protected function _detectFormat()
    {
        // prepare separator and pattern list
        $sep      = quotemeta($this->_separator);
        $patterns = array();
        $lengths  = array();

        // check for ISBN-10
        if ($this->_type == self::ISBN10 || $this->_type == self::AUTO) {
            if (empty($sep)) {
                $pattern = '/^[0-9]{9}[0-9X]{1}$/';
                $length  = 10;
            } else {
                $pattern = "/^[0-9]{1,7}[{$sep}]{1}[0-9]{1,7}[{$sep}]{1}[0-9]{1,7}[{$sep}]{1}[0-9X]{1}$/";
                $length  = 13;
            }

            $patterns[$pattern] = self::ISBN10;
            $lengths[$pattern]  = $length;
        }

        // check for ISBN-13
        if ($this->_type == self::ISBN13 || $this->_type == self::AUTO) {
            if (empty($sep)) {
                $pattern = '/^[0-9]{13}$/';
                $length  = 13;
            } else {
                $pattern = "/^[0-9]{1,9}[{$sep}]{1}[0-9]{1,5}[{$sep}]{1}[0-9]{1,9}[{$sep}]{1}[0-9]{1,9}[{$sep}]{1}[0-9]{1}$/";
                $length  = 17;
            }

            $patterns[$pattern] = self::ISBN13;
            $lengths[$pattern]  = $length;
        }

        // check pattern list
        foreach ($patterns as $pattern => $type) {
            if ((strlen($this->_value) == $lengths[$pattern]) && preg_match($pattern, $this->_value)) {
                return $type;
            }
        }

        return null;
    }