let rec str_contains s sub i =
  let nl = String.length s in
  let sl = String.length sub in
  if i + sl > nl then false
  else if String.sub s i sl = sub then true
  else str_contains s sub (i + 1)

let count_matching needle text =
  let lines = String.split_on_char '\n' text in
  List.fold_left (fun acc line ->
    if str_contains line needle 0 then acc + 1
    else acc
  ) 0 lines

;;

count_matching "fox" "the quick brown fox\nfox runs fast\nthe dog\nfoxes are clever"