Don't.
Use a regex for a quick sanity check, something like .@.., but almost all langauges / frameworks have better methods for checking an e-mail address. Use that.
It is possible to validate an e-mail address with a regex, but it is a long regex. Very long.
And in the end you will be none the wiser. You'll only know that the format is valid, but you still don't know if it's an active e-mail address. The only way to find out, is by sending a confirmation e-mail.
