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.
Mac OS X 10.2 or later