code: def register(
          self,
          pattern: str,
          handler: Any,
          flags: int = 0,
          channel: str = "*",
          subtype: Optional[str] = None,
      ) -> None:
          
          LOG.debug('Registering message endpoint "%s: %s"', pattern, handler)
          match = re.compile(pattern, flags)
  
          if subtype not in self._routes[channel]:
              self._routes[channel][subtype] = dict()
  
          if match in self._routes[channel][subtype]:
              self._routes[channel][subtype][match].append(handler)
          else:
              self._routes[channel][subtype][match] = [handler]
docs: Register a new handler for a specific :class:`slack.events.Message`.
  
          The routing is based on regex pattern matching the message text and the incoming slack channel.
  
          Args:
              pattern: Regex pattern matching the message text.
              handler: Callback
              flags: Regex flags.
              channel: Slack channel ID. Use * for any.
              subtype: Message subtype