code: def _get_path_pattern_tornado45(self, router=None):
          
          if router is None:
              router = self.application.default_router
          for rule in router.rules:
              if rule.matcher.match(self.request) is not None:
                  if isinstance(rule.matcher, routing.PathMatches):
                      return rule.matcher.regex.pattern
                  elif isinstance(rule.target, routing.Router):
                      return self._get_path_pattern_tornado45(rule.target)
docs: Return the path pattern used when routing a request. (Tornado>=4.5)
  
          :param tornado.routing.Router router: (Optional) The router to scan.
              Defaults to the application's router.
  
          :rtype: str