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