Mastering Python Regular Expression has a promising title, and I certainly wanted to bring my Python RE skills to true mastery, but I was sorely disappointed by this book. It consists of little more than the authors’ rephrasing of the official Python API documentation. Regular expressions are notoriously complex and require loads of examples and try-it-yourself assignments to get down, but this book only presents each aspect of REs in an extremely concise and deathly boring fashion before moving on. Before you know it, the book has finished (it is only 110 pages!), and you'll hardly be any more comfortable with regex than when you started.
The book definitely has a dodgy air about it. The last part of the book is essentially an advertisement for the commercial software RegexBuddy. I’ll be sure to avoid any offerings from Packt Publishing in the future. As an example of just how little effort was put into this book from the publisher's part, the non-native English of the two Spanish-speaking authors was never polished by an editor. Consequently, the book reads as very unprofessional, sometimes even annoying, e.g. “The IEEE thought their POSIX standard has tried to standardize and give better Unicode support to the regular expression syntax and behaviors. This is called the POSIX flavor of the regular expressions.”
As a general introduction to regular expressions, you would be much better off with O’Reilly's Mastering Regular Expressions (though it uses Perl for teaching, the particular regular-expression dialect it uses is very similar to Python's own). When it comes to exploiting REs in Python, pretty much any API documentation or online tutorial will be just as good, if not much better, than this book.