Regular Expression Pocket Reference, 2nd Ed. by Tony Stubblebine. Published by O'Reilly, ISBN 0-596-51427-1
This is a handy little guide for common regular expression odds and ends. The intro chapter does a nice job of giving you the regex elevator speech and hits all the important fundamentals: different engines, metacharacters, and general pattern rules. There’s also a regex “Cookbook” section; however, that bit is only two pages and the “recipes” there are so rudimentary that they’re nearly useless and would have been better left to a different book.
Each chapter is concise, well-written and hits the major things you’ll need to refresh your memory on from time to time.
Overall it’s a very well done book.