let remove_substr str substr =
    Str.global_replace (Str.regexp substr) "" str;;

let remove_substrs str substrs =
    List.fold_left remove_substr str substrs;;

let get_domain_name str =
    let trimmed = remove_substrs str ["https://"; "http://"; "www"] in
    let domain_frags = String.split_on_char '.' trimmed in
    List.find (fun frag -> String.length frag > 0) domain_frags;;

let is_four_chars_or_less url =
    let name = get_domain_name url in
    (String.length name) <= 4;;

List.filter is_four_chars_or_less ["www.google.com"; "https://stfj.net"]