1  =>(a|b{0,3}){0,1}
 2  a
 3  aa
 4  b
 5  bb
 6  bbb
 7  bbbb
 8  ab
 9  ba
10  =>([0-9]{0,3}|([0-9]{0}|[0-9]{0,3})){0,3}
11  0
12  00
13  123
14  abc