I'm not saying that checking email address with regex is good way of doing it, but there is countless examples of people doing it and it would help a lot if everyone would use one standardized regex for that.
/.*@.*/
Even if there was a standardised regex for all emails, which would not break when new TLDs are released, or new unicode characters are supported, or whatever, there would still be no guarantee that someemailaddress@domain.com is actually valid!
Your best bet is to simply allow users to enter anything (with perhaps some minor regex check like I did above), perhaps ask them to enter it twice, and perhaps send an account validation email.