On PHP.net today, there's a note about an addition to the O'Reilly classic book (an asset to any developer's library) "Mastering Regular Expressions" - a full 48-page chapter on PHP.
The book is described as:
Mastering Regular Expressions quickly covers the basics of regular-expression syntax, then delves into the mechanics of expression-processing, common pitfalls, performance issues, and implementation-specific differences. Written in an engaging style and sprinkled with solutions to complex real-world problems, Mastering Regular Expressions offers a wealth information that you can put to immediate use.
This new edition is the third release for the book.