Introducing Regular Expressions by Michael Fitzgerald
Summary: Grab it, read it, learn it
Basically, if you have never ever used regular expressions before, this book is for you. It will guide you through the most basic and most common patterns used within regular expressions world. There are lots of examples and lots of tools presented inside the book.
It goes like this. Michael starts with setting reader a high target by introducing complex example just at the beginning. But this is just a means of setting finish line. Throughout the book he will show you how to reach this target by starting from most common, very basic examples and then by gradually increasing the complexity of presented expressions.
There are few tools presented that can make your life easier. In various flavors (web based, standalone) for various systems (Windows, Linux, Mac). This is a benefit, because by trying all of them (you will be forced to do it if you want to follow examples) you will be able to find one that fits you best.
One drawback of the book. There are no clear statements which expressions work for particular syntax (e.g. Perl, Java, Ruby, etc.). This may be a little stopper if you try given expression in slightly different environment than one presented in a book.
If you are looking for comprehensive list of various expressions in the context of various syntaxes take a look here instead: Regular Expressions Cookbook by Jan Goyvaerts and Steven Levithan.