With a desperate need to debug a lengthy regular expression destined for use with the excellent RegexKitLite library, I have quickly put together a Mac OS X application.
Reginald is a kindly old gentleman devoted to assisting you with those tricky regular expressions.
Provide some sample input, and your regular expression, and Reginald will provide you with colour-coded output and a list of all your matches and the corresponding capture groups for your exploration. Select a match or capture group in the list to the right, and the corresponding text will be selected in the panel to the left.
It will run on Mac OS X 10.6 and above.