Reggy is a small OS X cocoa/objective-c application to very quickly visualize what a given regular expression will match given a test string. It will also tell you if you have errors in your regular expression.
What's New in version 1.3:
added preference to apply sub-match/capture group coloring: each match or capture group gets a slightly different shade of color --- especially nice for nesting matches
added preference to automatically use the clipboard contents on startup for the regular expression and/or the testing string
added menu items to paste the clipboard directly into