rex

Rex, regis is third declension masculine. The x of the nominative is not a separate letter so much as a spelling of c plus s, which is why the stem underneath is reg-: regis, regem, rege, reges, regum, regibus. Once you see that, the whole family falls out: rego, "I rule", regnum, "kingdom", regina, "queen", rectus, "straight, ruled".

For a Roman of the Republic rex was not a neutral job title. The expulsion of the kings in 509 BC was the founding story of the state, and to be accused of aiming at kingship, regnum adfectare, was a charge that got men killed. That is the political air around Caesar's refusal of the crown, and it is why Roman writers reach for princeps, imperator or dictator and leave rex for foreigners and for the distant past.

French roi, Spanish rey and Italian re all descend from it, and English took regal, regime, reign, rector and, at one remove, royal.