code: func ParseUIDGID(str string) (uid uint32, gid uint32, err error) {
  	if str == "" {
  		return 0, 0, nil
  	}
  	parts := strings.SplitN(str, ":", 2)
  	if len(parts) == 1 {
  		return 0, 0, errors.New("groups ID is not provided")
  	}
  	if uid, err = parseUID(parts[0]); err != nil {
  		return 0, 0, err
  	}
  	if gid, err = parseUID(parts[1]); err != nil {
  		return 0, 0, err
  	}
  	return
  }
docs: // ParseUIDGID takes the fast path to parse UID and GID if and only if they are both provided