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.
Reginald is built on RegexKitLite, and so uses the ICU syntax.
It will run on Mac OS X 10.6 and above.
Michael Tyson <michael at atastypixel.com>, A Tasty Pixel