Where does “ρήγας” come from?

ρήγας (Greek) comes from Ancient Greek ῥήξ, from Latin rēx, from Proto-Italic rēks, from Proto-Indo-European h₃rḗǵs, from Proto-Indo-European h₃reǵ- — to straighten, to right oneself; right; just.

ρήγας (Greek): king in a pack of playing cards