Regal

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late Middle English: from Old French, or from Latin regalis, from rex, reg- ‘king’.

wiktionary

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From Middle English regal, from Old French regal(“regal, royal”), from Latin rēgālis(“royal, kingly”), from rex(“king”); also regere(“to rule”). Doublet of royal(“belonging to a monarch”) and real(“unit of currency”). Cognate with Spanish real.

From Middle French régale, possibly from Old French regol(“a gutter, channel”).

etymonline

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regal (adj.)

"kingly, pertaining to a king," late 14c., from Old French regal "royal" (12c., Modern French réal) and directly from Latin regalis "royal, kingly; of or belonging to a king, worthy of a king," from rex (genitive regis) "king," from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule." Related: Regally.