Peter Bowyer's entry on The Regex Coach - interactive regular expressions alerted me to the very excellent Regex Coach.

The Regex Coach allows you to experiment with your regular expression (regex) and quickly determine how it would match your target string. It would be a very useful tool for trying out all any examples in any tutorials or books you may be reading. Also serves as a good testing platform for verifying your regex interactively (as opposed to writing it in your script, running it, and then checking the results). I wish I had this when I was learning regex.