Nominative (subject) singular
rex
Vocative (speaking to) singular
rex
Accusative (object) singular
reg-em
Genitive (of) singular
reg-is
Dative (to/for) singular
reg-i
Ablative (by, with, from) singular
reg-e
Nominative (subject) plural
reg-es
Vocative (speaking to) plural
reg-es
Accusative (object) plural
reg-es
Genitive (of) plural
reg-um
Dative (to/for) plural
reg-ibus
Ablative (by, with, from) plural
reg-ibus