public function hasValidFormat()
    {
        $is_ok = (preg_match('/^[0-9]+$/', $this->number) && preg_match('/([K0-9])$/', $this->vn) && strlen($this->number) > $this->minChars && strlen($this->number) < $this->maxChars);
        if (!$is_ok && $this->useExceptions) {
            throw new InvalidFormatException("R.U.T. '{$this->number}' with verification code '{$this->vn}' has an invalid format");
        }

        return $is_ok;
    }