function macfmt($s) {
        if (preg_match('/^([0-9a-z]{2})([0-9a-z]{2})([0-9a-z]{2})([0-9a-z]{2})([0-9a-z]{2})([0-9a-z]{2})$/', $s, $m))
            $retstr = $m[1] . ":" . $m[2] . ":" . $m[3] . ":" . $m[4] . ":" . $m[5] . ":" . $m[6]; 
        else
            $retstr = $s;
        return $retstr;
    }