Even the most seasoned programmers can have difficulties with Regular Expressions, especially when one uses more than one programming language or doesn’t use them on a regular basis.  Here are a few links that I use whenever I am attacking a piece of software that requires a RegEx.

RegEx Powertoy - Uses AJAX to instantly render the results of your regular expression.  Very useful for debugging.  Just remember, it does not render special escapes for quotes that may be required for specific programming languages.

Matchmaking with Regular Expressions - An excellent Java World article on Regular Expression basics that I use as a reference every time I need a refresher!

Common Web Development Tasks - For common web site tasks such as commenting and callbacks, take a look at this Smashing Magazine post.

What do you use to refresh your regular expression knowledge?