Regexp: (a|b{0,3}){0,1} a: Ok aa: Fail b: Ok bb: Ok bbb: Ok bbbb: Fail ab: Fail ba: Fail Regexp: ([0-9]{0,3}|([0-9]{0}|[0-9]{0,3})){0,3} 0: Ok 00: Ok 123: Ok abc: Fail
