Five stars out of five
I'm scared of regular expressions. They vex me constantly at work whether trying to figure out someone else's Apache RewriteRules or Perl or just trying myself to do mildly clever things in vi and sed. They are a headache.
A coworker let me browse his copy of this wonderful little book and I was hooked. I actually tried to buy one that day on the way home, but they were out of stock and so I permitted Amazon to ship me one.
In addition to brief explanations of the different types of regexes in the wild (based on the comprehensive Mastering title, also from ORA), there are detailed quick references to all of the different implementations including several common languages (Perl, PHP, C#, and several others) and software packages (including sed, Apache, vi and many others). All of their various quicks and "features" are explained briefly and there are some examples.
No one hacking around in Unix or doing much programming should be without this book, unless they are already a regex wizard, and I think even they'd find it handy.