Steve Mansour has compiled a nice reference (http://sitescooper.org/tao_regexps.html) for Perl Regular Expressions.

If you find other good reference sources, post them here.

Another good RegEx resource is the book O'Reilly's Mastering Regular Expressions (http://www.oreilly.com/catalog/regex/ and http://www.oreilly.com/catalog/regex2/).

The following comes straight out of the Sitescooper docs and has several more good Perl resources in addition to the Tao one:
For more information on regular expressions in general, and the Perl variety that sitescooper uses, I recommend checking out some of these pages. First of all, Steve Litt at Troubleshooters.com has put together a great guide here: Steve Litt's PERLs of Wisdom: PERL Regular Expressions (http://www.troubleshooters.com/codecorn/littperl/perlreg.htm). Highly recommended. Next up, there's A Tao Of Regular Expressions (http://sitescooper.org/tao_regexps.html), it's quite good. Also, weblogger Jorn Barger has written up a good page of to regexps at Regular Expressions Resources on the Web (http://www.robotwisdom.com/net/regexres.html). The "Perl" section is most relevant to sitescooper's regexp format. The About.com guide to Perl has a great tutorial on them (http://perl.about.com/compute/perl/library/weekly/aa092599.htm) too.

Another great RegEx resource, especially for beginners:

http://www.regular-expressions.info/tutorial.html

A great tool to learn RegEx is The RegEx Coach (http://www.weitz.de/regex-coach/).

The Regex Coach is a graphical application for Linux and Windows which can be used to experiment with (Perl-compatible) regular expressions interactively. It has the following features:

- It shows whether a regular expression matches a particular target string.
- It can also show which parts of the target string correspond to captured register groups or to arbitrary parts of the regular expression.
- It can "walk" through the target string one match at a time.
- It can simulate Perl's split and s/// (substitution) operators.
- It tries to describe the regular expression in plain English.
- It can show a graphical representation of the regular expression's parse tree.
- It can single-step through the matching process as performed by the regex engine.
- Everything happens in "real time", i.e. as soon as you make a change somewhere in the application all other parts are instantly updated.

Best of all, it's free!!

Another great resource is the Regular Expression Library (http://www.regxlib.com/DisplayPatterns.aspx), which has hundreds of patterns figured out for you already. Good use if you don't want to reinvent the wheel :p

Learning Perl the Hard Way (http://www.greenteapress.com/perl/) is a free ebook available as Adobe PDF. Chapter 2 focuses on Regular Expression. Good read!