At the Powertoys Weblog, Sara Ford posted about a very cool tool which is available at gotdotnet: Regex Builder.
At gotdotnet it is described as follows:
A tool for building and testing Regular Expressions. It allows you to manipulate the expression and your source text, and shows you a tree with all of the Matches, Groups, and Captures found in the text.
I think this tool is easier to use for RegEx beginners than for instance The Regulator, because its GUI is pretty simple and there’s some standard information available at startup. The Regulator might scare you off by offering a fairly complex GUI. For more experienced users I’d recommend The Regulator. With this tool you can even search RegexLib.com.
A few other cool features The Regulator offers: code generation and the possibility to compile your regular expression(s) to an assembly!