An Etymological Dictionary of the German Language/Regel
Regel, f., ‘rule, regulation, principle,’ from MidHG. rëgel, rëgele, OHG. rëgula, f., ‘rule, especially of an order’; borrowed in this latter sense during the OHG. period when the monastic system was adopted (see Kloster, Münster, Abt, and Mönch), from Lat. rëgula, pronounced rĕgula in MidLat. (Lat. ê would be changed into OHG. î, see Feier, Kreide, and Pein). This pronunciation is also implied by AS. rĕgul, m., and OFr. riule, ‘rule’ (E. rule, from MidE. reule, is derived from OFr. reule, Lat. regula).