It isn’t very often that you come across a tool that is truly unique in what it does and is so complete that you abandon all other tools you’ve accumulated over time in favor of that one tool that just works.
I came across a regular expression builder that had me deleting all of my bookmarks for other builders. It is amazingly complete for everything I’ve ever needed in the world of regex. http://RegExr.com it is built using adobe flex and has an amazingly easy to user interface.
This tool has a real time view of what your regular expression is doing. It handles both matching and replacing.