Latin 'rex' (king) from PIE *reg- (to rule) gave us 'regal,' 'royal,' 'reign,' 'rule,' and 'rich.'
A Latin word meaning 'king, ruler, sovereign,' and the source of English words relating to kingship, rule, and governance, including 'regal,' 'reign,' 'royal,' 'regent,' and 'realm.'
From Proto-Italic *rēks, from the Proto-Indo-European root *h₃reǵ- meaning 'to straighten, to direct, to rule.' Latin rēx (stem rēg-) was a third-declension masculine noun meaning 'king.' The PIE root is remarkably productive: it gave Sanskrit rājan ('king'), Old Irish rí ('king'), Gothic reiks ('ruler'), and, through the Latin verb regere ('to direct, to rule'), an enormous family of English words. After the Romans expelled their last king
The words 'royal' and 'regal' are doublets — both mean 'of a king' and both come from Latin rēx, but they arrived in English by different routes. 'Royal' came through Old French roial (from Vulgar Latin *rēgālis), while 'regal' was borrowed directly from Latin rēgālis. Even more surprisingly, 'rich' and 'Reich' descend from the same PIE root *h₃reǵ- through Germanic: Gothic reiks meant 'ruler,' and the adjective *rīkijaz meant 'powerful,' becoming English 'rich' — originally meaning not wealthy but powerful, kingly