(*
    * Deletes all invalid characters starting from last valid one
    * valid characters passes the 'test_char' function
*)

let trim w =
    let rec aux ww =
        match ww with
        | [] -> []
        | h :: t -> if test_char h then List.rev (h :: t) else aux t
        in
        aux (List.rev w)
    ;;

(* Testing, let's say we accept just a-z A-Z *)
trim ['c', 'b', 'd', '!', '$'];; (* outputs: ['c', 'b', 'd'] *)
trim ['c', '$', 'd'];; (* outputs: ['c', '$', 'd'] *)