I've been working with regular expressions a lot lately. I mostly use them in the process of parsing free form or "slightly structured" text files. Most of the work is done in c#, using the .Net 2.0 regex parser.

I've become hooked on Expresso for development and testing of my parsing strings. I've read a lot of pro's and con's about several different regex tools, but in my book, Expresso is easy to download, install and learn. If you need to work with regular expressions, you might give this editor a try.