This book is great if you want to learn about what you can do with regex, how they work, how you can improve them (efficiency & accuracy) and what kind of pitfalls there are.

My only gripe is about the Ruby reference on the cover. In the top right corner it says: "For Perl, PHP, Java, .NET, Ruby and more!" Perl, Java, .NET and PHP each get their own chapters (together 200 pages). Ruby, however, only participates in feature comparisons and benchmarks. If I had checked the ToC I would've known this, so I won't hold it against the book that much.

The Ruby thing is a bummer, but since the book is great, it will get all five stars.

The book is robust and goes into alot detail. I liked the comparing and contrasting between the various RegEx implementations. I had a difficult time digesting some of the detail. In particular, the analogies confused me and I felt the author went overboard with them. Many times, I had to backtrack to understand what was being discussed. However, regular expressions is a complex topic and the author did a good job easing into the concepts. An additional plus was with inline page citation - this helped to find the page corresponding to the topic being discussed.
I would have liked shorter chapters with chapter summaries and more diagrams in place of analogies. Ultimately, Mastering Regular Expressions is a good book referencing a complex topic.

I went through several books and online tutorials and never found anything that did a good comprehensive job of explaining regular expressions. This book does. If you are having trouble "getting it", I highly encourage reading this book. You will be extremely enlightened even after the first few chapters.

Quite a comprehensive guide to regular expressions. Gets very detailed in the areas that it covers. However, definitely not a 'beginner's guide'. I highly recommend that you are already comfortable with the basis of regular expressions before picking up this book. You will get way more out of it if this is not your first introduction to it. Comfort and proficiency with Perl would also be a big help. Title is correct though, this is a guide to 'mastering' regular expressions, not learning them from the beginning.

How deep down the rabbit hole do you really NEED to go? I had a serious need to get on top of regular expressions to solve one particular problem. I looked at several online tutorials which didn't take me where I needed to go, so I ordered Mastering Regular Expressions after reading the Amazon reviews. I always look at the negative reviews first. In spite of the negative reviews I ordered the book with an open mind.
When the book arrived I began reading it with enthusiasm. In the preface there is a small section on "How to Read This Book". I bought into the author's suggestion to read the book's first six chapters first. I was captivated through the first three chapters, and then somewhere in chapter 4 I began to get very weary with information overload. After putting the book down for a couple of days I decided to skip the rest and use what I needed to write the one regular expression I had need of. The book did successfully help me accomplish this, so I gave it 3 stars. Not only did it give me the information I needed that the online tutorials didn't, it also gave me the confidence I needed. For that, which I am grateful, I would have liked to have given it more stars. I think many of those in need of learning about regular expression could be well served by a "lite-edition" of this book. Perhaps someday when I have the time and the need I may try to wade through the rest of the book, but as it is now Mastering Regular Expressions took me far farther down the rabbit hole than I really needed or wanted to go.
If you need to get on top of Regular Expressions, I would recommend this book, however just be ready to be taken far deeper than the average coder probably needs to go.