Regular-Expressions.info - Regex Tutorial, Examples and Reference - Regexp Patterns
Regular Expressions Cheat Sheet - Cheat Sheets - ILoveJackDaniels.com

Those are a couple of my favourite's. Google 'regular expressions' to find more... the cheat sheet there is as simple as it gets, though might I suggest if you want to get a good handle on how it's working, you may want to find some example patterns and break them down to learn how they're matching what they're matching and why. Then create some random practice strings of your own and try to match various parts of them.
-m