Understanding and using the power of regular expressions, or simply regex, is a must for every smart programmer. Django uses them quite often on its elegant URLConf system.

I've been using regular expressions for a very long time and have read many books about them but if I have to recommend just one I'll go for the classic Mastering Regular Expressions by Jeffrey Friedl.

And nothing better to test you're on the right track while writing your own regular expressions than Kodos, a simple but nice Python based tool, easy to install from any Debian based distro with a quick sudo apt-get install kodos.