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 15