docs: def main(self, config_filename, regex):
          """
          :param str config_filename: The config filename.
          :param str regex: The regular expression for columns which we want to use.
  
          :rtype: int
          """
          self._read_configuration_file(config_filename)
  
          if self._constants_filename:
              self._io.title('Constants')
  
              self.connect()
              self._get_old_columns()
              self._get_columns()
              self._enhance_columns()
              self._merge_columns()
              self._write_columns()
              self._get_labels(regex)
              self._fill_constants()
              self.__write_constant_class()
              self.disconnect()
              self.__log_number_of_constants()
          else:
              self._io.log_verbose('Constants not enabled')
  
          return 0