The only hardcopy book I still have is "Mastering Regular Expressions", by Jeffrey E. F. Friedl. It is absolutely the best book I have read on regular expressions because it has an introduction, an extended introduction, an overview of the features and flavors, the mechanics involved, practical techniques, and how to craft an efficient expression. The book also contains regular expression processing for Java and .NET. It is well organized and will guide you on your quest to use regular expressions to rescue you from seemingly impossible tasks.