Okay, but just to point out: having an apostrophe in one's name is not that exotic...

• O'Brien
• O'Connor
• D'Angelo
• O'Dowd
• O'Malley
• O'Hara
• D'Arcy
• D'Addario

So someone could quite conceivably have Chris.O'Dowd@BBC.co.uk or something like that.
Anyway, if anyone's looking for how to get around this in the future, an alternate solution to changing PhabricatorUserEmail.php would be to go into the DB phabricator_user, in table user_email, and change it directly in there, thus bypassing validation.
If you did want to change the code, though, look into http://php.net/manual/en/filter.examples.validation.php - FILTER_VALIDATE_EMAIL and FILTER_SANITIZE_EMAIL.