First one to be back is the simple but very useful regular expression tester. What it does is simply dump the contents of a pattern match and its subpattern matches. Developers might find this a useful tool not only to test their regular expressions, but also to see the way subpatterns are counted to use backreferences.

At the tester links to PCRE documentation (Perl Compatible Regular Expressions) are available. Also I would like to point to a handy cheat sheet on general regular expressions by Dave Child and another cheat sheet on JavaScript regular expressions from Visibone.

This tool is accessible again at regex.edesign.nl and can be downloaded there as wel (use the src link).