PCRE

PCRE stands for Perl Compatible Regular Expressions. It is a library that implements regular expression matching with syntax and semantics closely resembling Perl 5. PCRE provides a C API that can be embedded in applications and is used by many software projects to perform pattern matching, searching, and text manipulation.

The PCRE project has evolved through two major lines: PCRE1, the original engine, and PCRE2, a complete rewrite intended to improve performance, stability, and maintainability. PCRE2 offers a redesigned API, better support for large patterns and Unicode, and a Just-In-Time (JIT) compiler to accelerate matching. PCRE1 remains in use in numerous existing codebases for compatibility, and many systems provide adapters to work with PCRE2 where feasible.

Key features include support for Perl-like syntax such as backreferences, lookahead and lookbehind assertions, subpatterns, named captures, conditional expressions, atomic groups, and possessive quantifiers. Unicode handling is available via character properties and various input encodings. PCRE patterns are compiled to an internal form and can be used for matching, searching, and replacing text. PCRE2 adds a modernized API and performance improvements, including optional JIT compilation, which can significantly speed up repeated matching tasks.

Usage and ecosystem: PCRE is widely embedded in software and development stacks, including web servers, scripting languages, and utilities. It is used by languages and platforms such as PHP, Apache, and Nginx, among others. Licensing is permissive, with a BSD-style license that permits use, modification, and redistribution in open-source and proprietary software.