I just stumbled across this new regex tester extension for firefox. It places a tool called "Regular Expressions Tester" under the "Tools" menu (or "More Tools" if you use that extension, as I do).
I thought this might be of use to some trying to create new filters (like myself). I know I'm definitely going to play with this....
Here's the link if you're interested: https://addons.mozilla.org/firefox/2077/
-Andy