Cost: Free, links back
Found Via: www.addedbytes.com
Whether you're searching strings or validating input fields, the use of regular expressions will save you a great deal of time. I found a great cheat sheet from AddedBytes.com that should save you a great deal of time. It's a free PDF download, so get to it!
By the way, if you'd like to watch a video tutorial that gives a crash-course in using regular expressions, be sure to pay a visit to the ThemeForest Blog. While you're there, you might as well subscribe to the RSS feed to stay updated on the latest articles and video tutorials!