let rec token buf =
  match%sedlex buf with
    | white_space -> token buf
    | "a" -> token buf
    | "b" -> 2
    | eof -> 0
    | _ -> failwith "Unknown token"

let () = 
  let lexbuf = Sedlexing.Utf8.from_string "ab" in
  print_newline (print_int (token lexbuf))