Regular Expression Library
I was reading a book review on Slashdot today and came across this gem in the comments section: RegExLib.com Regular Expression Library.

As the name suggests, RegExLib is a library full of useful regular expressions ready for cutting and pasting into one's code.
Divided into 8 categories, there are matching and validation patterns to suit the needs of most common web applications.

If I'd known about this site last week I could've saved myself time writing an email validator... So in future I'll definitely be checking for regexes here first.
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