def regon(self):
        """
        Returns 9 character Polish National Business Registry Number,
        Polish: Rejestr Gospodarki Narodowej - REGON.

        https://pl.wikipedia.org/wiki/REGON
        """
        voivodeship_number = self.random_int(0, 49) * 2 + 1
        regon_digits = [int(voivodeship_number / 10), voivodeship_number % 10]

        for _ in range(6):
            regon_digits.append(self.random_digit())

        regon_digits.append(regon_checksum(regon_digits))

        return ''.join(str(digit) for digit in regon_digits)