regex: regular expressions

The regex module provides an implementation of regular expressions which adheres
closely to the POSIX Extended Regular Expressions (ERE) specification.

See https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04

This module refers to a regular expression "match" as a [[result]]. The POSIX
match disambiguation rules are used; the longest of the leftmost matches is
returned. This implementation computes matches in linear time.

Compiling an expression:

	const re = regex::compile(`[Hh]a(rriet|ppy)`)!;
	defer regex::finish(&re);

Testing an expression against a string:

	assert(regex::test(&re, "Harriet is happy"));

Finding a match for an expression in a string:

	const result = regex::find(&re, "Harriet is happy");
	defer regex::result_free(result);
	for (let i = 0z; i < len(result); i += 1) {
		fmt::printf("{} ", result[i].content)!;
	};
	fmt::println()!;
	// -> Harriet rriet

Finding all matches for an expression in a string:

	const results = regex::findall(&re, "Harriet is happy");
	defer regex::result_freeall(results);
	for (let i = 0z; i < len(results); i += 1) {
		for (let j = 0z; j < len(results[i]); j += 1) {
			fmt::printf("{} ", results[i][j].content)!;
		};
		fmt::println()!;
	};
	// -> Harriet rriet; happy ppy

Replacing matches for an expression:

	const re = regex::compile(`happy`)!;
	const result = regex::replace(&re, "Harriet is happy", `cute`)!;
	// -> Harriet is cute

Replacing with capture group references:

	const re = regex::compile(`[a-z]+-([a-z]+)-[a-z]+`)!;
	const result = regex::replace(&re, "cat-dog-mouse; apple-pear-plum",
		`\1`)!;
	// -> dog; pear
