Jeffrey Friedl’s Mastering Regular Expressions will be coming out in a third edition, with new information on PHP and Java 1.5+.

Everyone loves this book, and with regular expressions being an essential yet rather opaque topic, I can see why a good, comprehensive book on the subject is popular. I never owned a copy, partly because I always felt I had a good handle on regexps. (See the Builderoo source code. :) ) Maybe I'll take another look when the new edition comes out.