Regex tip
Regex tip 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.
If you use an RSS reader such as Google Reader but don't use Twitter, you can subscribe to the RegexTip RSS feed.
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.
See my list of Twitter accounts for my personal account and my other daily tip accounts.