digits
22233344455566677778889999
   letters
abcdefghijklmnopqrstuvwxyz
   5{.valid
┌─┬──┬───┬───┬──┐
│a│aa│aaa│aam│ab│
└─┴──┴───┴───┴──┘
   5{.reps
┌─┬──┬───┬───┬──┐
│2│22│222│226│22│
└─┴──┴───┴───┴──┘