regex to select 2nd option from group value (a|b)

my regex i wrote is (a|b)-(?(1)(1)|(2)) what it is supposed to do:

if a then a-1 allowed

if b then b-2 allowed

having group 1 = a or b and group 2 = 1 or 2

i have tried using ((?'a'a)|b)-(?(a)(1)|(2)) but it gave me different groups for numbers being group 3 or 4

how can i keep numbers group to 2