Regular expressions are a complicated topic. Did this document help you understand them? Were there parts that were unclear, or Problems you encountered that weren't covered here? If so, please send suggestions for improvements to firstname.lastname@example.org.
The most complete book on regular expressions is almost certainly Jeffrey Friedl's Mastering Regular Expressions, published by O'Reilly. Unfortunately, the Python material in this book dates from before the re module -- all the examples use the old regex module -- but it covers writing good regular expressions in great detail.