ReX-T 2.0 is now released as first beta. Which means it probably still has a few bugs; please give me feedback if you find one!

ReX-T is a helper application that lets you develop, test and try out Regular Expressions, which can then be used in REALbasic, TextWrangler, BBEdit or any other application that supports PCRE-style regular expressions.

A Windows version is in the making, too.

Get more info (and a link to download it) on the Product Page.