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.
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.