Using regular expressions to validate data

function valid_email ($email) {
    $parts = explode("@", $email);
    if (count($parts) != 2 ) return false;
    if (strlen($parts[0]) > 64) return false;
    if (strlen($parts[1]) > 255) return false;

    $atom = '[[:alnum:]_!#$%&\'*+\/=?^`{|}~-]+';
    $dotatom = '(\.' . $atom . ')*';
    $address = '(^' . $atom . $dotatom . '$)';
    $char = '([^\\\\"])';
    $esc  = '(\\\\[\\\\"])';
    $text = '(' . $char . '|' . $esc . ')+';
    $quoted = '(^"' . $text . '"$)';
    $local_part = '/' . $address . '|' . $quoted . '/';
    $local_match = preg_match($local_part, $parts[0]);
    if ($local_match === false 
        || $local_match != 1) return false;
    $hostname =         '([[:alnum:]]([-[:alnum:]]{0,62}[[:alnum:]])?)';
    $hostnames = '(' . $hostname . '(\.' . $hostname . ')*)';
    $top = '\.[[:alnum:]]{2,6}';
    $domain_part = '/^' . $hostnames . $top . '$/';
    $domain_match = preg_match($domain_part, $parts[1]);
    if ($domain_match === false 
        || $domain_match != 1) return false;
    return true;
}