CL-PPCRE is a portable regular expression library for Common Lisp which has the following features:

• It is compatible with Perl.
• It is fast.
• It is portable, i.e. the code aims to be strictly ANSI-compliant and is known to work with lots of different CL implementations.
• It is thread-safe.
• It comes with convenient features like a SPLIT function, a couple of DO-like loop constructs, and a regex-based APROPOS feature similar to the one found in Emacs.
• In addition to specifying regular expressions as strings like in Perl you can also use S-expressions which obviously is more Lisp-y.
• Is it is fully documented.
• It comes with a BSD-style license so you can basically do with it whatever you want.

CL-PPCRE has been used successfully in various applications like BioLingua, LoGS, or The Regex Coach.

A nice tool (shameless self-plug, actually) to learn regular expressions is The Regex Coach, which uses CL-PPCRE and is of course also written in Lisp.

http://weitz.de/cl-ppcre/

Mastering Regular Expressions by Jeffrey Friedl