 <?php

$regex = "/([a-z0-9_]+|[a-z0-9_]+\.[a-z0-9_]+)@(([a-z0-9]|[a-z0-9]+\.[a-z0-9]+)+\.([a-z]{2,4}))/i";

// usage

if(!preg_match($regex, $email)) {
    echo "Invalid email address";
}
else {
    echo "email is valid";
}

?>