Regular expressions without a doubt are one of the best things you can learn to use properly. There are only two books you need to really unlock everything you need to know about them:
Get them both. Read them cover to cover (something I very rarely do with technical books!), and you will know kung fu.
Edit: also, FYI, regular expressions seem really complicated from the outside, but once you get to grips with them you realise they're not really that complicated at all - don't be put off :-)