load the RPID body into an AVP (use avp_write()) and check the body against an re via avp_check().
Or just use a search witha regexp :-) if you dont want to use avp+´s.
with the development version you have direct access to the rpid header, no need to use avps:
avp_check("$hdr(rpid)", "re/[0-1][0-9]+")
it's on a carrier-grade system. so i dont want use the development version. are there other ways?