code: public static RegionForwardingRuleId of(RegionId regionId, String rule) {
      return new RegionForwardingRuleId(regionId.getProject(), regionId.getRegion(), rule);
    }
docs: Returns a region forwarding rule identity given the region identity and the rule name. The
  forwarding rule name must be 1-63 characters long and comply with RFC1035. Specifically, the
  name must match the regular expression {@code [a-z]([-a-z0-9]*[a-z0-9])?} which means the first
  character must be a lowercase letter, and all following characters must be a dash, lowercase
  letter, or digit, except the last character, which cannot be a dash.
  
  @see <a href="https://www.ietf.org/rfc/rfc1035.txt">RFC1035</a>