Unless you want to use a very very long regular expressions you'll run into valid email addresses that are not covered (think Unicode). Also fake email addresses will pass as valid, so what is the point of validating if you can simply write test@test.com and get away with it?
The best way to validate email addresses is to send a confirmation email with a link to click. This will only work if the email address is valid: easy, and no need to use regex.
