As you're writing in PHP I'd advice you to use the PHP build-in validation for emails.
filter_var($value, FILTER_VALIDATE_EMAIL)If you're running a PHP version lower than 5.3.6, please be aware of this issue: Bug #53091: Crashes when I try to filter a text of > 2264 characters
If you want more information how this built-in validation works, see here: Does PHP's filter_var FILTER_VALIDATE_EMAIL actually work?