This excellent book goes further and deeper than many tutorials on regular expressions. You might be surprised with some of the things you'll learn from reading it.
Unlike many cookbooks, this one doesn't dive into the recipes right away. I thought this was a good call because regular expressions are a specialized topic, and most developers don't work with regular expressions on a daily basis so they probably have to be reminded of the building block concepts and syntax, and get prepared for a discussion of more advanced features. Chapter One provides a list of recommended tools for working with regular expressions. Chapter 2 is a concise but very thorough discussion of building block and more advanced regular expression concepts (e.g., possessive quantifier or atomic grouping, named capturing groups, lookahead and lookbehind, etc.), including a discussion of differences in engine implementations and feature support. Chapter 3 is a hundred-plus page tutorial on how to work with regular expressions using different programming and scripting languages, including potential gotchas and workarounds. Chapters Four through Eight contain the recipes for solving real-world problems, with tips on how to improve an initial solution's readability (e.g., use named capturing groups when possible, etc.) and/or efficiency.
I was initially skeptical about the authors' ambitious goal of covering so many regular expression flavors, thinking the discussions of differences in engine supported features might prove distracting. The book is written and organized so well, however, my fear did not materialize. In fact, I was pleasantly surprised to learn that: of the covered flavors, Microsoft's DotNet regex engine supports some of the most advanced features.
There's not much to dislike about this book but if I were asked to suggest one or two things that might be of value-add to readers, I would suggest making available for download files containing appropriate subject strings for testing the book's various recipes as a convenience to readers who learn best by doing and want to follow along as they read the recipes, and for the book to include, for easy reference, a feature-support comparison matrix of the covered flavors, much like the comparison table available in the regular-expressions.info website.