I hadn’t seen this one before. There are other regex testers out there but this one is neat because it helps guide you. There are examples and tooltip help to let you know what you’re doing. Very neat. There is a downloadable version as well apparently. gskinner has been an indirect sort of long-distance teacher for some time now. He has a tremendous amount of resources available on his site.
I found this tool really useful for testing regex in AS3. There’s a few of these around, but this one is very nicely done.. http://gskinner.com/RegExr/