What is a good regular expression tester for OS X?

_{(source: reggyapp.com)}

give Reggy a try! ...simple and effective!

NOTE: Though the last 1.3 release of the app is quite old (2007), a new version written in Objective-c is under development on Github

Maybe you can just use one of the online RegExp testers, like "RegExr".

I've found a nice Regexp tester for JavaScript at http://robrohan.com/projects/widgets/, which is available as a widget for OS X, and is even nice enough to give you numbered captures, something that's quite rare amongst RegExp testers.

However, to buy it you need to send some unspecified amount of money via paypal for it - I flipped a dollar in, partly because I didn't know whether it would be any good, and it turns out to be pretty nice.