如何在OCaml中去掉整型前后的括号？

let remove_brackets (s: string) : int option =
  match String.trim s with
  | "(" ^ num ^ ")" -> Some (int_of_string num)
  | _ -> None

let result = remove_brackets "(123)" in
match result with
| Some num -> print_int num
| None -> print_string "Invalid input"

let remove_brackets (s: string) : int option =
  let regex = Str.regexp "\\((\\([0-9]+\\))\\)" in
  if Str.string_match regex s 0 then
    Some (int_of_string (Str.matched_group 1 s))
  else
    None

let result = remove_brackets "(123)" in
match result with
| Some num -> print_int num
| None -> print_string "Invalid input"