RegexTip is a Twitter account that gives one tip per day for learning regular expressions. Most of the tips apply to many flavors of regular expressions: Perl, Python, .NET, etc.
Here are a few regular expression resources.
Four tips for learning regular expressions
Regular expressions in PowerShell and Perl
Regular expressions in Python and Perl
C++ TR1 regular expressions
If you’d like to learn about regular expressions in depth, the best book on regular expressions is Jeffrey Friedl’s Mastering Regular Expressions.
