An Etymological Dictionary of the German Language/Annotated/Regel
Regel, feminine, ‘rule, regulation, principle,’ from Middle High German rëgel, rëgele, Old High German rëgula, feminine, ‘rule, especially of an order’; borrowed in this latter sense during the Old High German period when the monastic system was adopted (see Kloster, Münster, Abt, and Mönch), from Latin rëgula, pronounced rĕgula in Middle Latin (Latin ê would be changed into Old High German î, see Feier, Kreide, and Pein). This pronunciation is also implied by Anglo-Saxon rĕgul, masculine, and Old French riule, ‘rule’ (English rule, from Middle English reule, is derived from Old French reule, Latin regula).