Beautiful tiny FSM encoded with pattern matching

swap_escape.ml

(* OCaml regular expressions are unusual in that they require
   parentheses and pipe symbol to be escaped to *gain* special
   meaning. swap_escape converts from (and, incidentally, to)
   the usual format. *)
let swap_escape input =
  let swap_chr (esc, str) chr =
    match esc, chr with
    (* Go to escape mode. *)
    | false, '\\'
    -> true,  str
    (* *Un*escape parens and pipe. *)
    | true,  '(' | true,  '|' | true,  ')'
    -> false, str ^ (String.of_char chr)
    (* Leave other escaped symbols as they are (escaped).
       Escape bare parens and pipe. *)
    | true,  _   | false, '(' | false, '|' | false, ')'
    -> false, str ^ "\\" ^ (String.of_char chr)
    (* Leave other unescaped symbols as they are (bare). *)
    | false, _
    -> false, str ^ (String.of_char chr)
  in
  snd (String.fold_left swap_chr (false, "") input)