If you use regular expressions to validate email addresses, make sure you allow '+' before the '@' character. This is perfectly valid according to the RFC and some email servers let you do fun things with it. Gmail, for example, delivers anything addressed to 'youremail+anythingyouwanthere at gmail' to 'youremail at gmail'. Combined with filters that automatically label and file items, you can get email delivered directly to a specific folder. I know some unix mail systems support this stuff, too.
Don't be lazy with your regular expressions.