a  conditional group may not be quantified.
       PCRE2 used to restrict the repetition of assertions, but  from  release
       10.35  the  only restriction is that an unlimited maximum repetition is
       changed to be one more than the minimum. For example, {3,}  is  treated
       as {3,4}.

   Alphabetic assertion names

       Traditionally,  symbolic  sequences such as (?= and (?<= have been used
       to specify lookaround assertions. Perl 5.28 introduced some  experimen-
       tal alphabetic alternatives which might be easier to remember. They all
       start  with  (* instead of (? and must be written using lower case let-
       ters. PCRE2 supports the following synonyms:

         (*positive_lookahead:  or (*pla: is the same as (?=
         (*negative_lookahead:  or (*nla: is the same as (?!
         (*positive_lookbehind: or (*plb: is the same as (?<=
         (*negative_lookbehind: or (*nlb: is the same as (?<!

       For example, (*pla:foo) is the same assertion as (?=foo). In  the  fol-
       lowing  sections, the various assertions are described using the origi-
       nal symbolic forms.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches a word followed by a semicolon, but does not include the  semi-
       colon in the match, and

         foo(?!bar)

       matches  any  occurrence  of  "foo" that is not followed by "bar". Note
       that the apparently similar pattern

         (?!foo)bar

       does not find an occurrence of "bar"  that  is  preceded  by  something
       other  than "foo"; it finds any occurrence of "bar" whatsoever, because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most convenient way to do it is with (?!) because an empty  string  al-
       ways  matches,  so  an assertion that requires there not to be an empty
       string must always fail.  The backtracking control verb (*FAIL) or (*F)
       is a synonym for (?!).

   Lookbehind assertions

       Lookbehind assertions start with (?<= for positive assertions and  (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does  find  an  occurrence  of "bar" that is not preceded by "foo". The
       contents of a lookbehind assertion are restricted such that there  must
       be  a known maximum to the lengths of all the strings it matches. There
       are two cases:

       If every top-level alternative matches a fixed length, for example

         (?<=colour|color)

       there is a limit of 65535 characters to the lengths, which do not  have
       to  be the same, as this example demonstrates. This is the only kind of
       lookbehind supported by PCRE2 versions earlier than 10.43  and  by  the
       alternative matching function pcre2_dfa_match().

       In  PCRE2 10.43 and later, pcre2_match() supports lookbehind assertions
       in which one or more top-level alternatives can  match  more  than  one
       string length, for example

         (?<=colou?r)

       The maximum matching length for any branch of the lookbehind is limited
       to  a value set by the calling program (default 255 characters). Unlim-
       ited repetition (for example \d*) is not supported. In some cases,  the
       escape  sequence \K (see above) can be used instead of a lookbehind as-
       sertion at the start of a pattern to get round  the  length  limit  re-
       striction.

       In  UTF-8  and  UTF-16 modes, PCRE2 does not allow the \C escape (which
       matches a single code unit even in a UTF mode) to appear in  lookbehind
       assertions,  because  it makes it impossible to calculate the length of
       the lookbehind. The \X and \R escapes, which can match  different  num-
       bers of code units, are never permitted in lookbehinds.

       "Subroutine"  calls  (see below) such as (?2) or (?&X) are permitted in
       lookbehinds, as long as the called capture  group  matches  a  limited-
       length  string. However, recursion, that is, a "subroutine" call into a
       group that is already active, is not supported.

       PCRE2 supports backreferences in lookbehinds, but only if certain  con-
       ditions  are met. The PCRE2_MATCH_UNSET_BACKREF option must not be set,
       there must be no use of (?| in the pattern (it creates duplicate  group
       numbers), and if the backreference is by name, the name must be unique.
       Of course, the referenced group must itself match a limited length sub-
       string.  The  following  pattern  matches words containing at least two
       characters that begin and end with the same character:

          \b(\w)\w++(?<=\1)

       Possessive quantifiers can be used in conjunction with  lookbehind  as-
       sertions  to  specify efficient matching at the end of subject strings.
       Consider a simple pattern such as

         abcd$

       when applied to a long string that does  not  match.  Because  matching
       proceeds  from  left to right, PCRE2 will look for each "a" in the sub-
       ject and then see if what follows matches the rest of the  pattern.  If
       the pattern is specified as

         ^.*abcd$

       the  initial .* matches the entire string at first, but when this fails
       (because there is no following "a"), it backtracks to match all but the
       last character, then all but the last two characters, and so  on.  Once
       again  the search for "a" covers the entire string, from right to left,
       so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

       there can be no backtracking for the .*+ item because of the possessive
       quantifier; it can match only the entire string. The subsequent lookbe-
       hind assertion does a single test on the last four  characters.  If  it
       fails,  the  match  fails  immediately. For long strings, this approach
       makes a significant difference to the processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches "foo" preceded by three digits that are not "999". Notice  that
       each  of  the  assertions is applied independently at the same point in
       the subject string. First there is a  check  that  the  previous  three
       characters  are  all  digits,  and  then there is a check that the same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded by six characters, the first of which are  digits  and  the  last
       three  of  which  are not "999". For example, it doesn't match "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This time the first assertion looks at the  preceding  six  characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches  an occurrence of "baz" that is preceded by "bar" which in turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is another pattern that matches "foo" preceded by three digits and  any
       three characters that are not "999".


NON-ATOMIC ASSERTIONS

       Traditional  lookaround assertions are atomic. That is, if an assertion
       is true, but there is a subsequent matching failure, there is no  back-
       tracking  into  the assertion. However, there are some cases where non-
       atomic positive assertions can be useful. PCRE2  provides  these  using
       the following syntax:

         (*non_atomic_positive_lookahead:  or (*napla: or (?*
         (*non_atomic_positive_lookbehind: or (*naplb: or (?<*

       Consider  the  problem  of finding the right-most word in a string that
       also appears earlier in the string, that is, it must  appear  at  least
       twice  in  total.  This pattern returns the required result as captured
       substring 1:

         ^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2}

       For a subject such as "word1 word2 word3 word2 word3 word4" the  result
       is  "word3".  How does it work? At the start, ^(?x) anchors the pattern
       and sets the "x" option, which causes white space (introduced for read-
       ability) to be ignored. Inside the assertion, the greedy  .*  at  first
       consumes the entire string, but then has to backtrack until the rest of
       the  assertion can match a word, which is captured by group 1. In other
       words, when the assertion first succeeds, it  captures  the  right-most
       word in the string.

       The  current  matching point is then reset to the start of the subject,
       and the rest of the pattern match checks for  two  occurrences  of  the
       captured  word,  using  an  ungreedy .*? to scan from the left. If this
       succeeds, we are done, but if the last word in the string does not  oc-
       cur  twice,  this  part  of  the pattern fails. If a traditional atomic
       lookahead (?= or (*pla: had been used, the assertion could not  be  re-
       entered, and the whole match would fail. The pattern would succeed only
       if the very last word in the subject was found twice.

       Using  a  non-atomic  lookahead, however, means that when the last word
       does not occur twice in the string, the  lookahead  can  backtrack  and
       find  the second-last word, and so on, until either the match succeeds,
       or all words have been tested.

       Two conditions must be met for a non-atomic assertion to be useful: the
       contents of one or more capturing groups must change after a  backtrack
       into  the  assertion,  and  there  must be a backreference to a changed
       group later in the pattern. If this is not the case, the  rest  of  the
       pattern  match  fails exactly as before because nothing has changed, so
       using a non-atomic assertion just wastes resources.

       There is one exception to backtracking into a non-atomic assertion.  If
       an  (*ACCEPT)  control verb is triggered, the assertion succeeds atomi-
       cally. That is, a subsequent match failure cannot  backtrack  into  the
       assertion.

       Non-atomic  assertions  are  not  supported by the alternative matching
       function pcre2_dfa_match(). They are supported by JIT, but only if they
       do not contain any control verbs such as (*ACCEPT). (This may change in
       future). Note that assertions that appear as conditions for conditional
       groups (see below) must be atomic.


SCAN SUBSTRING ASSERTIONS

       A special kind of assertion, not compatible with Perl, makes it  possi-
       ble to check the contents of a captured substring by matching it with a
       subpattern.   Because this involves capturing, this feature is not sup-
       ported by pcre2_dfa_match().

       A scan substring assertion starts with the  sequence  (*scan_substring:
       or (*scs: which is followed by a list of substring numbers (absolute or
       relative)  and/or  substring  names  enclosed in single quotes or angle
       brackets, all within parentheses. The rest of the item is  the  subpat-
       tern that is applied to the substring, as shown in these examples:

         (*scan_substring:(1)...)
         (*scs:(-2)...)
         (*scs:('AB')...)
         (*scs:(1,'AB',-2)...)

       The  list  of  groups is checked in the order they are given, and it is
       the contents of the first one that is found to be set that are scanned.
       When PCRE2_DUPNAMES is set and there are  ambiguous  group  names,  all
       groups  with  the same name are checked in numerical order. A scan sub-
       string assertion fails if none of the groups it  references  have  been
       set.

       The pattern match on the substring is always anchored, that is, it must
       match  from  the  start of the substring. There is no "bumpalong" if it
       does not match at the start. The end of the subject is temporarily  re-
       set  to be the end of the substring, so \Z, \z, and $ will match there.
       However, the start of the subject is  not  reset.  This  means  that  ^
       matches only if the substring is actually at the start of the main sub-
       ject,  but  it also means that lookbehind assertions into what precedes
       the substring are possible.

       Here is a very simple example: find a word that contains the  rare  (in
       English) sequence of letters "rh" not at the start:

         \b(\w++)(*scs:(1).+rh)

       The  first  group  captures  a word which is then scanned by the second
       group.  This example does not actually need this  heavyweight  feature;
       the same match can be achieved with:

         \b\w+?rh\w*\b

       When  things  are  more  complicated, however, scanning a captured sub-
       string can be a useful way to describe the required match. For  exmple,
       there  is  a  rather  complicated  pattern  in the PCRE2 test data that
       checks an entire subject string for a palindrome, that is, the sequence
       of letters is the same in both directions. Suppose you want  to  search
       for individual words of two or more characters such as "level" that are
       palindromes:

         (\b\w{2,}+\b)(*scs:(1)...palindrome-matching-pattern...)

       Within a substring scanning subpattern, references to other groups work
       as  normal.  Capturing  groups may appear, and will retain their values
       during ongoing matching if the assertion succeeds.


SCRIPT RUNS

       In concept, a script run is a sequence of characters that are all  from
       the  same  Unicode script such as Latin or Greek. However, because some
       scripts are commonly used together, and because  some  diacritical  and
       other  marks  are  used  with  multiple scripts, it is not that simple.
       There is a full description of the rules that PCRE2 uses in the section
       entitled "Script Runs" in the pcre2unicode documentation.

       If part of a pattern is enclosed between (*script_run: or (*sr:  and  a
       closing  parenthesis,  it  fails  if the sequence of characters that it
       matches are not a script run. After a failure, normal backtracking  oc-
       curs.  Script runs can be used to detect spoofing attacks using charac-
       ters that look the same, but are from  different  scripts.  The  string
       "paypal.com"  is an infamous example, where the letters could be a mix-
       ture of Latin and Cyrillic. This pattern ensures that the matched char-
       acters in a sequence of non-spaces that follow white space are a script
       run:

         \s+(*sr:\S+)

       To be sure that they are all from the Latin  script  (for  example),  a
       lookahead can be used:

         \s+(?=\p{Latin})(*sr:\S+)

       This works as long as the first character is expected to be a character
       in  that  script,  and  not (for example) punctuation, which is allowed
       with any script. If this is not the case, a more creative lookahead  is
       needed.  For  example, if digits, underscore, and dots are permitted at
       the start:

         \s+(?=[0-9_.]*\p{Latin})(*sr:\S+)


       In many cases, backtracking into a script run pattern fragment  is  not
       desirable.  The  script run can employ an atomic group to prevent this.
       Because this is a common requirement, a shorthand notation is  provided
       by (*atomic_script_run: or (*asr:

         (*asr:...) is the same as (*sr:(?>...))

       Note that the atomic group is inside the script run. Putting it outside
       would not prevent backtracking into the script run pattern.

       Support  for  script runs is not available if PCRE2 is compiled without
       Unicode support. A compile-time error is given if any of the above con-
       structs is encountered. Script runs are not supported by the  alternate
       matching  function,  pcre2_dfa_match() because they use the same mecha-
       nism as capturing parentheses.

       Warning: The (*ACCEPT) control verb (see  below)  should  not  be  used
       within a script run group, because it causes an immediate exit from the
       group, bypassing the script run checking.


CONDITIONAL GROUPS

       It is possible to cause the matching process to obey a pattern fragment
       conditionally or to choose between two alternative fragments, depending
       on  the result of an assertion, or whether a specific capture group has
       already been matched. The two possible forms of conditional group are:

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If the condition is satisfied, the yes-pattern is used;  otherwise  the
       no-pattern  (if present) is used. An absent no-pattern is equivalent to
       an empty string (it always matches). If there are more than two  alter-
       natives  in the group, a compile-time error occurs. Each of the two al-
       ternatives may itself contain nested groups of any form, including con-
       ditional groups; the restriction to two alternatives  applies  only  at
       the  level of the condition itself. This pattern fragment is an example
       where the alternatives are complex:

         (?(1) (A|B|C) | (D | (?(2)E|F) | E) )


       There are five kinds of condition: references to capture groups, refer-
       ences to recursion, two pseudo-conditions called  DEFINE  and  VERSION,
       and assertions.

   Checking for a used capture group by number

       If  the  text between the parentheses consists of a sequence of digits,
       the condition is true if a capture group of that number has  previously
       matched.  If  there is more than one capture group with the same number
       (see the earlier section about duplicate group numbers), the  condition
       is  true if any of them have matched. An alternative notation, which is
       a PCRE2 extension, not supported by Perl, is to precede the digits with
       a plus or minus sign. In this case, the group number is relative rather
       than absolute. The most recently opened capture group (which  could  be
       enclosing  this  condition)  can be referenced by (?(-1), the next most
       recent by (?(-2), and so on. Inside loops it can also make sense to re-
       fer to subsequent groups.  The next capture group to be opened  can  be
       referenced  as  (?(+1), and so on. The value zero in any of these forms
       is not used; it provokes a compile-time error.

       Consider the following pattern, which  contains  non-significant  white
       space  to  make it more readable (assume the PCRE2_EXTENDED option) and
       to divide it into three parts for ease of discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The first part matches an optional opening  parenthesis,  and  if  that
       character is present, sets it as the first captured substring. The sec-
       ond  part  matches one or more characters that are not parentheses. The
       third part is a conditional group that tests whether or not  the  first
       capture  group  matched. If it did, that is, if subject started with an
       opening parenthesis, the condition is true, and so the  yes-pattern  is
       executed  and  a  closing parenthesis is required. Otherwise, since no-
       pattern is not present, the conditional group matches nothing. In other
       words, this pattern matches a sequence of  non-parentheses,  optionally
       enclosed in parentheses.

       If  you  were  embedding  this pattern in a larger one, you could use a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This makes the fragment independent of the parentheses  in  the  larger
       pattern.

   Checking for a used capture group by name

       Perl  uses  the  syntax  (?(<name>)...) or (?('name')...) to test for a
       used capture group by name. For compatibility with earlier versions  of
       PCRE1,  which had this facility before Perl, the syntax (?(name)...) is
       also recognized.  Note, however, that undelimited names  consisting  of
       the  letter  R followed by digits are ambiguous (see the following sec-
       tion). Rewriting the above example to use a named group gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

       If the name used in a condition of this kind is a duplicate,  the  test
       is  applied  to  all groups of the same name, and is true if any one of
       them has matched.

   Checking for pattern recursion

       "Recursion" in this sense refers to any subroutine-like call  from  one
       part  of  the  pattern to another, whether or not it is actually recur-
       sive. See the sections entitled "Recursive  patterns"  and  "Groups  as
       subroutines" below for details of recursion and subroutine calls.

       If  a  condition  is the string (R), and there is no capture group with
       the name R, the condition is true if matching is currently in a  recur-
       sion  or  subroutine call to the whole pattern or any capture group. If
       digits follow the letter R, and there is no group with that  name,  the
       condition  is  true  if  the  most recent call is into a group with the
       given number, which must exist somewhere in the overall  pattern.  This
       is a contrived example that is equivalent to a+b:

         ((?(R1)a+|(?1)b))

       However,  in  both  cases,  if there is a capture group with a matching
       name, the condition tests for its being set, as described in  the  sec-
       tion  above,  instead of testing for recursion. For example, creating a
       group with the name R1 by adding (?<R1>)  to  the  above  pattern  com-
       pletely changes its meaning.

       If a name preceded by ampersand follows the letter R, for example:

         (?(R&name)...)

       the  condition  is true if the most recent recursion is into a group of
       that name (which must exist within the pattern).

       This condition does not check the entire recursion stack. It tests only
       the current level. If the name used in a condition of this  kind  is  a
       duplicate,  the  test is applied to all groups of the same name, and is
       true if any one of them is the most recent recursion.

       At "top level", all these recursion test conditions are false.

   Defining capture groups for use by reference only

       If the condition is the string (DEFINE), the condition is always false,
       even if there is a group with the name DEFINE. In this case, there  may
       be only one alternative in the rest of the conditional group. It is al-
       ways  skipped if control reaches this point in the pattern; the idea of
       DEFINE is that it can be used to define subroutines that can be  refer-
       enced  from elsewhere. (The use of subroutines is described below.) For
       example, a pattern to match an IPv4 address  such  as  "192.168.23.245"
       could be written like this (ignore white space and line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The  first  part  of the pattern is a DEFINE group inside which another
       group named "byte" is defined. This matches an individual component  of
       an  IPv4  address  (a number less than 256). When matching takes place,
       this part of the pattern is skipped because DEFINE acts  like  a  false
       condition.  The  rest of the pattern uses references to the named group
       to match the four dot-separated components of an IPv4 address,  insist-
       ing on a word boundary at each end.

   Checking the PCRE2 version

       Programs  that link with a PCRE2 library can check the version by call-
       ing pcre2_config() with appropriate arguments.  Users  of  applications
       that  do  not have access to the underlying code cannot do this. A spe-
       cial "condition" called VERSION exists to allow such users to  discover
       which version of PCRE2 they are dealing with by using this condition to
       match  a string such as "yesno". VERSION must be followed either by "="
       or ">=" and a version number.  For example:

         (?(VERSION>=10.4)yes|no)

       This pattern matches "yes" if the PCRE2 version is greater or equal  to
       10.4,  or  "no"  otherwise.  The  fractional part of the version number
       could be ommited.

   Assertion conditions

       If the condition is not in any of the  above  formats,  it  must  be  a
       parenthesized  assertion.  This may be a positive or negative lookahead
       or lookbehind assertion. However, it must be a traditional  atomic  as-
       sertion, not one of the non-atomic assertions.

       Consider  this  pattern,  again containing non-significant white space,
       and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The condition is a positive lookahead assertion  that  matches  an  op-
       tional sequence of non-letters followed by a letter. In other words, it
       tests for the presence of at least one letter in the subject. If a let-
       ter  is  found,  the  subject is matched against the first alternative;
       otherwise it is  matched  against  the  second.  This  pattern  matches
       strings  in  one  of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
       letters and dd are digits.

       When an assertion that is a condition contains capture groups, any cap-
       turing that occurs in a matching branch  is  retained  afterwards,  for
       both  positive and negative assertions, because matching always contin-
       ues after the assertion, whether it succeeds or  fails.  (Compare  non-
       conditional  assertions, for which captures are retained only for posi-
       tive assertions that succeed.)


COMMENTS

       There are two ways of including comments in patterns that are processed
       by PCRE2. In both cases, the start of the comment  must  not  be  in  a
       character  class,  nor  in  the middle of any other sequence of related
       characters such as (?: or a group name or number or a Unicode  property
       name. The characters that make up a comment play no part in the pattern
       matching.

       The  sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses are not permitted. If  the
       PCRE2_EXTENDED  or  PCRE2_EXTENDED_MORE  option  is set, an unescaped #
       character also introduces a comment, which in this  case  continues  to
       immediately  after  the next newline character or character sequence in
       the pattern. Which characters are interpreted as newlines is controlled
       by an option passed to the compiling function or by a special  sequence
       at the start of the pattern, as described in the section entitled "New-
       line conventions" above. Note that the end of this type of comment is a
       literal  newline  sequence in the pattern; escape sequences that happen
       to represent a newline do not count. For example, consider this pattern
       when PCRE2_EXTENDED is set, and the default newline convention (a  sin-
       gle linefeed character) is in force:

         abc #comment \n still comment

       On  encountering  the # character, pcre2_compile() skips along, looking
       for a newline in the pattern. The sequence \n is still literal at  this
       stage,  so  it does not terminate the comment. Only an actual character
       with the code value 0x0a (the default newline) does so.


RECURSIVE PATTERNS

       Consider the problem of matching a string in parentheses, allowing  for
       unlimited  nested  parentheses.  Without the use of recursion, the best
       that can be done is to use a pattern that  matches  up  to  some  fixed
       depth  of  nesting.  It  is not possible to handle an arbitrary nesting
       depth.

       For some time, Perl has provided a facility that allows regular expres-
       sions to recurse (amongst other things). It does this by  interpolating
       Perl  code in the expression at run time, and the code can refer to the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously, PCRE2 cannot support the interpolation  of  Perl  code.  In-
       stead,  it supports special syntax for recursion of the entire pattern,
       and also for individual capture group recursion. After its introduction
       in PCRE1 and Python, this kind of recursion was subsequently introduced
       into Perl at release 5.10.

       A special item that consists of (? followed by a  number  greater  than
       zero  and  a  closing parenthesis is a recursive subroutine call of the
       capture group of the given number, provided that it occurs inside  that
       group.  (If  not,  it  is a non-recursive subroutine call, which is de-
       scribed in the next section.) The special item (?R) or (?0) is a recur-
       sive call of the entire regular expression.

       This PCRE2 pattern solves the nested parentheses  problem  (assume  the
       PCRE2_EXTENDED option is set so that white space is ignored):

         \( ( [^()]++ | (?R) )* \)

       First  it matches an opening parenthesis. Then it matches any number of
       substrings which can either be a sequence of non-parentheses, or a  re-
       cursive match of the pattern itself (that is, a correctly parenthesized
       substring).   Finally there is a closing parenthesis. Note the use of a
       possessive quantifier to avoid  backtracking  into  sequences  of  non-
       parentheses.

       If  this  were  part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

         ( \( ( [^()]++ | (?1) )* \) )

       We have put the pattern into parentheses, and caused the  recursion  to
       refer to them instead of the whole pattern.

       In  a  larger  pattern,  keeping  track  of  parenthesis numbers can be
       tricky. This is made easier by the use of relative references.  Instead
       of (?1) in the pattern above you can write (?-2) to refer to the second
       most  recently  opened  parentheses  preceding  the recursion. In other
       words, a negative number counts capturing  parentheses  leftwards  from
       the point at which it is encountered.

       Be  aware  however, that if duplicate capture group numbers are in use,
       relative references refer to the earliest group  with  the  appropriate
       number. Consider, for example:

         (?|(a)|(b)) (c) (?-2)

       The first two capture groups (a) and (b) are both numbered 1, and group
       (c)  is  number  2. When the reference (?-2) is encountered, the second
       most recently opened parentheses has the number 1, but it is the  first
       such group (the (a) group) to which the recursion refers. This would be
       the  same if an absolute reference (?1) was used. In other words, rela-
       tive references are just a shorthand for computing a group number.

       It is also possible to refer to subsequent capture groups,  by  writing
       references  such  as  (?+2). However, these cannot be recursive because
       the reference is not inside the parentheses that are  referenced.  They
       are  always  non-recursive  subroutine  calls, as described in the next
       section.

       An alternative approach is to use named parentheses.  The  Perl  syntax
       for  this  is  (?&name);  PCRE1's earlier syntax (?P>name) is also sup-
       ported. We could rewrite the above example as follows:

         (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If there is more than one group with the same name, the earliest one is
       used.

       The example pattern that we have been looking at contains nested unlim-
       ited repeats, and so the use of a possessive  quantifier  for  matching
       strings  of  non-parentheses  is important when applying the pattern to
       strings that do not match. For example, when this pattern is applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it yields "no match" quickly. However, if a  possessive  quantifier  is
       not  used, the match runs for a very long time indeed because there are
       so many different ways the + and * repeats can carve  up  the  subject,
       and all have to be tested before failure can be reported.

       At  the  end  of a match, the values of capturing parentheses are those
       from the outermost level. If you want to obtain intermediate values,  a
       callout function can be used (see below and the pcre2callout documenta-
       tion). If the pattern above is matched against

         (ab(cd)ef)

       the  value  for  the  inner capturing parentheses (numbered 2) is "ef",
       which is the last value taken on at the top level. If a  capture  group
       is  not  matched  at  the top level, its final captured value is unset,
       even if it was (temporarily) set at a deeper level during the  matching
       process.

       Do  not  confuse  the (?R) item with the condition (R), which tests for
       recursion.  Consider this pattern, which matches text in  angle  brack-
       ets,  allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are  permit-
       ted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In  this  pattern,  (?(R) is the start of a conditional group, with two
       different alternatives for the recursive and non-recursive  cases.  The
       (?R) item is the actual recursive call.

   Differences in recursion processing between PCRE2 and Perl

       Some former differences between PCRE2 and Perl no longer exist.

       Before  release 10.30, recursion processing in PCRE2 differed from Perl
       in that a recursive subroutine call was always  treated  as  an  atomic
       group.  That is, once it had matched some of the subject string, it was
       never re-entered, even if it contained untried alternatives  and  there
       was  a  subsequent matching failure. (Historical note: PCRE implemented
       recursion before Perl did.)

       Starting with release 10.30, recursive subroutine calls are  no  longer
       treated as atomic. That is, they can be re-entered to try unused alter-
       natives  if  there  is a matching failure later in the pattern. This is
       now compatible with the way Perl works. If you want a  subroutine  call
       to be atomic, you must explicitly enclose it in an atomic group.

       Supporting backtracking into recursions simplifies certain types of re-
       cursive pattern. For example, this pattern matches palindromic strings:

         ^((.)(?1)\2|.?)$

       The  second  branch  in the group matches a single central character in
       the palindrome when there are an odd number of characters,  or  nothing
       when  there  are  an even number of characters, but in order to work it
       has to be able to try the second case when  the  rest  of  the  pattern
       match fails. If you want to match typical palindromic phrases, the pat-
       tern  has  to  ignore  all  non-word characters, which can be done like
       this:

         ^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$

       If run with the PCRE2_CASELESS option,  this  pattern  matches  phrases
       such  as "A man, a plan, a canal: Panama!". Note the use of the posses-
       sive quantifier *+ to avoid backtracking  into  sequences  of  non-word
       characters. Without this, PCRE2 takes a great deal longer (ten times or
       more)  to  match typical phrases, and Perl takes so long that you think
       it has gone into a loop.

       Another way in which PCRE2 and Perl used to differ in  their  recursion
       processing  is  in  the  handling of captured values. Formerly in Perl,
       when a group was called recursively or as a subroutine  (see  the  next
       section), it had no access to any values that were captured outside the
       recursion,  whereas  in  PCRE2 these values can be referenced. Consider
       this pattern:

         ^(.)(\1|a(?2))

       This pattern matches "bab". The first capturing parentheses match  "b",
       then in the second group, when the backreference \1 fails to match "b",
       the second alternative matches "a" and then recurses. In the recursion,
       \1  does now match "b" and so the whole match succeeds. This match used
       to fail in Perl, but in later versions (I tried 5.024) it now works.

   Groups as subroutines

       If the syntax for a recursive group call (either by number or by  name)
       is  used  outside the parentheses to which it refers, it operates a bit
       like a subroutine in a programming  language.  More  accurately,  PCRE2
       treats the referenced group as an independent subpattern which it tries
       to  match at the current matching position. The called group may be de-
       fined before or after the reference. A numbered reference  can  be  ab-
       solute or relative, as in these examples:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility" as well as the  other
       two  strings.  Another  example  is  given  in the discussion of DEFINE
       above.

       Like recursions, subroutine calls used to be  treated  as  atomic,  but
       this  changed  at  PCRE2 release 10.30, so backtracking into subroutine
       calls can now occur. However, any capturing parentheses  that  are  set
       during the subroutine call revert to their previous values afterwards.

       Processing  options such as case-independence are fixed when a group is
       defined, so if it is used as  a  subroutine,  such  options  cannot  be
       changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It  matches  "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called group.

       The behaviour of backtracking control verbs in groups  when  called  as
       subroutines is described in the section entitled "Backtracking verbs in
       subroutines" below.

   Recursion and subroutines with returned capture groups

       Since  PCRE2  10.47,  recursion and subroutine calls may also specify a
       list of capture groups to return. This is a PCRE2 syntax extension  not
       supported  by  Perl.  The pattern matching recurses into the referenced
       expression as described above, however, when the recursion  returns  to
       the  calling expression the subgroups captured during the recursion can
       be retained when the calling expression's context is restored.

       When used as a subroutine, this allows the subroutine's capture  groups
       to be used as return values.

       Only the specific capture groups listed by the caller will be retained,
       using the following syntax:

         (?R(grouplist))       recurse whole pattern, returning capture groups
         (?n(grouplist))       )
         (?+n(grouplist))      )
         (?-n(grouplist))      ) call subroutine, returning capture groups
         (?&name(grouplist))   )
         (?P>name(grouplist))  )

       The  list  of  capture  groups "grouplist" is a comma-separated list of
       (absolute or relative) group numbers, and group names enclosed in  sin-
       gle quotes or angle brackets.

       Here  is  an  example which first uses the DEFINE condition to create a
       re-usable routine for matching a weekday, then  calls  that  subroutine
       and retains the groups it captures for use later:

         (?x: # ignore whitespace for clarity
           # Define the routine "weekendday" which matches Saturday or
           # Sunday, and returns the Sat/Sun prefix as \k<short>.
           (?(DEFINE) (?<weekendday>
               (?|(?<short>Sat)urday|(?<short>Sun)day) ) )
           # Call the routine. Matches "Saturday,Sat" or "Sunday,Sun".
           (?&weekendday(<short>)),\k<short> )

       This  feature  is  not  available using the Oniguruma syntax \g<...> or
       \g'...'  below.

   Oniguruma subroutine syntax

       For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for calling a group as a subroutine, possibly re-
       cursively.  Here  are  two  of the examples used above, rewritten using
       this syntax:

         (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
         (sens|respons)e and \g'1'ibility

       PCRE2 supports an extension to Oniguruma: if a number is preceded by  a
       plus or a minus sign it is taken as a relative reference. For example:

         (abc)(?i:\g<-1>)

       Note  that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a backreference; the latter is  a  subroutine
       call.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl  code to be obeyed in the middle of matching a regular expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE2 provides a similar feature, but of course it  cannot  obey  arbi-
       trary  Perl  code. The feature is called "callout". The caller of PCRE2
       provides an external function by putting its entry  point  in  a  match
       context  using  the function pcre2_set_callout(), and then passing that
       context to pcre2_match() or pcre2_dfa_match(). If no match  context  is
       passed,  or  if  the callout entry point is set to NULL, callout points
       will be passed over silently during matching. To disallow  callouts  in
       the pattern syntax, you may use the PCRE2_EXTRA_NEVER_CALLOUT option.

       Within  a  regular expression, (?C<arg>) indicates a point at which the
       external function is to be called. There  are  two  kinds  of  callout:
       those  with a numerical argument and those with a string argument. (?C)
       on its own with no argument is treated as (?C0). A  numerical  argument
       allows  the  application  to  distinguish  between  different callouts.
       String arguments were added for release 10.20 to make it  possible  for
       script  languages that use PCRE2 to embed short scripts within patterns
       in a similar way to Perl.

       During matching, when PCRE2 reaches a callout point, the external func-
       tion is called. It is provided with the number or  string  argument  of
       the  callout, the position in the pattern, and one item of data that is
       also set in the match block. The callout function may cause matching to
       proceed, to backtrack, or to fail.

       By default, PCRE2 implements a  number  of  optimizations  at  matching
       time,  and  one  side-effect is that sometimes callouts are skipped. If
       you need all possible callouts to happen, you need to set options  that
       disable  the relevant optimizations. More details, including a complete
       description of the programming interface to the callout  function,  are
       given in the pcre2callout documentation.

   Callouts with numerical arguments

       If  you  just  want  to  have  a means of identifying different callout
       points, put a number less than 256 after the  letter  C.  For  example,
       this pattern has two callout points:

         (?C1)abc(?C2)def

       If  the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
       callouts are automatically installed before each item in  the  pattern.
       They  are all numbered 255. If there is a conditional group in the pat-
       tern whose condition is an assertion, an additional callout is inserted
       just before the condition. An explicit callout may also be set at  this
       position, as in this example:

         (?(?C9)(?=a)abc|def)

       Note that this applies only to assertion conditions, not to other types
       of condition.

   Callouts with string arguments

       A  delimited  string may be used instead of a number as a callout argu-
       ment. The starting delimiter must be one of ` ' " ^ % #  $  {  and  the
       ending delimiter is the same as the start, except for {, where the end-
       ing  delimiter  is  }.  If  the  ending  delimiter is needed within the
       string, it must be doubled. For example:

         (?C'ab ''c'' d')xyz(?C{any text})pqr

       The doubling is removed before the string  is  passed  to  the  callout
       function.


BACKTRACKING CONTROL

       There  are  a  number  of  special "Backtracking Control Verbs" (to use
       Perl's terminology) that modify the behaviour  of  backtracking  during
       matching.  They are generally of the form (*VERB) or (*VERB:NAME). Some
       verbs take either form, and may behave differently depending on whether
       or not a name argument is present. The names are  not  required  to  be
       unique within the pattern.

       By  default,  for  compatibility  with  Perl, a name is any sequence of
       characters that does not include a closing parenthesis. The name is not
       processed in any way, and it is  not  possible  to  include  a  closing
       parenthesis   in  the  name.   This  can  be  changed  by  setting  the
       PCRE2_ALT_VERBNAMES option, but the result is no  longer  Perl-compati-
       ble.

       When  PCRE2_ALT_VERBNAMES  is  set,  backslash processing is applied to
       verb names and only an unescaped  closing  parenthesis  terminates  the
       name.  However, the only backslash items that are permitted are \Q, \E,
       and sequences such as \x{100} that define character code points.  Char-
       acter type escapes such as \d are faulted.

       A closing parenthesis can be included in a name either as \) or between
       \Q  and  \E. In addition to backslash processing, if the PCRE2_EXTENDED
       or PCRE2_EXTENDED_MORE option is also set,  unescaped  white  space  in
       verb names is skipped, and #-comments are recognized, exactly as in the
       rest of the pattern.  PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not af-
       fect verb names unless PCRE2_ALT_VERBNAMES is also set.

       The  maximum  length of a name is 255 in the 8-bit library and 65535 in
       the 16-bit and 32-bit libraries. If the name is empty, that is, if  the
       closing  parenthesis immediately follows the colon, the effect is as if
       the colon were not there. Any number of these verbs may occur in a pat-
       tern. Except for (*ACCEPT), they may not be quantified.

       Since these verbs are specifically related  to  backtracking,  most  of
       them  can be used only when the pattern is to be matched using the tra-
       ditional matching function or JIT, because they use backtracking  algo-
       rithms.  With  the  exception  of (*FAIL), which behaves like a failing
       negative assertion, the backtracking control verbs cause  an  error  if
       encountered by the DFA matching function.

       The  behaviour  of  these  verbs in repeated groups, assertions, and in
       capture groups called as subroutines (whether or  not  recursively)  is
       documented below.

   Optimizations that affect backtracking verbs

       PCRE2 contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may  know  the minimum length of matching subject, or that a particular
       character must be present. When one of these optimizations bypasses the
       running of a match,  any  included  backtracking  verbs  will  not,  of
       course, be processed. You can suppress the start-of-match optimizations
       by  setting  the PCRE2_NO_START_OPTIMIZE option when calling pcre2_com-
       pile(), by calling pcre2_set_optimize() with a PCRE2_START_OPTIMIZE_OFF
       directive, or by starting the pattern with  (*NO_START_OPT).  There  is
       more  discussion  of  this  option in the section entitled "Compiling a
       pattern" in the pcre2api documentation.

       Experiments with Perl suggest that it too  has  similar  optimizations,
       and like PCRE2, turning them off can change the result of a match.

   Verbs that act immediately

       The following verbs act as soon as they are encountered.

          (*ACCEPT) or (*ACCEPT:NAME)

       This  verb causes the match to end successfully, skipping the remainder
       of the pattern. However, when it is inside  a  capture  group  that  is
       called as a subroutine, only that group is ended successfully. Matching
       then continues at the outer level. If (*ACCEPT) in triggered in a posi-
       tive  assertion,  the  assertion succeeds; in a negative assertion, the
       assertion fails.

       If (*ACCEPT) is inside capturing parentheses, the data so far  is  cap-
       tured. For example:

         A((?:A|B(*ACCEPT)|C)D)

       This  matches  "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
       tured by the outer parentheses.

       (*ACCEPT) is the only backtracking verb that is allowed to  be  quanti-
       fied  because  an  ungreedy  quantification with a minimum of zero acts
       only when a backtrack happens. Consider, for example,

         (A(*ACCEPT)??B)C

       where A, B, and C may be complex expressions. After matching  "A",  the
       matcher  processes  "BC"; if that fails, causing a backtrack, (*ACCEPT)
       is triggered and the match succeeds. In both cases, all but C  is  cap-
       tured.  Whereas  (*COMMIT) (see below) means "fail on backtrack", a re-
       peated (*ACCEPT) of this type means "succeed on backtrack".

       Warning: (*ACCEPT) should not be used within a script  run  group,  be-
       cause  it causes an immediate exit from the group, bypassing the script
       run checking.

         (*FAIL) or (*FAIL:NAME)

       This verb causes a matching failure, forcing backtracking to occur.  It
       may  be  abbreviated  to  (*F).  It is equivalent to (?!) but easier to
       read. The Perl documentation notes that it is probably useful only when
       combined with (?{}) or (??{}). Those are, of course, Perl features that
       are not present in PCRE2. The nearest equivalent is  the  callout  fea-
       ture, as for example in this pattern:

         a+(?C)(*FAIL)

       A  match  with the string "aaaa" always fails, but the callout is taken
       before each backtrack happens (in this example, 10 times).

       (*ACCEPT:NAME) and (*FAIL:NAME) behave the  same  as  (*MARK:NAME)(*AC-
       CEPT)  and  (*MARK:NAME)(*FAIL),  respectively,  that  is, a (*MARK) is
       recorded just before the verb acts.

   Recording which path was taken

       There is one verb whose main purpose is to track how a  match  was  ar-
       rived  at,  though  it also has a secondary use in conjunction with ad-
       vancing the match starting point (see (*SKIP) below).

         (*MARK:NAME) or (*:NAME)

       A name is always required with this verb. For all the other  backtrack-
       ing control verbs, a NAME argument is optional.

       When  a  match  succeeds, the name of the last-encountered mark name on
       the matching path is passed back to the caller as described in the sec-
       tion entitled "Other information about the match" in the pcre2api docu-
       mentation. This applies to all instances of (*MARK)  and  other  verbs,
       including those inside assertions and atomic groups. However, there are
       differences  in  those  cases  when (*MARK) is used in conjunction with
       (*SKIP) as described below.

       The mark name that was last encountered on the matching path is  passed
       back.  A verb without a NAME argument is ignored for this purpose. Here
       is an example of pcre2test output, where the "mark"  modifier  requests
       the retrieval and outputting of (*MARK) data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this exam-
       ple  it indicates which of the two alternatives matched. This is a more
       efficient way of obtaining this information than putting each  alterna-
       tive in its own capturing parentheses.

       If  a  verb  with a name is encountered in a positive assertion that is
       true, the name is recorded and passed back if it  is  the  last-encoun-
       tered. This does not happen for negative assertions or failing positive
       assertions.

       After  a  partial match or a failed match, the last encountered name in
       the entire match process is returned. For example:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
         data> XP
         No match, mark = B

       Note that in this unanchored example the  mark  is  retained  from  the
       match attempt that started at the letter "X" in the subject. Subsequent
       match attempts starting at "P" and then with an empty string do not get
       as far as the (*MARK) item, but nevertheless do not reset it.

       If  you  are  interested  in  (*MARK)  values after failed matches, you
       should probably either set the PCRE2_NO_START_OPTIMIZE option  or  call
       pcre2_set_optimize()  with  a  PCRE2_START_OPTIMIZE_OFF  directive (see
       above) to ensure that the match is always attempted.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching con-
       tinues with what follows, but if there is a subsequent  match  failure,
       causing  a  backtrack  to the verb, a failure is forced. That is, back-
       tracking cannot pass to the left of the  verb.  However,  when  one  of
       these  verbs  appears inside an atomic group or in an atomic lookaround
       assertion that is true, its effect is confined to that  group,  because
       once  the  group has been matched, there is never any backtracking into
       it. Backtracking from beyond an atomic assertion or group  ignores  the
       entire group, and seeks a preceding backtracking point.

       These  verbs  differ  in exactly what kind of failure occurs when back-
       tracking reaches them. The behaviour described below  is  what  happens
       when  the  verb is not in a subroutine or an assertion. Subsequent sec-
       tions cover these special cases.

         (*COMMIT) or (*COMMIT:NAME)

       This verb causes the whole match to fail outright if there is  a  later
       matching failure that causes backtracking to reach it. Even if the pat-
       tern  is  unanchored,  no further attempts to find a match by advancing
       the starting point take place. If (*COMMIT) is  the  only  backtracking
       verb that is encountered, once it has been passed pcre2_match() is com-
       mitted to finding a match at the current starting point, or not at all.
       For example:

         a+(*COMMIT)b

       This  matches  "xxaab" but not "aacaab". It can be thought of as a kind
       of dynamic anchor, or "I've started, so I must finish."

       The behaviour of (*COMMIT:NAME) is not the same  as  (*MARK:NAME)(*COM-
       MIT).  It is like (*MARK:NAME) in that the name is remembered for pass-
       ing back to the caller. However, (*SKIP:NAME) searches only  for  names
       that are set with (*MARK), ignoring those set by any of the other back-
       tracking verbs.

       If  there  is more than one backtracking verb in a pattern, a different
       one that follows (*COMMIT) may be triggered first,  so  merely  passing
       (*COMMIT) during a match does not always guarantee that a match must be
       at this starting point.

       Note that (*COMMIT) at the start of a pattern is not the same as an an-
       chor,  unless  PCRE2's  start-of-match optimizations are turned off, as
       shown in this output from pcre2test:

           re> /(*COMMIT)abc/
         data> xyzabc
          0: abc
         data>
         re> /(*COMMIT)abc/no_start_optimize
         data> xyzabc
         No match

       For the first pattern, PCRE2 knows that any match must start with  "a",
       so  the optimization skips along the subject to "a" before applying the
       pattern to the first set of data. The match attempt then succeeds.  The
       second  pattern disables the optimization that skips along to the first
       character. The pattern is now applied  starting  at  "x",  and  so  the
       (*COMMIT)  causes  the  match to fail without trying any other starting
       points.

         (*PRUNE) or (*PRUNE:NAME)

       This verb causes the match to fail at the current starting position  in
       the subject if there is a later matching failure that causes backtrack-
       ing  to  reach it. If the pattern is unanchored, the normal "bumpalong"
       advance to the next starting character then happens.  Backtracking  can
       occur  as  usual to the left of (*PRUNE), before it is reached, or when
       matching to the right of (*PRUNE), but if there  is  no  match  to  the
       right,  backtracking cannot cross (*PRUNE). In simple cases, the use of
       (*PRUNE) is just an alternative to an atomic group or possessive  quan-
       tifier, but there are some uses of (*PRUNE) that cannot be expressed in
       any  other  way. In an anchored pattern (*PRUNE) has the same effect as
       (*COMMIT).

       The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to the caller. However, (*SKIP:NAME) searches only for names  set  with
       (*MARK), ignoring those set by other backtracking verbs.

         (*SKIP)

       This  verb, when given without a name, is like (*PRUNE), except that if
       the pattern is unanchored, the "bumpalong" advance is not to  the  next
       character, but to the position in the subject where (*SKIP) was encoun-
       tered.  (*SKIP)  signifies that whatever text was matched leading up to
       it cannot be part of a successful match if there is a  later  mismatch.
       Consider:

         a+(*SKIP)b

       If  the  subject  is  "aaaac...",  after  the first match attempt fails
       (starting at the first character in the  string),  the  starting  point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifier does not have the same effect as this example; although it would
       suppress  backtracking  during  the first match attempt, the second at-
       tempt would start at the second character instead  of  skipping  on  to
       "c".

       If  (*SKIP) is used to specify a new starting position that is the same
       as the starting position of the current match, or (by  being  inside  a
       lookbehind)  earlier, the position specified by (*SKIP) is ignored, and
       instead the normal "bumpalong" occurs.

         (*SKIP:NAME)

       When (*SKIP) has an associated name, its behaviour  is  modified.  When
       such  a  (*SKIP) is triggered, the previous path through the pattern is
       searched for the most recent (*MARK) that has the same name. If one  is
       found,  the  "bumpalong" advance is to the subject position that corre-
       sponds to that (*MARK) instead of to where (*SKIP) was encountered.  If
       no (*MARK) with a matching name is found, the (*SKIP) is ignored.

       The  search  for a (*MARK) name uses the normal backtracking mechanism,
       which means that it does not  see  (*MARK)  settings  that  are  inside
       atomic groups or assertions, because they are never re-entered by back-
       tracking. Compare the following pcre2test examples:

           re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/
         data: abc
          0: a
          1: a
         data:
           re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/
         data: abc
          0: b
          1: b

       In  the first example, the (*MARK) setting is in an atomic group, so it
       is not seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored.
       This allows the second branch of the pattern to be tried at  the  first
       character  position.  In the second example, the (*MARK) setting is not
       in an atomic group. This allows (*SKIP:X) to find the (*MARK)  when  it
       backtracks, and this causes a new matching attempt to start at the sec-
       ond  character.  This  time, the (*MARK) is never seen because "a" does
       not match "b", so the matcher immediately jumps to the second branch of
       the pattern.

       Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME).  It
       ignores names that are set by other backtracking verbs.

         (*THEN) or (*THEN:NAME)

       This  verb  causes  a skip to the next innermost alternative when back-
       tracking reaches it. That  is,  it  cancels  any  further  backtracking
       within  the  current  alternative.  Its name comes from the observation
       that it can be used for a pattern-based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If the COND1 pattern matches, FOO is tried (and possibly further  items
       after  the  end  of the group if FOO succeeds); on failure, the matcher
       skips to the second alternative and tries COND2,  without  backtracking
       into  COND1.  If that succeeds and BAR fails, COND3 is tried. If subse-
       quently BAZ fails, there are no more alternatives, so there is a  back-
       track  to  whatever came before the entire group. If (*THEN) is not in-
       side an alternation, it acts like (*PRUNE).

       The behaviour of (*THEN:NAME) is not the same  as  (*MARK:NAME)(*THEN).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to  the  caller. However, (*SKIP:NAME) searches only for names set with
       (*MARK), ignoring those set by other backtracking verbs.

       A group that does not contain a | character is just a part of  the  en-
       closing  alternative;  it is not a nested alternation with only one al-
       ternative. The effect of (*THEN) extends beyond such a group to the en-
       closing alternative.  Consider this pattern, where A, B, etc. are  com-
       plex  pattern  fragments  that  do not contain any | characters at this
       level:

         A (B(*THEN)C) | D

       If A and B are matched, but there is a failure in C, matching does  not
       backtrack into A; instead it moves to the next alternative, that is, D.
       However,  if  the  group containing (*THEN) is given an alternative, it
       behaves differently:

         A (B(*THEN)C | (*FAIL)) | D

       The effect of (*THEN) is now confined to the inner group. After a fail-
       ure in C, matching moves to (*FAIL), which causes the  whole  group  to
       fail  because  there  are  no  more  alternatives to try. In this case,
       matching does backtrack into A.

       Note that a conditional group is not considered as having two  alterna-
       tives,  because  only one is ever used. In other words, the | character
       in a conditional group has a different meaning. Ignoring  white  space,
       consider:

         ^.*? (?(?=a) a | b(*THEN)c )

       If the subject is "ba", this pattern does not match. Because .*? is un-
       greedy,  it initially matches zero characters. The condition (?=a) then
       fails, the character "b" is matched, but "c" is  not.  At  this  point,
       matching  does  not  backtrack to .*? as might perhaps be expected from
       the presence of the | character. The conditional group is part  of  the
       single  alternative  that comprises the whole pattern, and so the match
       fails. (If there was a backtrack into .*?, allowing it  to  match  "b",
       the match would succeed.)

       The  verbs just described provide four different "strengths" of control
       when subsequent matching fails. (*THEN) is the weakest, carrying on the
       match at the next alternative. (*PRUNE) comes next, failing  the  match
       at  the  current starting position, but allowing an advance to the next
       character (for an unanchored pattern). (*SKIP) is similar, except  that
       the advance may be more than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

   More than one backtracking verb

       If  more  than  one  backtracking verb is present in a pattern, the one
       that is backtracked onto first acts. For example,  consider  this  pat-
       tern, where A, B, etc. are complex pattern fragments:

         (A(*COMMIT)B(*THEN)C|ABD)

       If  A matches but B fails, the backtrack to (*COMMIT) causes the entire
       match to fail. However, if A and B match, but C fails, the backtrack to
       (*THEN) causes the next alternative (ABD) to be tried.  This  behaviour
       is  consistent,  but is not always the same as Perl's. It means that if
       two or more backtracking verbs appear in succession, all but  the  last
       of them has no effect. Consider this example:

         ...(*COMMIT)(*PRUNE)...

       If there is a matching failure to the right, backtracking onto (*PRUNE)
       causes  it to be triggered, and its action is taken. There can never be
       a backtrack onto (*COMMIT).

   Backtracking verbs in repeated groups

       PCRE2 sometimes differs from Perl in its handling of backtracking verbs
       in repeated groups. For example, consider:

         /(a(*COMMIT)b)+ac/

       If the subject is "abac", Perl matches  unless  its  optimizations  are
       disabled,  but  PCRE2  always fails because the (*COMMIT) in the second
       repeat of the group acts.

   Backtracking verbs in assertions

       (*FAIL) in any assertion has its normal effect: it forces an  immediate
       backtrack.  The  behaviour  of  the other backtracking verbs depends on
       whether or not the assertion is standalone or acting as  the  condition
       in a conditional group.

       (*ACCEPT)  in  a  standalone positive assertion causes the assertion to
       succeed without any further processing; captured  strings  and  a  mark
       name  (if  set) are retained. In a standalone negative assertion, (*AC-
       CEPT) causes the assertion to fail without any further processing; cap-
       tured substrings and any mark name are discarded.

       If the assertion is a condition, (*ACCEPT) causes the condition  to  be
       true  for  a  positive assertion and false for a negative one; captured
       substrings are retained in both cases.

       The remaining verbs act only when a later failure causes a backtrack to
       reach them. This means that, for the Perl-compatible assertions,  their
       effect is confined to the assertion, because Perl lookaround assertions
       are atomic. A backtrack that occurs after such an assertion is complete
       does  not  jump  back  into  the  assertion.  Note in particular that a
       (*MARK) name that is set in an assertion is not "seen" by  an  instance
       of (*SKIP:NAME) later in the pattern.

       PCRE2  now  supports non-atomic positive assertions and also "scan sub-
       string" assertions, as described in the sections  entitled  "Non-atomic
       assertions"  and  "Scan  substring  assertions" above. These assertions
       must be standalone (not used as conditions). They are not Perl-compati-
       ble. For these assertions, a later backtrack does jump  back  into  the
       assertion,  and  therefore  verbs such as (*COMMIT) can be triggered by
       backtracks from later in the pattern.

       The effect of (*THEN) is not allowed to escape beyond an assertion.  If
       there  are no more branches to try, (*THEN) causes a positive assertion
       to be false, and a negative assertion to be true. This  behaviour  dif-
       fers from Perl when the assertion has only one branch.

       The  other  backtracking verbs are not treated specially if they appear
       in a standalone positive assertion. In a  conditional  positive  asser-
       tion, backtracking (from within the assertion) into (*COMMIT), (*SKIP),
       or  (*PRUNE) causes the condition to be false. However, for both stand-
       alone and conditional negative assertions, backtracking into (*COMMIT),
       (*SKIP), or (*PRUNE) causes the assertion to be true, without consider-
       ing any further alternative branches.

   Backtracking verbs in subroutines

       These behaviours occur whether or not the group is called recursively.

       (*ACCEPT) in a group called as a subroutine causes the subroutine match
       to succeed without any further processing. Matching then continues  af-
       ter  the  subroutine call. Perl documents this behaviour. Perl's treat-
       ment of the other verbs in subroutines is different in some cases.

       (*FAIL) in a group called as a subroutine has  its  normal  effect:  it
       forces an immediate backtrack.

       (*COMMIT),  (*SKIP),  and  (*PRUNE)  cause the subroutine match to fail
       when triggered by being backtracked to in a group called as  a  subrou-
       tine. There is then a backtrack at the outer level.

       (*THEN), when triggered, skips to the next alternative in the innermost
       enclosing  group that has alternatives (its normal behaviour). However,
       if there is no such group within the subroutine's group, the subroutine
       match fails and there is a backtrack at the outer level.


EBCDIC ENVIRONMENTS

       Differences in the way PCRE behaves when it is running in an EBCDIC en-
       vironment are covered in this section.

   Escape sequences

       When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..}  is  not  supported.
       \a, \e, \f, \n, \r, and \t generate the appropriate EBCDIC code values.
       The \c escape is processed as specified for Perl in the perlebcdic doc-
       ument.  The  only characters that are allowed after \c are A-Z, a-z, or
       one of @, [, \, ], ^, _, or ?. Any other character provokes a  compile-
       time  error.  The  sequence  \c@ encodes character code 0; after \c the
       letters (in either case) encode characters 1-26 (hex 01 to hex 1A);  [,
       \,  ], ^, and _ encode characters 27-31 (hex 1B to hex 1F), and \c? be-
       comes either 255 (hex FF) or 95 (hex 5F).

       Thus, apart from \c?, these escapes generate the  same  character  code
       values  as they do in an ASCII or Unicode environment, though the mean-
       ings of the values mostly differ. For  example,  \cG  always  generates
       code value 7, which is BEL in ASCII but DEL in EBCDIC.

       The  sequence  \c? generates DEL (127, hex 7F) in an ASCII environment,
       but because 127 is not a control character in  EBCDIC,  Perl  makes  it
       generate  the  APC character. Unfortunately, there are several variants
       of EBCDIC. In most of them the APC character has  the  value  255  (hex
       FF),  but  in  the one Perl calls POSIX-BC its value is 95 (hex 5F). If
       certain other characters have POSIX-BC values, PCRE2 makes \c? generate
       95; otherwise it generates 255.

   Character classes

       In character classes there is a special case in EBCDIC environments for
       ranges whose end points are both specified as literal  letters  in  the
       same  case.  For compatibility with Perl, EBCDIC code points within the
       range that are not letters are omitted. For example, [h-k] matches only
       four characters, even though the EBCDIC codes for h and k are 0x88  and
       0x92, a range of 11 code points. However, if the range is specified nu-
       merically,  for  example,  [\x88-\x92] or [h-\x92], all code points are
       included.


SEE ALSO

       pcre2api(3),   pcre2callout(3),    pcre2matching(3),    pcre2syntax(3),
       pcre2(3).


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 25 October 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                     25 October 2025                PCRE2PATTERN(3)
------------------------------------------------------------------------------


PCRE2PERFORM(3)            Library Functions Manual            PCRE2PERFORM(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 PERFORMANCE

       Two  aspects  of performance are discussed below: memory usage and pro-
       cessing time. The way you express your pattern as a regular  expression
       can affect both of them.


COMPILED PATTERN MEMORY USAGE

       Patterns are compiled by PCRE2 into a reasonably efficient interpretive
       code,  so  that most simple patterns do not use much memory for storing
       the compiled version. However, there is one case where the memory usage
       of a compiled pattern can be unexpectedly  large.  If  a  parenthesized
       group  has  a quantifier with a minimum greater than 1 and/or a limited
       maximum, the whole group is repeated in the compiled code. For example,
       the pattern

         (abc|def){2,4}

       is compiled as if it were

         (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical aside: It is done this way so that backtrack  points  within
       each of the repetitions can be independently maintained.)

       For  regular expressions whose quantifiers use only small numbers, this
       is not usually a problem. However, if the numbers are large,  and  par-
       ticularly  if  such repetitions are nested, the memory usage can become
       an embarrassment. For example, the very simple pattern

         ((ab){1,1000}c){1,3}

       uses over 50KiB when compiled using the 8-bit library.  When  PCRE2  is
       compiled  with its default internal pointer size of two bytes, the size
       limit on a compiled pattern is 65535 code units in the 8-bit and 16-bit
       libraries, and this is reached with the above pattern if the outer rep-
       etition is increased from 3 to 4. PCRE2 can be compiled to  use  larger
       internal  pointers  and thus handle larger compiled patterns, but it is
       better to try to rewrite your pattern to use less memory if you can.

       One way of reducing the memory usage for such patterns is to  make  use
       of PCRE2's "subroutine" facility. Re-writing the above pattern as

         ((ab)(?2){0,999}c)(?1){0,2}

       reduces  the memory requirements to around 16KiB, and indeed it remains
       under 20KiB even with the outer repetition increased to  100.  However,
       this kind of pattern is not always exactly equivalent, because any cap-
       tures  within  subroutine calls are lost when the subroutine completes.
       If this is not a problem, this kind of  rewriting  will  allow  you  to
       process  patterns that PCRE2 cannot otherwise handle. The matching per-
       formance of the two different versions of the pattern are  roughly  the
       same.  (This applies from release 10.30 - things were different in ear-
       lier releases.)


STACK AND HEAP USAGE AT RUN TIME

       From release 10.30, the interpretive (non-JIT) version of pcre2_match()
       uses very little system stack at run time. In earlier  releases  recur-
       sive  function  calls  could  use a great deal of stack, and this could
       cause problems, but this usage has been eliminated. Backtracking  posi-
       tions  are now explicitly remembered in memory frames controlled by the
       code.

       The size of each frame depends on the size of pointer variables and the
       number of capturing parenthesized groups in the pattern being  matched.
       On a 64-bit system the frame size for a pattern with no captures is 128
       bytes. For each capturing group the size increases by 16 bytes.

       Until  release  10.41,  an initial 20KiB frames vector was allocated on
       the system stack, but this still caused some  issues  for  multi-thread
       applications  where  each  thread  has a very small stack. From release
       10.41 backtracking memory frames are always held  in  heap  memory.  An
       initial heap allocation is obtained the first time any match data block
       is  passed  to  pcre2_match().  This  is remembered with the match data
       block and re-used if that block is used for another match. It is  freed
       when the match data block itself is freed.

       The  size  of the initial block is the larger of 20KiB or ten times the
       pattern's frame size, unless the heap limit is less than this, in which
       case the heap limit is used. If the initial  block  proves  to  be  too
       small during matching, it is replaced by a larger block, subject to the
       heap  limit.  The  heap limit is checked only when a new block is to be
       allocated. Reducing the heap limit between calls to pcre2_match()  with
       the same match data block does not affect the saved block.

       In  contrast  to  pcre2_match(),  pcre2_dfa_match()  does use recursive
       function calls, but only for processing atomic groups,  lookaround  as-
       sertions, and recursion within the pattern. The original version of the
       code  used  to  allocate  quite large internal workspace vectors on the
       stack, which caused some problems for  some  patterns  in  environments
       with  small  stacks.  From release 10.32 the code for pcre2_dfa_match()
       has been re-factored to use heap memory  when  necessary  for  internal
       workspace  when  recursing,  though  recursive function calls are still
       used.

       The "match depth" parameter can be used to limit the depth of  function
       recursion,  and  the  "match  heap"  parameter  to limit heap memory in
       pcre2_dfa_match().


PROCESSING TIME

       Certain items in regular expression patterns are processed  more  effi-
       ciently than others. It is more efficient to use a character class like
       [aeiou]   than   a   set   of  single-character  alternatives  such  as
       (a|e|i|o|u). In general, the simplest construction  that  provides  the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains  a  lot  of useful general discussion about optimizing regular
       expressions for efficient performance. This document contains a few ob-
       servations about PCRE2.

       Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is
       slow,  because  PCRE2 has to use a multi-stage table lookup whenever it
       needs a character's property. If you can find  an  alternative  pattern
       that does not use character properties, it will probably be faster.

       By  default,  the  escape  sequences  \b, \d, \s, and \w, and the POSIX
       character classes such as [:alpha:]  do  not  use  Unicode  properties,
       partly for backwards compatibility, and partly for performance reasons.
       However,  you  can  set  the PCRE2_UCP option or start the pattern with
       (*UCP) if you want Unicode character properties to be  used.  This  can
       double  the  matching  time  for  items  such  as \d, when matched with
       pcre2_match(); the performance loss is less with a DFA  matching  func-
       tion, and in both cases there is not much difference for \b.

       When  a pattern begins with .* not in atomic parentheses, nor in paren-
       theses that are the subject of a backreference,  and  the  PCRE2_DOTALL
       option  is  set,  the pattern is implicitly anchored by PCRE2, since it
       can match only at the start of a subject string.  If  the  pattern  has
       multiple top-level branches, they must all be anchorable. The optimiza-
       tion  can be disabled by the PCRE2_NO_DOTSTAR_ANCHOR option, and is au-
       tomatically disabled if the pattern contains (*PRUNE) or (*SKIP).

       If PCRE2_DOTALL is not set, PCRE2 cannot make  this  optimization,  be-
       cause  the  dot metacharacter does not then match a newline, and if the
       subject string contains newlines, the pattern may match from the  char-
       acter immediately following one of them instead of from the very start.
       For example, the pattern

         .*second

       matches  the subject "first\nand second" (where \n stands for a newline
       character), with the match starting at the seventh character. In  order
       to  do  this, PCRE2 has to retry the match starting after every newline
       in the subject.

       If you are using such a pattern with subject strings that do  not  con-
       tain   newlines,   the   best   performance   is  obtained  by  setting
       PCRE2_DOTALL, or starting the pattern with ^.* or ^.*? to indicate  ex-
       plicit  anchoring.  That saves PCRE2 from having to scan along the sub-
       ject looking for a newline to restart at.

       Beware of patterns that contain nested indefinite  repeats.  These  can
       take  a  long time to run when applied to a string that does not match.
       Consider the pattern fragment

         ^(a+)*

       This can match "aaaa" in 16 different ways, and this  number  increases
       very  rapidly  as the string gets longer. (The * repeat can match 0, 1,
       2, 3, or 4 times, and for each of those cases other than 0 or 4, the  +
       repeats  can  match  different numbers of times.) When the remainder of
       the pattern is such that the entire match is going to fail,  PCRE2  has
       in  principle to try every possible variation, and this can take an ex-
       tremely long time, even for relatively short strings.

       An optimization catches some of the more simple cases such as

         (a+)*b

       where a literal character follows. Before  embarking  on  the  standard
       matching  procedure, PCRE2 checks that there is a "b" later in the sub-
       ject string, and if there is not, it fails the match immediately.  How-
       ever,  when  there  is no following literal this optimization cannot be
       used. You can see the difference by comparing the behaviour of

         (a+)*\d

       with the pattern above. The former gives  a  failure  almost  instantly
       when  applied  to  a  whole  line of "a" characters, whereas the latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier. This can often reduce  mem-
       ory requirements as well. As another example, consider this pattern:

         ([^<]|<(?!inet))+

       It  matches  from wherever it starts until it encounters "<inet" or the
       end of the data, and is the kind of pattern that  might  be  used  when
       processing an XML file. Each iteration of the outer parentheses matches
       either  one  character that is not "<" or a "<" that is not followed by
       "inet". However, each time a parenthesis is processed,  a  backtracking
       position  is  passed,  so this formulation uses a memory frame for each
       matched character. For a long string, a lot of memory is required. Con-
       sider now this  rewritten  pattern,  which  matches  exactly  the  same
       strings:

         ([^<]++|<(?!inet))+

       This runs much faster, because sequences of characters that do not con-
       tain "<" are "swallowed" in one item inside the parentheses, and a pos-
       sessive  quantifier  is  used to stop any backtracking into the runs of
       non-"<" characters. This version also uses a lot  less  memory  because
       entry  to  a  new  set of parentheses happens only when a "<" character
       that is not followed by "inet" is encountered (and we  assume  this  is
       relatively rare).

       This example shows that one way of optimizing performance when matching
       long  subject strings is to write repeated parenthesized subpatterns to
       match more than one character whenever possible.

   SETTING RESOURCE LIMITS

       You can set limits on the amount of processing that  takes  place  when
       matching,  and  on  the amount of heap memory that is used. The default
       values of the limits are very large, and unlikely ever to operate. They
       can be changed when PCRE2 is built, and  they  can  also  be  set  when
       pcre2_match()  or pcre2_dfa_match() is called. For details of these in-
       terfaces, see the pcre2build documentation  and  the  section  entitled
       "The match context" in the pcre2api documentation.

       The  pcre2test  test program has a modifier called "find_limits" which,
       if applied to a subject line, causes it to  find  the  smallest  limits
       that allow a pattern to match. This is done by repeatedly matching with
       different limits.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 06 December 2022
       Copyright (c) 1997-2022 University of Cambridge.


PCRE2 10.48                    06 December 2022                PCRE2PERFORM(3)
------------------------------------------------------------------------------


PCRE2POSIX(3)              Library Functions Manual              PCRE2POSIX(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


SYNOPSIS

       #include <pcre2posix.h>

       int pcre2_regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int pcre2_regexec(const regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t pcre2_regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void pcre2_regfree(regex_t *preg);


DESCRIPTION

       This  set of functions provides a POSIX-style API for the PCRE2 regular
       expression 8-bit library. There are no POSIX-style wrappers for PCRE2's
       16-bit and 32-bit libraries. See the pcre2api documentation for  a  de-
       scription  of  PCRE2's native API, which contains much additional func-
       tionality.

       IMPORTANT NOTE: The functions described here are NOT  thread-safe,  and
       should  not  be used in multi-threaded applications. They are also lim-
       ited to processing subjects that are not bigger than 2GB. Use  the  na-
       tive API instead.

       These  functions  are  wrapper functions that ultimately call the PCRE2
       native API. Their prototypes are defined  in  the  pcre2posix.h  header
       file, and they all have unique names starting with pcre2_. However, the
       pcre2posix.h  header  also  contains macro definitions that convert the
       standard POSIX names such  regcomp()  into  pcre2_regcomp()  etc.  This
       means  that a program can use the usual POSIX names without running the
       risk of accidentally linking with POSIX functions from a different  li-
       brary.

       On  Unix-like systems the PCRE2 POSIX library is called libpcre2-posix,
       so can be accessed by adding -lpcre2-posix to the command  for  linking
       an application. Because the POSIX functions call the native ones, it is
       also necessary to add -lpcre2-8.

       On Windows systems, if you are linking to a DLL version of the library,
       it  is  recommended  that PCRE2POSIX_SHARED is defined before including
       the pcre2posix.h header, as it will allow for a more efficient  way  to
       invoke the functions by adding the __declspec(dllimport) decorator.

       Although  they were not defined as prototypes in pcre2posix.h, releases
       10.33 to 10.36 of the library contained functions with the POSIX  names
       regcomp()  etc.  These simply passed their arguments to the PCRE2 func-
       tions. These functions were provided for backwards  compatibility  with
       earlier  versions  of  PCRE2, which had only POSIX names. However, this
       has proved troublesome in situations where a program links with several
       libraries, some of which use PCRE2's POSIX interface while  others  use
       the  real  POSIX functions.  For this reason, the POSIX names have been
       removed since release 10.37.

       Calling the header file pcre2posix.h avoids  any  conflict  with  other
       POSIX  libraries.  It can, of course, be renamed or aliased as regex.h,
       which is the "correct" name, if there is  no  clash.  It  provides  two
       structure  types,  regex_t  for compiled internal forms, and regmatch_t
       for returning captured substrings. It also defines some constants whose
       names start with "REG_"; these are used for setting options and identi-
       fying error codes.


USING THE POSIX FUNCTIONS

       Note that these functions are just POSIX-style wrappers for PCRE2's na-
       tive API.  They do not give POSIX  regular  expression  behaviour,  and
       they are not thread-safe or even POSIX compatible.

       Those  POSIX  option bits that can reasonably be mapped to PCRE2 native
       options have been implemented. In addition, the option REG_EXTENDED  is
       defined  with  the  value  zero. This has no effect, but since programs
       that are written to the POSIX interface often use  it,  this  makes  it
       easier  to  slot in PCRE2 as a replacement library. Other POSIX options
       are not even defined.

       There are also some options that are not defined by POSIX.  These  have
       been  added  at  the  request  of users who want to make use of certain
       PCRE2-specific features via the POSIX calling interface or to  add  BSD
       or GNU functionality.

       When  PCRE2  is  called via these functions, it is only the API that is
       POSIX-like in style. The syntax and semantics of  the  regular  expres-
       sions  themselves  are  still  those of Perl, subject to the setting of
       various PCRE2 options, as described below. "POSIX-like in style"  means
       that  the  API  approximates  to  the POSIX definition; it is not fully
       POSIX-compatible, and in multi-unit encoding  domains  it  is  probably
       even less compatible.

       The  descriptions  below use the actual names of the functions, but, as
       described above, the standard POSIX names (without the  pcre2_  prefix)
       may also be used.


COMPILING A PATTERN

       The function pcre2_regcomp() is called to compile a pattern into an in-
       ternal  form. By default, the pattern is a C string terminated by a bi-
       nary zero (but see REG_PEND below). The preg argument is a pointer to a
       regex_t structure that is used as a base for storing information  about
       the  compiled  regular  expression.  It  is  also  used  for input when
       REG_PEND is set. The regex_t structure used by pcre2_regcomp()  is  de-
       fined  in  pcre2posix.h  and  is  not the same as the structure used by
       other libraries that provide POSIX-style matching.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

       The PCRE2_DOTALL option is set when the regular  expression  is  passed
       for  compilation  to  the  native function. Note that REG_DOTALL is not
       part of the POSIX standard.

         REG_ICASE

       The PCRE2_CASELESS option is set when the regular expression is  passed
       for compilation to the native function.

         REG_NEWLINE

       The PCRE2_MULTILINE option is set when the regular expression is passed
       for  compilation  to the native function. Note that this does not mimic
       the defined POSIX behaviour for REG_NEWLINE  (see  the  following  sec-
       tion).

         REG_NOSPEC

       The  PCRE2_LITERAL  option is set when the regular expression is passed
       for compilation to the native function. This disables all meta  charac-
       ters  in the pattern, causing it to be treated as a literal string. The
       only other options that are  allowed  with  REG_NOSPEC  are  REG_ICASE,
       REG_NOSUB,  REG_PEND,  and REG_UTF. Note that REG_NOSPEC is not part of
       the POSIX standard.

         REG_NOSUB

       When  a  pattern  that  is  compiled  with  this  flag  is  passed   to
       pcre2_regexec()  for  matching, the nmatch and pmatch arguments are ig-
       nored, and no captured strings are returned. Versions of the PCRE2  li-
       brary  prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile op-
       tion, but this no longer happens because it disables the use  of  back-
       references.

         REG_PEND

       If  this option is set, the reg_endp field in the preg structure (which
       has the type const char *) must be set to point to the character beyond
       the end of the pattern before calling pcre2_regcomp(). The pattern  it-
       self  may  now  contain binary zeros, which are treated as data charac-
       ters. Without REG_PEND, a binary zero terminates the  pattern  and  the
       re_endp field is ignored. This is a GNU extension to the POSIX standard
       and  should be used with caution in software intended to be portable to
       other systems.

         REG_UCP

       The PCRE2_UCP option is set when the regular expression is  passed  for
       compilation  to  the  native function. This causes PCRE2 to use Unicode
       properties when matching \d, \w,  etc.,  instead  of  just  recognizing
       ASCII values. Note that REG_UCP is not part of the POSIX standard.

         REG_UNGREEDY

       The  PCRE2_UNGREEDY option is set when the regular expression is passed
       for compilation to the native function. Note that REG_UNGREEDY  is  not
       part of the POSIX standard.

         REG_UTF

       The  PCRE2_UTF  option is set when the regular expression is passed for
       compilation to the native function. This causes the pattern itself  and
       all  data  strings used for matching it to be treated as UTF-8 strings.
       Note that REG_UTF is not part of the POSIX standard.

       In the absence of these flags, no options  are  passed  to  the  native
       function.  This means that the regex is compiled with PCRE2 default se-
       mantics.  In  particular,  the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way.  Note  that  setting
       PCRE2_MULTILINE has only some of the effects specified for REG_NEWLINE.
       It  does not affect the way newlines are matched by the dot metacharac-
       ter (they are not) or by a negative class such as [^a] (they are).

       The yield of pcre2_regcomp() is zero on success,  and  non-zero  other-
       wise.  The preg structure is filled in on success, and one other member
       of  the  structure (as well as re_endp) is public: re_nsub contains the
       number of capturing subpatterns in the regular expression. Various  er-
       ror codes are defined in the header file.

       NOTE: If the yield of pcre2_regcomp() is non-zero, you must not attempt
       to use the contents of the preg structure. If, for example, you pass it
       to  pcre2_regexec(), the result is undefined and your program is likely
       to crash.


MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of
       things.  It is not possible to get PCRE2 to obey POSIX  semantics,  but
       then PCRE2 was never intended to be a POSIX engine. The following table
       lists  the  different  possibilities for matching newline characters in
       Perl and PCRE2:

                                 Default   Change with

         . matches newline          no     PCRE2_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE2_DOLLAR_ENDONLY
         $ matches \n in middle     no     PCRE2_MULTILINE
         ^ matches \n in middle     no     PCRE2_MULTILINE

       This is the equivalent table for a POSIX-compatible pattern matcher:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       This behaviour is not what happens when PCRE2 is called via  its  POSIX
       API.  By  default, PCRE2's behaviour is the same as Perl's, except that
       there is no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both  PCRE2
       and Perl, there is no way to stop newline from matching [^a].

       Default  POSIX newline handling can be obtained by setting PCRE2_DOTALL
       and PCRE2_DOLLAR_ENDONLY when  calling  pcre2_compile()  directly,  but
       there is no way to make PCRE2 behave exactly as for the REG_NEWLINE ac-
       tion.  When  using  the  POSIX  API,  passing  REG_NEWLINE  to  PCRE2's
       pcre2_regcomp()  function  causes  PCRE2_MULTILINE  to  be  passed   to
       pcre2_compile(), and REG_DOTALL passes PCRE2_DOTALL. There is no way to
       pass PCRE2_DOLLAR_ENDONLY.


MATCHING A PATTERN

       The function pcre2_regexec() is called to match a compiled pattern preg
       against  a  given string, which is by default terminated by a zero byte
       (but see REG_STARTEND below), subject to the options in eflags.   These
       can be:

         REG_NOTBOL

       The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match-
       ing function.

         REG_NOTEMPTY

       The  PCRE2_NOTEMPTY  option  is  set  when calling the underlying PCRE2
       matching function. Note that REG_NOTEMPTY is  not  part  of  the  POSIX
       standard.  However, setting this option can give more POSIX-like behav-
       iour in some situations.

         REG_NOTEOL

       The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match-
       ing function.

         REG_STARTEND

       When this option  is  set,  the  subject  string  starts  at  string  +
       pmatch[0].rm_so  and  ends  at  string  + pmatch[0].rm_eo, which should
       point to the first character beyond the string. There may be binary ze-
       ros within the subject string, and indeed, using  REG_STARTEND  is  the
       only way to pass a subject string that contains a binary zero.

       Whatever  the  value  of  pmatch[0].rm_so,  the  offsets of the matched
       string and any captured substrings are  still  given  relative  to  the
       start  of  string  itself. (Before PCRE2 release 10.30 these were given
       relative to string + pmatch[0].rm_so, but this differs from  other  im-
       plementations.)

       This  is  a  BSD  extension,  compatible with but not specified by IEEE
       Standard 1003.2 (POSIX.2), and should be used with caution in  software
       intended  to  be  portable to other systems. Note that a non-zero rm_so
       does not imply REG_NOTBOL; REG_STARTEND affects only the  location  and
       length  of  the string, not how it is matched. Setting REG_STARTEND and
       passing pmatch as NULL are mutually exclusive; the error REG_INVARG  is
       returned.

       If  the pattern was compiled with the REG_NOSUB flag, no data about any
       matched strings  is  returned.  The  nmatch  and  pmatch  arguments  of
       pcre2_regexec()  are  ignored  (except  possibly as input for REG_STAR-
       TEND).

       The value of nmatch may be zero, and the value pmatch may be NULL  (un-
       less  REG_STARTEND  is  set);  in  both  these  cases no data about any
       matched strings is returned.

       Otherwise, the portion of the string that was  matched,  and  also  any
       captured substrings, are returned via the pmatch argument, which points
       to  an  array  of  nmatch structures of type regmatch_t, containing the
       members rm_so and rm_eo. These contain the byte  offset  to  the  first
       character of each substring and the offset to the first character after
       the  end of each substring, respectively. The 0th element of the vector
       relates to the entire portion of string that  was  matched;  subsequent
       elements relate to the capturing subpatterns of the regular expression.
       Unused entries in the array have both structure members set to -1.

       regmatch_t  as  well  as  the  regoff_t  typedef it uses are defined in
       pcre2posix.h and are not warranted to have the same size or  layout  as
       other  similarly  named  types from other libraries that provide POSIX-
       style matching.

       A successful match yields a zero return; various error  codes  are  de-
       fined  in the header file, of which REG_NOMATCH is the "expected" fail-
       ure code.


ERROR MESSAGES

       The pcre2_regerror() function maps a  non-zero  errorcode  from  either
       pcre2_regcomp()  or  pcre2_regexec() to a printable message. If preg is
       not NULL, the error should have arisen from the use of that  structure.
       A  message  terminated  by  a  binary  zero is placed in errbuf. If the
       buffer is too short, only the first errbuf_size - 1 characters  of  the
       error message are used. The yield of the function is the size of buffer
       needed  to hold the whole message, including the terminating zero. This
       value is greater than errbuf_size if the message was truncated.


MEMORY USAGE

       Compiling a regular expression causes memory to be allocated and  asso-
       ciated  with the preg structure. The function pcre2_regfree() frees all
       such memory, after which preg may no longer be used as a  compiled  ex-
       pression.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 27 November 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                    27 November 2024                  PCRE2POSIX(3)
------------------------------------------------------------------------------


PCRE2SAMPLE(3)             Library Functions Manual             PCRE2SAMPLE(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 SAMPLE PROGRAM

       A  simple, complete demonstration program to get you started with using
       PCRE2 is supplied in the file pcre2demo.c in the src directory  in  the
       PCRE2 distribution. A listing of this program is given in the pcre2demo
       documentation. If you do not have a copy of the PCRE2 distribution, you
       can save this listing to recreate the contents of pcre2demo.c.

       The  demonstration  program compiles the regular expression that is its
       first argument, and matches it against the subject string in its second
       argument. No PCRE2 options are set, and default  character  tables  are
       used. If matching succeeds, the program outputs the portion of the sub-
       ject  that  matched,  together  with  the contents of any captured sub-
       strings.

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

       The code in pcre2demo.c is an 8-bit program that uses the  PCRE2  8-bit
       library.  It  handles  strings  and characters that are stored in 8-bit
       code units.  By default, one character corresponds to  one  code  unit,
       but  if  the  pattern starts with "(*UTF)", both it and the subject are
       treated as UTF-8 strings, where characters  may  occupy  multiple  code
       units.

       If  PCRE2  is installed in the standard include and library directories
       for your operating system, you should be able to compile the demonstra-
       tion program using a command like this:

         cc -o pcre2demo pcre2demo.c -lpcre2-8

       If PCRE2 is installed elsewhere, you may need to add additional options
       to the command line. For example, on a Unix-like system that has  PCRE2
       installed  in /usr/local, you can compile the demonstration program us-
       ing a command like this:

         cc -o pcre2demo -I/usr/local/include pcre2demo.c \
            -L/usr/local/lib -lpcre2-8

       Once you have built the demonstration program, you can run simple tests
       like this:

         ./pcre2demo 'cat|dog' 'the cat sat on the mat'
         ./pcre2demo -g 'cat|dog' 'the dog sat on the cat'
         ./pcre2demo -i 'cat' 'the dog sat on the CAT'

       Note that there is a  much  more  comprehensive  test  program,  called
       pcre2test,  which supports many more facilities for testing regular ex-
       pressions using all three PCRE2 libraries (8-bit, 16-bit,  and  32-bit,
       though  not all three need be installed). The pcre2demo program is pro-
       vided as a relatively simple coding example.

       If you try to run pcre2demo when PCRE2 is not installed in the standard
       library directory, you may get an error like  this  on  some  operating
       systems (e.g. Solaris):

         ld.so.1: pcre2demo: fatal: libpcre2-8.so.0: open failed: No such file
       or directory

       This  is  caused  by the way shared library support works on those sys-
       tems. You need to add

         -R/usr/local/lib

       (for example) to the compile command to get round this problem.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 28 February 2025
       Copyright (c) 1997-2016 University of Cambridge.


PCRE2 10.48                    28 February 2025                 PCRE2SAMPLE(3)
------------------------------------------------------------------------------
PCRE2SERIALIZE(3)          Library Functions Manual          PCRE2SERIALIZE(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


SAVING AND RE-USING PRECOMPILED PCRE2 PATTERNS

       int32_t pcre2_serialize_decode(pcre2_code **codes,
         int32_t number_of_codes, const uint8_t *bytes,
         pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(const pcre2_code **codes,
         int32_t number_of_codes, uint8_t **serialized_bytes,
         PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);

       If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run. However, if you are using the just-in-time  optimization  feature,
       it is not possible to save and reload the JIT data, because it is posi-
       tion-dependent.  The  host  on  which the patterns are reloaded must be
       running the same version of PCRE2, with the same code unit  width,  and
       must  also have the same endianness, pointer width and PCRE2_SIZE type.
       For example, patterns compiled on a 32-bit system using PCRE2's  16-bit
       library cannot be reloaded on a 64-bit system, nor can they be reloaded
       using the 8-bit library.

       Note  that  "serialization" in PCRE2 does not convert compiled patterns
       to an abstract format like Java or .NET serialization.  The  serialized
       output  is really just a bytecode dump, which is why it can only be re-
       loaded in the same environment as the one that created  it.  Hence  the
       restrictions  mentioned  above.   Applications  that are not statically
       linked with a fixed version of PCRE2 must be prepared to recompile pat-
       terns from their sources, in order to be immune to PCRE2 upgrades.


SECURITY CONCERNS

       The facility for saving and restoring compiled patterns is intended for
       use within individual applications.  As  such,  the  data  supplied  to
       pcre2_serialize_decode()  is expected to be trusted data, not data from
       arbitrary external sources.  There  is  only  some  simple  consistency
       checking, not complete validation of what is being re-loaded. Corrupted
       data may cause undefined results. For example, if the length field of a
       pattern in the serialized data is corrupted, the deserializing code may
       read beyond the end of the byte stream that is passed to it.


SAVING COMPILED PATTERNS

       Before compiled patterns can be saved they must be serialized, which in
       PCRE2  means converting the pattern to a stream of bytes. A single byte
       stream may contain any number of compiled patterns, but they  must  all
       use  the same character tables. A single copy of the tables is included
       in the byte stream (its size is 1088 bytes). For more details of  char-
       acter  tables,  see the section on locale support in the pcre2api docu-
       mentation.

       The function pcre2_serialize_encode() creates a serialized byte  stream
       from  a  list of compiled patterns. Its first two arguments specify the
       list, being a pointer to a vector of pointers to compiled patterns, and
       the length of the vector. The third and fourth arguments point to vari-
       ables which are set to point to the created byte stream and its length,
       respectively. The final argument is a pointer  to  a  general  context,
       which  can  be  used  to specify custom memory management functions. If
       this argument is NULL, malloc() is used to obtain memory for  the  byte
       stream. The yield of the function is the number of serialized patterns,
       or one of the following negative error codes:

         PCRE2_ERROR_BADDATA      the number of patterns is zero or less
         PCRE2_ERROR_BADMAGIC     mismatch of id bytes in one of the patterns
         PCRE2_ERROR_NOMEMORY     memory allocation failed
         PCRE2_ERROR_MIXEDTABLES  the patterns do not all use the same tables
         PCRE2_ERROR_NULL         the 1st, 3rd, or 4th argument is NULL

       PCRE2_ERROR_BADMAGIC  means  either that a pattern's code has been cor-
       rupted, or that a slot in the vector does not point to a compiled  pat-
       tern.

       Once a set of patterns has been serialized you can save the data in any
       appropriate  manner. Here is sample code that compiles two patterns and
       writes them to a file. It assumes that the variable fd refers to a file
       that is open for output. The error checking that should be present in a
       real application has been omitted for simplicity.

         int errorcode;
         uint8_t *bytes;
         PCRE2_SIZE erroroffset;
         PCRE2_SIZE bytescount;
         pcre2_code *list_of_codes[2];
         list_of_codes[0] = pcre2_compile("first pattern",
           PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
         list_of_codes[1] = pcre2_compile("second pattern",
           PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
         errorcode = pcre2_serialize_encode(list_of_codes, 2, &bytes,
           &bytescount, NULL);
         errorcode = fwrite(bytes, 1, bytescount, fd);

       Note that the serialized data is binary data that may  contain  any  of
       the  256  possible  byte values. On systems that make a distinction be-
       tween binary and non-binary data, be sure that the file is  opened  for
       binary output.

       Serializing  a  set  of patterns leaves the original data untouched, so
       they can still be used for matching. Their memory  must  eventually  be
       freed in the usual way by calling pcre2_code_free(). When you have fin-
       ished with the byte stream, it too must be freed by calling pcre2_seri-
       alize_free().  If  this function is called with a NULL argument, it re-
       turns immediately without doing anything.


RE-USING PRECOMPILED PATTERNS

       In order to re-use a set of saved patterns you must first make the  se-
       rialized  byte stream available in main memory (for example, by reading
       from a file). The management of this memory block is up to the applica-
       tion. You can use the pcre2_serialize_get_number_of_codes() function to
       find out how many compiled patterns are in the serialized data  without
       actually decoding the patterns:

         uint8_t *bytes = <serialized data>;
         int32_t number_of_codes = pcre2_serialize_get_number_of_codes(bytes);

       The pcre2_serialize_decode() function reads a byte stream and recreates
       the compiled patterns in new memory blocks, setting pointers to them in
       a  vector.  The  first two arguments are a pointer to a suitable vector
       and its length, and the third argument points to a byte stream. The fi-
       nal argument is a pointer to a general context, which can  be  used  to
       specify custom memory management functions for the decoded patterns. If
       this argument is NULL, malloc() and free() are used. After deserializa-
       tion, the byte stream is no longer needed and can be discarded.

         pcre2_code *list_of_codes[2];
         uint8_t *bytes = <serialized data>;
         int32_t number_of_codes =
           pcre2_serialize_decode(list_of_codes, 2, bytes, NULL);

       If  the  vector  is  not  large enough for all the patterns in the byte
       stream, it is filled with those that fit, and  the  remainder  are  ig-
       nored.  The yield of the function is the number of decoded patterns, or
       one of the following negative error codes:

         PCRE2_ERROR_BADDATA    second argument is zero or less
         PCRE2_ERROR_BADMAGIC   mismatch of id bytes in the data
         PCRE2_ERROR_BADMODE    mismatch of code unit size or PCRE2 version
         PCRE2_ERROR_BADSERIALIZEDDATA  other sanity check failure
         PCRE2_ERROR_MEMORY     memory allocation failed
         PCRE2_ERROR_NULL       first or third argument is NULL

       PCRE2_ERROR_BADMAGIC may mean that the data is corrupt, or that it  was
       compiled on a system with different endianness.

       Decoded patterns can be used for matching in the usual way, and must be
       freed  by  calling pcre2_code_free(). However, be aware that there is a
       potential race issue if you are using multiple patterns that  were  de-
       coded  from a single byte stream in a multithreaded application. A sin-
       gle copy of the character tables is used by all  the  decoded  patterns
       and a reference count is used to arrange for its memory to be automati-
       cally  freed when the last pattern is freed, but there is no locking on
       this reference count. Therefore, if you want to call  pcre2_code_free()
       for  these  patterns  in  different  threads, you must arrange your own
       locking, and ensure that pcre2_code_free()  cannot  be  called  by  two
       threads at the same time.

       If  a pattern was processed by pcre2_jit_compile() before being serial-
       ized, the JIT data is discarded and so is no longer available  after  a
       save/restore  cycle.  You can, however, process a restored pattern with
       pcre2_jit_compile() if you wish.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 19 January 2024
       Copyright (c) 1997-2018 University of Cambridge.


PCRE2 10.48                     19 January 2024              PCRE2SERIALIZE(3)
------------------------------------------------------------------------------


PCRE2SYNTAX(3)             Library Functions Manual             PCRE2SYNTAX(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY

       The  full  syntax and semantics of the regular expression patterns that
       are supported by PCRE2 are described in the pcre2pattern documentation.
       This document contains a quick-reference summary of the pattern  syntax
       followed by the syntax of replacement strings in substitution function.
       The full description of the latter is in the pcre2api documentation.


QUOTING

         \x         where x is non-alphanumeric is a literal x
         \Q...\E    treat enclosed characters as literal

       Note that white space inside \Q...\E is always treated as literal, even
       if PCRE2_EXTENDED is set, causing most other white space to be ignored.
       Note  also  that  PCRE2's handling of \Q...\E has some differences from
       Perl's. See the pcre2pattern documentation for details.


BRACED ITEMS

       With one exception, wherever brace characters { and } are  required  to
       enclose  data for constructions such as \g{2} or \k{name}, space and/or
       horizontal tab characters that follow { or precede }  are  allowed  and
       are ignored. In the case of quantifiers, they may also appear before or
       after  the comma. The exception is \u{...} which is not Perl-compatible
       and is recognized only when PCRE2_EXTRA_ALT_BSUX is set. This is an EC-
       MAScript compatibility feature, and follows ECMAScript's behaviour.


ESCAPED CHARACTERS

       This table applies to ASCII and Unicode environments.  An  unrecognized
       escape sequence causes an error.

         \a         alarm, that is, the BEL character (hex 07)
         \cx        "control-x", where x is a non-control ASCII character
         \e         escape (hex 1B)
         \f         form feed (hex 0C)
         \n         newline (hex 0A)
         \r         carriage return (hex 0D)
         \t         tab (hex 09)
         \0dd       character with octal code 0dd
         \ddd       character with octal code ddd, or backreference
         \o{ddd..}  character with octal code ddd..
         \N{U+hh..} character with Unicode code point hh.. (Unicode mode only)
         \xhh       character with hex code hh
         \x{hh..}   character with hex code hh..

       \N{U+hh..} is synonymous with \x{hh..} but is not supported in environ-
       ments  that  use  EBCDIC code (mainly IBM mainframes). Note that \N not
       followed by an opening curly bracket has a different meaning  (see  be-
       low).

       If PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX is set ("ALT_BSUX mode"), the
       following are also recognized:

         \U         the character "U"
         \uhhhh     character with hex code hhhh
         \u{hh..}   character with hex code hh.. but only for EXTRA_ALT_BSUX

       When  \x  is not followed by {, one or two hexadecimal digits are read,
       but in ALT_BSUX mode \x must be followed by two hexadecimal  digits  to
       be  recognized  as a hexadecimal escape; otherwise it matches a literal
       "x".  Likewise, if \u (in ALT_BSUX mode) is not followed by four  hexa-
       decimal  digits or (in EXTRA_ALT_BSUX mode) a sequence of hex digits in
       curly brackets, it matches a literal "u".

       Note that \0dd is always an octal code. The treatment of backslash fol-
       lowed by a non-zero digit is complicated; for details see  the  section
       "Non-printing  characters" in the pcre2pattern documentation, where de-
       tails of escape processing in EBCDIC environments are also given.


CHARACTER TYPES

         .          any character except newline;
                      in dotall mode, any character whatsoever
         \C         one code unit, even in UTF mode (best avoided)
         \d         a decimal digit
         \D         a character that is not a decimal digit
         \h         a horizontal white space character
         \H         a character that is not a horizontal white space character
         \N         a character that is not a newline
         \p{xx}     a character with the xx property
         \P{xx}     a character without the xx property
         \R         a newline sequence
         \s         a white space character
         \S         a character that is not a white space character
         \v         a vertical white space character
         \V         a character that is not a vertical white space character
         \w         a "word" character
         \W         a "non-word" character
         \X         a Unicode extended grapheme cluster

       \C is dangerous because it may leave the current matching point in  the
       middle of a UTF-8 or UTF-16 character. The application can lock out the
       use  of  \C  by  setting the PCRE2_NEVER_BACKSLASH_C option. It is also
       possible to build PCRE2 with the use of \C permanently disabled.

       By default, \d, \s, and \w match only ASCII characters, even  in  UTF-8
       mode or in the 16-bit and 32-bit libraries. However, if locale-specific
       matching  is  happening,  \s and \w may also match characters with code
       points in the range 128-255. If the PCRE2_UCP option is set, the behav-
       iour of these escape sequences is changed to use Unicode properties and
       they match many more characters, but there  are  some  option  settings
       that  can  restrict individual sequences to matching only ASCII charac-
       ters.

       Property descriptions in \p and \P are matched caselessly; hyphens, un-
       derscores, and ASCII white space characters are ignored, in  accordance
       with  Unicode's  "loose matching" rules. For example, \p{Bidi_Class=al}
       is the same as \p{ bidi class = AL }.


GENERAL CATEGORY PROPERTIES FOR \p and \P

         C          Other
         Cc         Control
         Cf         Format
         Cn         Unassigned
         Co         Private use
         Cs         Surrogate

         L          Letter
         Lc         Cased letter, the union of Ll, Lu, and Lt
         L&         Synonym of Lc
         Ll         Lower case letter
         Lm         Modifier letter
         Lo         Other letter
         Lt         Title case letter
         Lu         Upper case letter

         M          Mark
         Mc         Spacing mark
         Me         Enclosing mark
         Mn         Non-spacing mark

         N          Number
         Nd         Decimal number
         Nl         Letter number
         No         Other number

         P          Punctuation
         Pc         Connector punctuation
         Pd         Dash punctuation
         Pe         Close punctuation
         Pf         Final punctuation
         Pi         Initial punctuation
         Po         Other punctuation
         Ps         Open punctuation

         S          Symbol
         Sc         Currency symbol
         Sk         Modifier symbol
         Sm         Mathematical symbol
         So         Other symbol

         Z          Separator
         Zl         Line separator
         Zp         Paragraph separator
         Zs         Space separator

       From release 10.45, when caseless matching is set, Ll, Lu, and  Lt  are
       all equivalent to Lc.


PCRE2 SPECIAL CATEGORY PROPERTIES FOR \p and \P

         Xan        Alphanumeric: union of properties L and N
         Xps        POSIX space: property Z or tab, NL, VT, FF, CR
         Xsp        Perl space: property Z or tab, NL, VT, FF, CR
         Xuc        Universally-named character: one that can be
                      represented by a Universal Character Name
         Xwd        Perl word: property Xan or underscore

       Perl and POSIX space are now the same. Perl added VT to its space char-
       acter set at release 5.18.


BINARY PROPERTIES FOR \p AND \P

       Unicode  defines  a  number  of  binary properties, that is, properties
       whose only values are true or false. You can obtain  a  list  of  those
       that  are  recognized  by \p and \P, along with their abbreviations, by
       running this command:

         pcre2test -LP


SCRIPT MATCHING WITH \p AND \P

       Many script names and their 4-letter abbreviations  are  recognized  in
       \p{sc:...}  or  \p{scx:...} items, or on their own with \p (and also \P
       of course). You can obtain a list of these scripts by running this com-
       mand:

         pcre2test -LS


THE BIDI_CLASS PROPERTY FOR \p AND \P

         \p{Bidi_Class:<class>}   matches a character with the given class
         \p{BC:<class>}           matches a character with the given class

       The recognized classes are:

         AL          Arabic letter
         AN          Arabic number
         B           paragraph separator
         BN          boundary neutral
         CS          common separator
         EN          European number
         ES          European separator
         ET          European terminator
         FSI         first strong isolate
         L           left-to-right
         LRE         left-to-right embedding
         LRI         left-to-right isolate
         LRO         left-to-right override
         NSM         non-spacing mark
         ON          other neutral
         PDF         pop directional format
         PDI         pop directional isolate
         R           right-to-left
         RLE         right-to-left embedding
         RLI         right-to-left isolate
         RLO         right-to-left override
         S           segment separator
         WS          white space


CHARACTER CLASSES

         [...]       positive character class
         [^...]      negative character class
         [x-y]       range (can be used for hex characters)
         [[:xxx:]]   positive POSIX named set
         [[:^xxx:]]  negative POSIX named set

         alnum       alphanumeric
         alpha       alphabetic
         ascii       0-127
         blank       space or tab
         cntrl       control character
         digit       decimal digit
         graph       printing, excluding space
         lower       lower case letter
         print       printing, including space
         punct       printing, excluding alphanumeric
         space       white space
         upper       upper case letter
         word        same as \w
         xdigit      hexadecimal digit

       In PCRE2, POSIX character set names recognize only ASCII characters  by
       default,  but  some of them use Unicode properties if PCRE2_UCP is set.
       You can use \Q...\E inside a character class.

       When PCRE2_ALT_EXTENDED_CLASS is set, UTS#18 extended character classes
       may be used, allowing nested character classes, combined using set  op-
       erators.

         [x&&[^y]]   UTS#18 extended character class

         x||y        set union (OR)
         x&&y        set intersection (AND)
         x--y        set difference (AND NOT)
         x~~y        set symmetric difference (XOR)


PERL EXTENDED CHARACTER CLASSES

         (?[...])                Perl extended character class
         (?[\p{Thai} & \p{Nd}])  operators; white space ignored
         (?[(x - y) & z])        parentheses for grouping

         (?[ [^3] & \p{Nd} ])    [...] is a nested ordinary class
         (?[ [:alpha:] - [z] ])  POSIX set is allowed outside [...]
         (?[  \d  -  [3]  ])          backslash-escaped set is allowed outside
       [...]
         (?[ !\n & [:ascii:] ])  backslash-escaped character is  allowed  out-
       side [...]
                             all  other  characters or ranges must be enclosed
       in [...]

         x|y, x+y                set union (OR)
         x&y                     set intersection (AND)
         x-y                     set difference (AND NOT)
         x^y                     set symmetric difference (XOR)
         !x                      set complement (NOT)

       Inside a Perl extended character class, [...] switches mode to  be  in-
       terpreted  as  an  ordinary character class. Outside of a nested [...],
       the only items permitted are backslash-escapes, POSIX sets,  operators,
       and  parentheses. Inside a nested ordinary class, ^ has its usual mean-
       ing (inverts the class when used as the first character); outside of  a
       nested class, ^ is the XOR operator.


QUANTIFIERS

         ?           0 or 1, greedy
         ?+          0 or 1, possessive
         ??          0 or 1, lazy
         *           0 or more, greedy
         *+          0 or more, possessive
         *?          0 or more, lazy
         +           1 or more, greedy
         ++          1 or more, possessive
         +?          1 or more, lazy
         {n}         exactly n
         {n,m}       at least n, no more than m, greedy
         {n,m}+      at least n, no more than m, possessive
         {n,m}?      at least n, no more than m, lazy
         {n,}        n or more, greedy
         {n,}+       n or more, possessive
         {n,}?       n or more, lazy
         {,m}        zero up to m, greedy
         {,m}+       zero up to m, possessive
         {,m}?       zero up to m, lazy


ANCHORS AND SIMPLE ASSERTIONS

         \b          word boundary
         \B          not a word boundary
         ^           start of subject
                       also after an internal newline in multiline mode
                       (after any newline if PCRE2_ALT_CIRCUMFLEX is set)
         \A          start of subject
         $           end of subject
                       also before newline at end of subject
                       also before internal newline in multiline mode
         \Z          end of subject
                       also before newline at end of subject
         \z          end of subject
         \G          first matching position in subject


REPORTED MATCH POINT SETTING

         \K          set reported start of match

       From  release 10.38 \K is not permitted by default in lookaround asser-
       tions, for compatibility with Perl.  However,  if  the  PCRE2_EXTRA_AL-
       LOW_LOOKAROUND_BSK option is set, the previous behaviour is re-enabled.
       When this option is set, \K is honoured in positive assertions, but ig-
       nored in negative ones.


ALTERNATION

         expr|expr|expr...


CAPTURING

         (...)           capture group
         (?<name>...)    named capture group (Perl)
         (?'name'...)    named capture group (Perl)
         (?P<name>...)   named capture group (Python)
         (?:...)         non-capture group
         (?|...)         non-capture group; reset group numbers for
                          capture groups in each alternative

       In  non-UTF  modes, names may contain underscores and ASCII letters and
       digits; in UTF modes, any Unicode letters and  Unicode  decimal  digits
       are permitted. In both cases, a name must not start with a digit.


ATOMIC GROUPS

         (?>...)         atomic non-capture group
         (*atomic:...)   atomic non-capture group


COMMENT

         (?#....)        comment (not nestable)


OPTION SETTING
       Changes  of these options within a group are automatically cancelled at
       the end of the group.

         (?a)            all ASCII options
         (?aD)           restrict \d to ASCII in UCP mode
         (?aS)           restrict \s to ASCII in UCP mode
         (?aW)           restrict \w to ASCII in UCP mode
         (?aP)           restrict all POSIX classes to ASCII in UCP mode
         (?aT)           restrict POSIX digit classes to ASCII in UCP mode
         (?i)            caseless
         (?J)            allow duplicate named groups
         (?m)            multiline
         (?n)            no auto capture
         (?r)            restrict caseless to either ASCII or non-ASCII
         (?s)            single line (dotall)
         (?U)            default ungreedy (lazy)
         (?x)            ignore white space except in classes or \Q...\E
         (?xx)           as (?x) but also ignore space and tab in classes
         (?-...)         unset the given option(s)
         (?^)            unset imnrsx options

       (?aP) implies (?aT) as well, though this has no additional effect. How-
       ever, it means that (?-aP) also implies (?-aT) and disables  all  ASCII
       restrictions for POSIX classes.

       Unsetting  x or xx unsets both. Several options may be set at once, and
       a mixture of setting and unsetting such as (?i-x) is allowed, but there
       may be only one hyphen. Setting (but no unsetting) is allowed after (?^
       for example (?^in). An option setting may appear at the start of a non-
       capture group, for example (?i:...).

       The following are recognized only at the very start of a pattern or af-
       ter one of the newline or \R sequences or options with similar  syntax.
       More  than  one of them may appear. For the first three, d is a decimal
       number.

         (*LIMIT_DEPTH=d)     set the backtracking limit to d
         (*LIMIT_HEAP=d)      set the heap size limit to d * 1024 bytes
         (*LIMIT_MATCH=d)     set the match limit to d
         (*CASELESS_RESTRICT) set PCRE2_EXTRA_CASELESS_RESTRICT when matching
         (*NOTEMPTY)          set PCRE2_NOTEMPTY when matching
         (*NOTEMPTY_ATSTART)  set PCRE2_NOTEMPTY_ATSTART when matching
         (*NO_AUTO_POSSESS)   no auto-possessification (PCRE2_NO_AUTO_POSSESS)
         (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR)
         (*NO_JIT)            disable JIT optimization
         (*NO_START_OPT)      no start-match optimization  (PCRE2_NO_START_OP-
       TIMIZE)
         (*TURKISH_CASING)    set PCRE2_EXTRA_TURKISH_CASING when matching
         (*UTF)               set appropriate UTF mode for the library in use
         (*UCP)                set  PCRE2_UCP  (use  Unicode properties for \d
       etc)

       Note that LIMIT_DEPTH, LIMIT_HEAP, and LIMIT_MATCH can only reduce  the
       value   of   the   limits   set  by  the  caller  of  pcre2_match()  or
       pcre2_dfa_match(), not increase them. LIMIT_RECURSION  is  an  obsolete
       synonym for LIMIT_DEPTH. The application can lock out the use of (*UTF)
       and  (*UCP)  by setting the PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options,
       respectively, at compile time.


NEWLINE CONVENTION

       These are recognized only at the very start of the pattern or after op-
       tion settings with a similar syntax.

         (*CR)           carriage return only
         (*LF)           linefeed only
         (*CRLF)         carriage return followed by linefeed
         (*ANYCRLF)      all three of the above
         (*ANY)          any Unicode newline sequence
         (*NUL)          the NUL character (binary zero)


WHAT \R MATCHES

       These are recognized only at the very start of the pattern or after op-
       tion setting with a similar syntax.

         (*BSR_ANYCRLF)  CR, LF, or CRLF
         (*BSR_UNICODE)  any Unicode newline sequence


LOOKAHEAD AND LOOKBEHIND ASSERTIONS

         (?=...)                     )
         (*pla:...)                  ) positive lookahead
         (*positive_lookahead:...)   )

         (?!...)                     )
         (*nla:...)                  ) negative lookahead
         (*negative_lookahead:...)   )

         (?<=...)                    )
         (*plb:...)                  ) positive lookbehind
         (*positive_lookbehind:...)  )

         (?<!...)                    )
         (*nlb:...)                  ) negative lookbehind
         (*negative_lookbehind:...)  )

       Each top-level branch of a lookbehind must have a limit for the  number
       of  characters it matches. If any branch can match a variable number of
       characters, the maximum for each branch is limited to a  value  set  by
       the  caller  of  pcre2_compile()  or defaulted. The default is set when
       PCRE2 is built (ultimate default 255). If every branch matches a  fixed
       number of characters, the limit for each branch is 65535 characters.


NON-ATOMIC LOOKAROUND ASSERTIONS

       These assertions are specific to PCRE2 and are not Perl-compatible.

         (?*...)                                )
         (*napla:...)                           ) synonyms
         (*non_atomic_positive_lookahead:...)   )

         (?<*...)                               )
         (*naplb:...)                           ) synonyms
         (*non_atomic_positive_lookbehind:...)  )


SUBSTRING SCAN ASSERTION
       This feature is not Perl-compatible.

         (*scan_substring:(grouplist)...)  scan captured substring
         (*scs:(grouplist)...)             scan captured substring

       The  comma-separated list "grouplist" may identify groups in any of the
       following ways:

         n       absolute reference
         +n      relative reference
         -n      relative reference
         <name>  name
         'name'  name


SCRIPT RUNS

         (*script_run:...)           ) script run, can be backtracked into
         (*sr:...)                   )

         (*atomic_script_run:...)    ) atomic script run
         (*asr:...)                  )


BACKREFERENCES

         \n              reference by number (can be ambiguous)
         \gn             reference by number
         \g{n}           reference by number
         \g+n            relative reference by number (PCRE2 extension)
         \g-n            relative reference by number
         \g{+n}          relative reference by number (PCRE2 extension)
         \g{-n}          relative reference by number
         \k<name>        reference by name (Perl)
         \k'name'        reference by name (Perl)
         \g{name}        reference by name (Perl)
         \k{name}        reference by name (.NET)
         (?P=name)       reference by name (Python)


SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)

         (?R)            recurse whole pattern
         (?n)            call subroutine by absolute number
         (?+n)           call subroutine by relative number
         (?-n)           call subroutine by relative number
         (?&name)        call subroutine by name (Perl)
         (?P>name)       call subroutine by name (Python)
         \g<name>        call subroutine by name (Oniguruma)
         \g'name'        call subroutine by name (Oniguruma)
         \g<n>           call subroutine by absolute number (Oniguruma)
         \g'n'           call subroutine by absolute number (Oniguruma)
         \g<+n>          call subroutine by relative number (PCRE2 extension)
         \g'+n'          call subroutine by relative number (PCRE2 extension)
         \g<-n>          call subroutine by relative number (PCRE2 extension)
         \g'-n'          call subroutine by relative number (PCRE2 extension)

       The variants using parentheses (?...) may also specify a list  of  cap-
       ture  groups  to  return, which shall be retained in the calling subex-
       pression if set during the recursion (this feature is not supported  by
       Perl).

         (?R(grouplist))       recurse whole pattern, returning capture groups
                                 (PCRE2 extension)
         (?n(grouplist))       )
         (?+n(grouplist))      ) call subroutine, returning capture groups
         (?-n(grouplist))      )   (PCRE2 extension)
         (?&name(grouplist))   )
         (?P>name(grouplist))  )

       The   comma-separated   list   "grouplist"  uses  the  same  syntax  as
       (*scan_substring:(grouplist)...), and may identify groups in any of the
       following ways:

         n       absolute reference
         +n      relative reference
         -n      relative reference
         <name>  name
         'name'  name


CONDITIONAL PATTERNS

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

         (?(n)                absolute reference condition
         (?(+n)               relative reference condition (PCRE2 extension)
         (?(-n)               relative reference condition (PCRE2 extension)
         (?(<name>)           named reference condition (Perl)
         (?('name')           named reference condition (Perl)
         (?(name)             named reference condition (PCRE2, deprecated)
         (?(R)                overall recursion condition
         (?(Rn)               specific numbered group recursion condition
         (?(R&name)           specific named group recursion condition
         (?(DEFINE)           define groups for reference
         (?(VERSION[>]=n[.m]) test PCRE2 version
         (?(assert)           assertion condition

       Note the ambiguity of (?(R) and (?(Rn) which might be  named  reference
       conditions  or  recursion  tests.  Such a condition is interpreted as a
       reference condition if the relevant named group exists.

       The parts within brackets for the VERSION conditional syntax  could  be
       ommited.   The  fractional  part of the version number defaults to 0 in
       that case.


BACKTRACKING CONTROL

       All backtracking control verbs may be in  the  form  (*VERB:NAME).  For
       (*MARK)  the  name is mandatory, for the others it is optional. (*SKIP)
       changes its behaviour if :NAME is present. The others just set  a  name
       for passing back to the caller, but this is not a name that (*SKIP) can
       see. The following act immediately they are reached:

         (*ACCEPT)       force successful match
         (*FAIL)         force backtrack; synonym (*F)
         (*MARK:NAME)    set name to be passed back; synonym (*:NAME)

       The  following  act only when a subsequent match failure causes a back-
       track to reach them. They all force a match failure, but they differ in
       what happens afterwards. Those that advance the start-of-match point do
       so only if the pattern is not anchored.

         (*COMMIT)       overall failure, no advance of starting point
         (*PRUNE)        advance to next starting character
         (*SKIP)         advance to current matching position
         (*SKIP:NAME)    advance to position corresponding to an earlier
                         (*MARK:NAME); if not found, the (*SKIP) is ignored
         (*THEN)         local failure, backtrack to next alternation

       The effect of one of these verbs in a group called as a  subroutine  is
       confined to the subroutine call.


CALLOUTS

         (?C)            callout (assumed number 0)
         (?Cn)           callout with numerical data n
         (?C"text")      callout with string data

       The allowed string delimiters are ` ' " ^ % # $ (which are the same for
       the  start  and the end), and the starting delimiter { matched with the
       ending delimiter }. To encode the ending delimiter within  the  string,
       double it.


REPLACEMENT STRINGS

       If the PCRE2_SUBSTITUTE_LITERAL option is set, a replacement string for
       pcre2_substitute()  is not interpreted. Otherwise, by default, the only
       special character is the dollar  character  in  one  of  the  following
       forms:

         $$                  insert a dollar character
         $n or ${n}          insert the contents of group n
         $<name>             insert the contents of named group
         $0 or $&            insert the entire matched substring
         $`                  insert the substring that precedes the match
         $'                  insert the substring that follows the match
         $_                  insert the entire input string
         $+                   insert  the highest-numbered capture group which
       matched
         $*MARK or ${*MARK}  insert a control verb name

       For ${n}, n can be a name or a number. If PCRE2_SUBSTITUTE_EXTENDED  is
       set, there is additional interpretation:

       1.  Backslash  is  an escape character, and the forms described in "ES-
       CAPED CHARACTERS" above are recognized. Also:

         \Q...\E can be used to suppress interpretation
         \l      force the next character to lower case
         \u      force the next character to upper case
         \L      force subsequent characters to lower case
         \U      force subsequent characters to upper case
         \u\L    force next character to upper case, then all lower
         \l\U    force next character to lower case, then all upper
         \E      end \L or \U case forcing
         \b      backspace character (note: as in character class in pattern)
         \v      vertical tab character (note: not the same as in a pattern)

       2. The Python form \g<n>, where the angle brackets are part of the syn-
       tax and n is either a group name or a number, is recognized as  an  al-
       ternative way of inserting the contents of a group, for example \g<3>.

       3. Capture substitution supports the following additional forms:

         ${n:-string}             default for unset group
         ${n:+string1:string2}    values for set/unset group

       The substitution strings themselves are expanded. Backslash can be used
       to escape colons and closing curly brackets.


SEE ALSO

       pcre2pattern(3),    pcre2api(3),   pcre2callout(3),   pcre2matching(3),
       pcre2(3).


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 14 October 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                     14 October 2025                 PCRE2SYNTAX(3)
------------------------------------------------------------------------------


PCRE2UNICODE(3)            Library Functions Manual            PCRE2UNICODE(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


UNICODE AND UTF SUPPORT

       PCRE2 is normally built with Unicode support, though if you do not need
       it,  you  can  build  it  without,  in  which  case the library will be
       smaller. With Unicode support, PCRE2 has knowledge of Unicode character
       properties and can process strings of text in UTF-8, UTF-16, and UTF-32
       format (depending on the code unit width), but this is not the default.
       Unless specifically requested, PCRE2 treats each code unit in a  string
       as one character.

       There  are two ways of telling PCRE2 to switch to UTF mode, where char-
       acters may consist of more than one code unit and the range  of  values
       is constrained. The program can call pcre2_compile() with the PCRE2_UTF
       option,  or  the  pattern may start with the sequence (*UTF).  However,
       the latter facility can be locked out by  the  PCRE2_NEVER_UTF  option.
       That  is,  the  programmer can prevent the supplier of the pattern from
       switching to UTF mode.

       Note  that  the  PCRE2_MATCH_INVALID_UTF  option  (see  below)   forces
       PCRE2_UTF to be set.

       In  UTF mode, both the pattern and any subject strings that are matched
       against it are treated as UTF strings instead of strings of  individual
       one-code-unit  characters. There are also some other changes to the way
       characters are handled, as documented below.


UNICODE PROPERTY SUPPORT

       When PCRE2 is built with Unicode support, the escape sequences  \p{..},
       \P{..}, and \X can be used. This is not dependent on the PCRE2_UTF set-
       ting.   The Unicode properties that can be tested are a subset of those
       that Perl supports. Currently they are limited to the general  category
       properties such as Lu for an upper case letter or Nd for a decimal num-
       ber, the derived properties Any and Lc (synonym L&), the Unicode script
       names such as Arabic or Han, Bidi_Class, Bidi_Control, and a few binary
       properties.

       The full lists are given in the pcre2pattern and pcre2syntax documenta-
       tion.  In  general,  only the short names for properties are supported.
       For example, \p{L} matches a letter. Its longer synonym, \p{Letter}, is
       not supported. Furthermore, in Perl, many properties may optionally  be
       prefixed  by "Is", for compatibility with Perl 5.6. PCRE2 does not sup-
       port this.


WIDE CHARACTERS AND UTF MODES

       Code points less than 256 can be specified in patterns by either braced
       or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3).
       Larger values have to use braced sequences. Unbraced octal code  points
       up to \777 are also recognized; larger ones can be coded using \o{...}.

       The  escape sequence \N{U+<hex digits>} is recognized as another way of
       specifying a Unicode character by code point in a UTF mode. It  is  not
       allowed in non-UTF mode.

       In  UTF  mode, repeat quantifiers apply to complete UTF characters, not
       to individual code units.

       In UTF mode, the dot metacharacter matches one UTF character instead of
       a single code unit.

       In UTF mode, capture group names are not restricted to ASCII,  and  may
       contain any Unicode letters and decimal digits, as well as underscore.

       The  escape  sequence \C can be used to match a single code unit in UTF
       mode, but its use can lead to some strange effects because it breaks up
       multi-unit characters (see the description of \C  in  the  pcre2pattern
       documentation). For this reason, there is a build-time option that dis-
       ables  support  for  \C completely. There is also a less draconian com-
       pile-time option for locking out the use of \C when a pattern  is  com-
       piled.

       The  use  of  \C  is not supported by the alternative matching function
       pcre2_dfa_match() when in UTF-8 or UTF-16 mode, that is, when a charac-
       ter may consist of more than one code unit. The  use  of  \C  in  these
       modes  provokes a match-time error. Also, the JIT optimization does not
       support \C in these modes. If JIT optimization is requested for a UTF-8
       or UTF-16 pattern that contains \C, it will not succeed,  and  so  when
       pcre2_match() is called, the matching will be carried out by the inter-
       pretive function.

       The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test
       characters  of  any  code  value,  but, by default, the characters that
       PCRE2 recognizes as digits, spaces, or word characters remain the  same
       set  as  in  non-UTF mode, all with code points less than 256. This re-
       mains true even when PCRE2 is built to include Unicode support, because
       to do otherwise would slow down matching in  many  common  cases.  Note
       that  this also applies to \b and \B, because they are defined in terms
       of \w and \W. If you want to test for a wider sense of,  say,  "digit",
       you  can  use  explicit Unicode property tests such as \p{Nd}. Alterna-
       tively, if you set the PCRE2_UCP option, the way that the character es-
       capes work is changed so that Unicode properties are used to  determine
       which  characters  match,  though  there are some options that suppress
       this for individual escapes. For details see  the  section  on  generic
       character types in the pcre2pattern documentation.

       Like  the  escapes,  characters  that  match  the POSIX named character
       classes are all low-valued characters unless the  PCRE2_UCP  option  is
       set, but there is an option to override this.

       In contrast to the character escapes and character classes, the special
       horizontal  and  vertical  white  space escapes (\h, \H, \v, and \V) do
       match all the appropriate Unicode characters, whether or not  PCRE2_UCP
       is set.


UNICODE CASE-EQUIVALENCE

       If  either  PCRE2_UTF  or PCRE2_UCP is set, upper/lower case processing
       makes use of Unicode properties except for characters whose code points
       are less than 128 and that have at most two case-equivalent values. For
       these, a direct table lookup is used for speed. A few  Unicode  charac-
       ters  such as Greek sigma have more than two code points that are case-
       equivalent, and these are treated specially. Setting PCRE2_UCP  without
       PCRE2_UTF  allows  Unicode-style  case processing for non-UTF character
       encodings such as UCS-2.

       There are two ASCII characters (S and K) that,  in  addition  to  their
       ASCII  lower case equivalents, have a non-ASCII one as well (long S and
       Kelvin sign).  Recognition of these non-ASCII characters as case-equiv-
       alent to their ASCII  counterparts  can  be  disabled  by  setting  the
       PCRE2_EXTRA_CASELESS_RESTRICT  option. When this is set, all characters
       in a case equivalence must either be ASCII or non-ASCII; there  can  be
       no mixing.

           Without PCRE2_EXTRA_CASELESS_RESTRICT:
             'k' = 'K' = U+212A (Kelvin sign)
             's' = 'S' = U+017F (long S)
           With PCRE2_EXTRA_CASELESS_RESTRICT:
             'k' = 'K'
             U+212A (Kelvin sign)  only case-equivalent to itself
             's' = 'S'
             U+017F (long S)       only case-equivalent to itself

       One  language family, Turkish and Azeri, has its own case-insensitivity
       rules, which can be  selected  by  setting  PCRE2_EXTRA_TURKISH_CASING.
       This  alters  the behaviour of the 'i', 'I', U+0130 (capital I with dot
       above), and U+0131 (small dotless i) characters.

           Without PCRE2_EXTRA_TURKISH_CASING:
             'i' = 'I'
             U+0130 (capital I with dot above)  only case-equivalent to itself
             U+0131 (small dotless i)           only case-equivalent to itself
           With PCRE2_EXTRA_TURKISH_CASING:
             'i' = U+0130 (capital I with dot above)
             U+0131 (small dotless i) = 'I'

       It is not allowed to  specify  both  PCRE2_EXTRA_CASELESS_RESTRICT  and
       PCRE2_EXTRA_TURKISH_CASING together.

       From  release  10.45  the Unicode letter properties Lu (upper case), Ll
       (lower case), and Lt (title case) are all treated as Lc (cased  letter)
       when  caseless  matching  is  set  by the PCRE2_CASELESS option or (?i)
       within the pattern.


SCRIPT RUNS

       The pattern constructs (*script_run:...) and  (*atomic_script_run:...),
       with  synonyms (*sr:...) and (*asr:...), verify that the string matched
       within the parentheses is a script run. In concept, a script run  is  a
       sequence  of characters that are all from the same Unicode script. How-
       ever, because some scripts are commonly used together, and because some
       diacritical and other marks are used with multiple scripts, it  is  not
       that simple.

       Every Unicode character has a Script property, mostly with a value cor-
       responding  to the name of a script, such as Latin, Greek, or Cyrillic.
       There are also three special values:

       "Unknown" is used for code points that have not been assigned, and also
       for the surrogate code points. In the PCRE2 32-bit library,  characters
       whose  code  points  are  greater  than the Unicode maximum (U+10FFFF),
       which are accessible only in non-UTF mode,  are  assigned  the  Unknown
       script.

       "Common"  is used for characters that are used with many scripts. These
       include punctuation, emoji, mathematical, musical,  and  currency  sym-
       bols, and the ASCII digits 0 to 9.

       "Inherited"  is used for characters such as diacritical marks that mod-
       ify a previous character. These are considered to take on the script of
       the character that they modify.

       Some Inherited characters are used with many scripts, but many of  them
       are  only  normally  used  with a small number of scripts. For example,
       U+102E0 (Coptic Epact thousands mark) is used only with Arabic and Cop-
       tic. In order to make it possible to check  this,  a  Unicode  property
       called Script Extension exists. Its value is a list of scripts that ap-
       ply to the character. For the majority of characters, the list contains
       just  one  script,  the  same  one as the Script property. However, for
       characters such as U+102E0 more than one Script is  listed.  There  are
       also  some  Common  characters that have a single, non-Common script in
       their Script Extension list.

       The next section describes the basic rules for deciding whether a given
       string of characters is a script run. Note,  however,  that  there  are
       some  special cases involving the Chinese Han script, and an additional
       constraint for decimal digits. These are  covered  in  subsequent  sec-
       tions.

   Basic script run rules

       A string that is less than two characters long is a script run. This is
       the  only  case  in  which an Unknown character can be part of a script
       run. Longer strings are checked using only the Script Extensions  prop-
       erty, not the basic Script property.

       If  a character's Script Extension property is the single value "Inher-
       ited", it is always accepted as part of a script run. This is also true
       for the property "Common", subject to the checking  of  decimal  digits
       described below. All the remaining characters in a script run must have
       at  least one script in common in their Script Extension lists. In set-
       theoretic terminology, the intersection of all the sets of scripts must
       not be empty.

       A simple example is an Internet name such as "google.com". The  letters
       are all in the Latin script, and the dot is Common, so this string is a
       script run.  However, the Cyrillic letter "o" looks exactly the same as
       the  Latin "o"; a string that looks the same, but with Cyrillic "o"s is
       not a script run.

       More interesting examples involve characters with more than one  script
       in their Script Extension. Consider the following characters:

         U+060C  Arabic comma
         U+06D4  Arabic full stop

       The  first  has the Script Extension list Arabic, Hanifi Rohingya, Syr-
       iac, and Thaana; the second has just Arabic and Hanifi  Rohingya.  Both
       of  them  could  appear  in  script runs of either Arabic or Hanifi Ro-
       hingya. The first could also appear in Syriac or  Thaana  script  runs,
       but the second could not.

   The Chinese Han script

       The  Chinese  Han  script  is  commonly  used in conjunction with other
       scripts for writing certain languages. Japanese uses the  Hiragana  and
       Katakana  scripts  together  with Han; Korean uses Hangul and Han; Tai-
       wanese Mandarin uses Bopomofo and Han.  These  three  combinations  are
       treated  as special cases when checking script runs and are, in effect,
       "virtual scripts". Thus, a script run may contain a  mixture  of  Hira-
       gana,  Katakana,  and Han, or a mixture of Hangul and Han, or a mixture
       of Bopomofo and Han, but not, for example,  a  mixture  of  Hangul  and
       Bopomofo  and  Han. PCRE2 (like Perl) follows Unicode's Technical Stan-
       dard  39   ("Unicode   Security   Mechanisms",   http://unicode.org/re-
       ports/tr39/) in allowing such mixtures.

   Decimal digits

       Unicode  contains  many sets of 10 decimal digits in different scripts,
       and some scripts (including the Common script) contain  more  than  one
       set.  Some  of these decimal digits them are visually indistinguishable
       from the common ASCII digits. In addition to the  script  checking  de-
       scribed  above,  if a script run contains any decimal digits, they must
       all come from the same set of 10 adjacent characters.


VALIDITY OF UTF STRINGS

       When the PCRE2_UTF option is set, the strings passed  as  patterns  and
       subjects are (by default) checked for validity on entry to the relevant
       functions. If an invalid UTF string is passed, a negative error code is
       returned.  The  code  unit offset to the offending character can be ex-
       tracted from the match data  block  by  calling  pcre2_get_startchar(),
       which is used for this purpose after a UTF error.

       In  some  situations, you may already know that your strings are valid,
       and therefore want to skip these checks in  order  to  improve  perfor-
       mance,  for  example in the case of a long subject string that is being
       scanned repeatedly.  If you set the PCRE2_NO_UTF_CHECK option  at  com-
       pile  time  or at match time, PCRE2 assumes that the pattern or subject
       it is given (respectively) contains only valid UTF code unit sequences.

       If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is  set,  the
       result  is undefined and your program may crash or loop indefinitely or
       give incorrect results. There is, however, one mode  of  matching  that
       can  handle  invalid  UTF  subject  strings. This is enabled by passing
       PCRE2_MATCH_INVALID_UTF to pcre2_compile() and is  discussed  below  in
       the  next  section.  The  rest  of  this  section  covers the case when
       PCRE2_MATCH_INVALID_UTF is not set.

       Passing PCRE2_NO_UTF_CHECK to pcre2_compile()  just  disables  the  UTF
       check  for  the  pattern; it does not also apply to subject strings. If
       you want to disable the check for a subject string you must  pass  this
       same option to pcre2_match() or pcre2_dfa_match().

       UTF-16 and UTF-32 strings can indicate their endianness by special code
       knows  as  a  byte-order  mark (BOM). The PCRE2 functions do not handle
       this, expecting strings to be in host byte order.

       Unless PCRE2_NO_UTF_CHECK is set, a UTF string is  checked  before  any
       other  processing  takes  place.  In  the  case  of  pcre2_match()  and
       pcre2_dfa_match() calls with a non-zero starting offset, the  check  is
       applied only to that part of the subject that could be inspected during
       matching,  and  there is a check that the starting offset points to the
       first code unit of a character or to the end of the subject.  If  there
       are  no  lookbehind  assertions in the pattern, the check starts at the
       starting offset.  Otherwise, it starts at the  length  of  the  longest
       lookbehind  before  the starting offset, or at the start of the subject
       if there are not that many characters before the starting offset.  Note
       that the sequences \b and \B are one-character lookbehinds.

       In  addition  to checking the format of the string, there is a check to
       ensure that all code points lie in the range U+0 to U+10FFFF, excluding
       the surrogate area. The so-called "non-character" code points  are  not
       excluded because Unicode corrigendum #9 makes it clear that they should
       not be.

       Characters  in  the "Surrogate Area" of Unicode are reserved for use by
       UTF-16, where they are used in pairs to encode code points with  values
       greater  than  0xFFFF. The code points that are encoded by UTF-16 pairs
       are available independently in the  UTF-8  and  UTF-32  encodings.  (In
       other  words, the whole surrogate thing is a fudge for UTF-16 which un-
       fortunately messes up UTF-8 and UTF-32.)

       Setting PCRE2_NO_UTF_CHECK at compile time does not disable  the  error
       that  is  given if an escape sequence for an invalid Unicode code point
       is encountered in the pattern. If you want to  allow  escape  sequences
       such  as  \x{d800}  (a  surrogate code point) you can set the PCRE2_EX-
       TRA_ALLOW_SURROGATE_ESCAPES extra option.  However,  this  is  possible
       only  in  UTF-8  and  UTF-32 modes, because these values are not repre-
       sentable in UTF-16.

   Errors in UTF-8 strings

       The following negative error codes are given for invalid UTF-8 strings:

         PCRE2_ERROR_UTF8_ERR1
         PCRE2_ERROR_UTF8_ERR2
         PCRE2_ERROR_UTF8_ERR3
         PCRE2_ERROR_UTF8_ERR4
         PCRE2_ERROR_UTF8_ERR5

       The string ends with a truncated UTF-8 character;  the  code  specifies
       how  many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
       characters to be no longer than 4 bytes, the  encoding  scheme  (origi-
       nally  defined  by  RFC  2279)  allows  for  up to 6 bytes, and this is
       checked first; hence the possibility of 4 or 5 missing bytes.

         PCRE2_ERROR_UTF8_ERR6
         PCRE2_ERROR_UTF8_ERR7
         PCRE2_ERROR_UTF8_ERR8
         PCRE2_ERROR_UTF8_ERR9
         PCRE2_ERROR_UTF8_ERR10

       The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
       the character do not have the binary value 0b10 (that  is,  either  the
       most significant bit is 0, or the next bit is 1).

         PCRE2_ERROR_UTF8_ERR11
         PCRE2_ERROR_UTF8_ERR12

       A  character that is valid by the RFC 2279 rules is either 5 or 6 bytes
       long; these code points are excluded by RFC 3629.

         PCRE2_ERROR_UTF8_ERR13

       A 4-byte character has a value greater than 0x10ffff; these code points
       are excluded by RFC 3629.

         PCRE2_ERROR_UTF8_ERR14

       A 3-byte character has a value in the  range  0xd800  to  0xdfff;  this
       range  of code points are reserved by RFC 3629 for use with UTF-16, and
       so are excluded from UTF-8.

         PCRE2_ERROR_UTF8_ERR15
         PCRE2_ERROR_UTF8_ERR16
         PCRE2_ERROR_UTF8_ERR17
         PCRE2_ERROR_UTF8_ERR18
         PCRE2_ERROR_UTF8_ERR19

       A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it  codes
       for  a  value that can be represented by fewer bytes, which is invalid.
       For example, the two bytes 0xc0, 0xae give the value 0x2e,  whose  cor-
       rect coding uses just one byte.

         PCRE2_ERROR_UTF8_ERR20

       The two most significant bits of the first byte of a character have the
       binary  value 0b10 (that is, the most significant bit is 1 and the sec-
       ond is 0). Such a byte can only validly occur as the second  or  subse-
       quent byte of a multi-byte character.

         PCRE2_ERROR_UTF8_ERR21

       The  first byte of a character has the value 0xfe or 0xff. These values
       can never occur in a valid UTF-8 string.

   Errors in UTF-16 strings

       The following  negative  error  codes  are  given  for  invalid  UTF-16
       strings:

         PCRE2_ERROR_UTF16_ERR1  Missing low surrogate at end of string
         PCRE2_ERROR_UTF16_ERR2  Invalid low surrogate follows high surrogate
         PCRE2_ERROR_UTF16_ERR3  Isolated low surrogate


   Errors in UTF-32 strings

       The  following  negative  error  codes  are  given  for  invalid UTF-32
       strings:

         PCRE2_ERROR_UTF32_ERR1  Surrogate character (0xd800 to 0xdfff)
         PCRE2_ERROR_UTF32_ERR2  Code point is greater than 0x10ffff


MATCHING IN INVALID UTF STRINGS

       You can run pattern matches on subject strings that may contain invalid
       UTF sequences if you  call  pcre2_compile()  with  the  PCRE2_MATCH_IN-
       VALID_UTF  option.  This  is  supported by pcre2_match(), including JIT
       matching, but not by pcre2_dfa_match(). When PCRE2_MATCH_INVALID_UTF is
       set, it forces PCRE2_UTF to be set as well.  Note,  however,  that  the
       pattern itself must be a valid UTF string.

       If  you  do not set PCRE2_MATCH_INVALID_UTF when calling pcre2_compile,
       and you are not certain that your subject strings  are  valid  UTF  se-
       quences,  you  should  not  make  use  of  the JIT "fast path" function
       pcre2_jit_match() because it bypasses sanity checks, including the  one
       for  UTF validity. An invalid string may cause undefined behaviour, in-
       cluding looping, crashing, or giving the wrong answer.

       Setting PCRE2_MATCH_INVALID_UTF does not  affect  what  pcre2_compile()
       generates,  but  if pcre2_jit_compile() is subsequently called, it does
       generate different code. If JIT is not used, the option affects the be-
       haviour of the interpretive code in pcre2_match(). When PCRE2_MATCH_IN-
       VALID_UTF is set at compile  time,  PCRE2_NO_UTF_CHECK  is  ignored  at
       match time.

       In  this  mode,  an  invalid  code  unit  sequence in the subject never
       matches any pattern item. It does not match  dot,  it  does  not  match
       \p{Any},  it does not even match negative items such as [^X]. A lookbe-
       hind assertion fails if it encounters an invalid sequence while  moving
       the  current  point backwards. In other words, an invalid UTF code unit
       sequence acts as a barrier which no match can cross.

       You can also think of this as the subject being split up into fragments
       of valid UTF, delimited internally by invalid code unit sequences.  The
       pattern  is  matched  fragment  by fragment. The result of a successful
       match, however, is given as code unit offsets  in  the  entire  subject
       string in the usual way. There are a few points to consider:

       The  internal  boundaries are not interpreted as the beginnings or ends
       of lines and so do not match circumflex or  dollar  characters  in  the
       pattern.

       If  pcre2_match()  is  called  with an offset that points to an invalid
       UTF-sequence, that sequence is skipped, and the  match  starts  at  the
       next valid UTF character, or the end of the subject.

       At internal fragment boundaries, \b and \B behave in the same way as at
       the  beginning  and end of the subject. For example, a sequence such as
       \bWORD\b would match an instance of WORD that is surrounded by  invalid
       UTF code units.

       Using  PCRE2_MATCH_INVALID_UTF, an application can run matches on arbi-
       trary data, knowing that any matched  strings  that  are  returned  are
       valid UTF. This can be useful when searching for UTF text in executable
       or other binary files.

       Note,  however,  that  the  16-bit  and  32-bit PCRE2 libraries process
       strings as sequences of uint16_t or uint32_t code points.  They  cannot
       find  valid  UTF  sequences  within an arbitrary string of bytes unless
       such sequences are suitably aligned.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 27 November 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.48                    27 November 2024                PCRE2UNICODE(3)
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