I've been using the regular expression applet provided by the apache project for some time now, to test my regular expressions. To my horror, it wasn't available today, so I put together my own silverlight version.

See it in action below or if you are interested, you can also download the code. The code is very simple so doesn't get clever with MVVM patterns or anything like that, but it's functional. There are a couple of features like the debug output that the apache version provides which I haven't bothered to implement, but there are other features such as support for back references that I have added, that the apache version doesn't provide. Enjoy.