code: func (r *Route) GetPathRegexp() (string, error) {
  	if r.err != nil {
  		return "", r.err
  	}
  	if r.regexp.path == nil {
  		return "", errors.New("mux: route does not have a path")
  	}
  	return r.regexp.path.regexp.String(), nil
  }
docs: // GetPathRegexp returns the expanded regular expression used to match route path.
  // This is useful for building simple REST API documentation and for instrumentation
  // against third-party services.
  // An error will be returned if the route does not define a path.