I'm not sure about the format myself. When the ref tags were first added to OSM it was in the "R-II 5511" format and that's what I went with. The original decision (https://web.archive.org/web/20240716215656/https://vladars.rs/sr-SP-Cyrl/Vlada/Ministarstva/msv/Documents/%D0%9E%D0%B4%D0%BB%D1%83%D0%BA%D0%B0%20%D0%BE%20%D1%80%D0%B0%D0%B7%D0%B2%D1%80%D1%81%D1%82%D0%B0%D0%B2%D0%B0%D1%9A%D1%83%20%D1%98%D0%B0%D0%B2%D0%BD%D0%B8%D1%85%20%D0%BF%D1%83%D1%82%D0%B5%D0%B2%D0%B0_406181601.pdf) format is like that on page 6 upper left table says "РЕГИОНАЛНИ ПУТЕВИ ПРВОГ РЕДА (Р-I)" which in latin is REGIONALNI PUTEVI PRVOG REDA (R-I)" and then lists plain numbers. The other pdf list them in the RII-5511 fomat that is more in line with the previous numers like 4-65 for example. Per on-the ground rule it's best to tag them like on road signs but that needs to be checked.
Cheers,
-Frank