RegexPlor is a tool for interactively exploring regular expressions. It provides a graphic frontend for experimenting with such regular expressions in a intuitive way. You can edit an input text as well as a regular expression as well as colors for matched text, etc. and study various effects at every keystroke. This is a much more convenient way of hunting down the one expression you're after, without typing any code at all in the usual trial-and-error method.
BEN Doesn't seem to do what it claims, though I'll confess I'm new to Regular Expressions.
I'm following the tutorials on www.regular-expressions.info and RegexPlor fails to match when it should. Same eamples run with egrep in Terminal or in TextMate match correctly.