Just my 2 cents, if this is for a signup page, email validation has spawned large debates and Regex's. The simple thing to do to validate someone's email is just to send them an email with a validation link during signup. Make it such that they can't login if they haven't been validated. Of course check for '@' and '.' .
If its just a form for checking whether an email address exists, well if the email isn't there in the database, it isn't there.