Or you could just use the simple RegEx as posted in the PHP Manual By Bobocop
$atom = '[-a-z0-9!#$%&\'*+/=?^_`{|}~]'; // allowed characters for part before "at" character
$domain = '([a-z0-9]([-a-z0-9]*[a-z0-9]+)?)'; // allowed characters for part after "at" character
$regex = '^' . $atom . '+' . // One or more atom characters.
'(\.' . $atom . '+)*'. // Followed by zero or more dot separated sets of one or more atom characters.
'@'. // Followed by an "at" character.
'(' . $domain . '{1,63}\.)+'. // Followed by one or max 63 domain characters (dot separated).
$domain . '{2,63}'. // Must be followed by one set consisting a period of two
'$'; // or max 63 domain characters.
But it's a good tutorial, although I'd just use the above code...
