protected function validate_iban($field, $input, $param = null)
    {
        if (!isset($input[$field]) || empty($input[$field])) {
            return;
        }

        $character = array(
            'A' => 10, 'C' => 12, 'D' => 13, 'E' => 14, 'F' => 15, 'G' => 16,
            'H' => 17, 'I' => 18, 'J' => 19, 'K' => 20, 'L' => 21, 'M' => 22,
            'N' => 23, 'O' => 24, 'P' => 25, 'Q' => 26, 'R' => 27, 'S' => 28,
            'T' => 29, 'U' => 30, 'V' => 31, 'W' => 32, 'X' => 33, 'Y' => 34,
            'Z' => 35,
        );

        if (!preg_match("/\A[A-Z]{2}\d{2} ?[A-Z\d]{4}( ?\d{4}){1,} ?\d{1,4}\z/", $input[$field])) {
            return array(
                'field' => $field,
                'value' => $input[$field],
                'rule' => __FUNCTION__,
                'param' => $param,
            );
        }

        $iban = str_replace(' ', '', $input[$field]);
        $iban = substr($iban, 4).substr($iban, 0, 4);
        $iban = strtr($iban, $character);

        if (bcmod($iban, 97) != 1) {
            return array(
                'field' => $field,
                'value' => $input[$field],
                'rule' => __FUNCTION__,
                'param' => $param,
            );
        }
    }