One of my basic beliefs - Regular Expression is God-Gifted.. Its hard to find something that can be so simple but so much useful.. All for these reasons, I end up using RegEx whenever I have some pattern matching business. And for that to be perfect, I need a cool RegEx validator.
Most of the times, I use the validator from GSkinner - http://gskinner.com/RegExr/ ..
It's built with flash. Have a cool interface. There are a lot of inbuilt patters you can use or try out. You can also save your own patterns.
It have options to test both matching of patterns and replacing using patterns.
And to add to all this, it has a desktop version you can download and use right away! Sweet! :)