Usually when checking for a valid email address in PHP I use:

preg_match("/^([a-zA-Z0-9])+([a-zA-Z0-9\._-])*@([a-zA-Z0-9_-])+([a-zA-Z0-9\._-]+)+$/",'bob@alice.com')

A much better approach is:

filter_var( 'bob@alice.com', FILTER_VALIDATE_EMAIL )

More at http://www.php.net/manual/en/function.filter-var.php