tips and tricks for learning, building and using regular expressions
IBM developerWorks tutorials and articles
“Hone your regexp pattern-building skills” is a hands-on tutorial taking you through the classic UNIX and Linux command-line tools.
Other tutorials and articles
“Finding Text in Context”: an article about grep’s context output.
Regexp examples and one-liners
regexlib.com, a searchable library of thousands of regular expressions
For further reference
If it lives up to its name, the Regular Expressions Cookbook is bound to be a classic.
Need more? The dmoz listing for regexps is long.