Perl compatible regular expressions syntax and library
PCREs refers to an extended regular expression syntax and the corresponding free and open source regex matching library that implements it as used by pcregrep, grep -P and a plethora of utilities and programming languages (zsh, wget, snort, php...).
It closely emulates perl 5 regular expressions with a few extensions of its own.
Use this tag for questions related to the syntax of those regular expressions or questions related to using the PCRE library.