How do I compile this?

ls.ml

open Core.Std

module Regex = Re2.Regex

let ls pattern = 
  let pat = Regex.create_exn pattern in
  let matcher = Regex.matches pat in
  Sys.ls_dir "." 
  |> List.filter ~f:matcher
  |> List.iter ~f:(fun s -> print_string s; print_newline ())

let () =
  match In_channel.input_line stdin with
  | None -> (print_string "No Input"; print_newline ())
  | Some pat -> ls pat