"Perl compatible regular expressions". PCRE provides the pattern matching functionality required for the "matches" display filter operator.
You probably don't need this. GRegex, part of GLib 2.14 and later is a wrapper around PCRE. Wireshark's configure script will use it instead of PCRE if it is present.
If this library isn't already installed or available as a package for your platform, you can get it at: http://www.pcre.org/.
You can get the latest version at: http://gnuwin32.sourceforge.net/packages/pcre.htm