The PCRE library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl 5. PCRE has its own native API as well as a set of wrapper functions that correspond to the POSIX regular expression API.
Oniguruma is a modern and flexible regular expressions library. It encompasses features from different regular expression implementations that traditionally exist in different languages. Character encoding can be specified per regular expression object.
RE2 is a fast, safe, thread-friendly alternative to backtracking regular expression engines like those used in PCRE, Perl, and Python. It is a C++ library with wrappers for many languages including Perl, Python, Erlang and more.