(EDIT 1: Account for invalid values.)
(EDIT 2: Trim empty strings when splitting.)
(EDIT 3: Included explanations.)

defmodule Test do
  def extract_id(url) when is_binary(url) do
    url
    |> String.split(~w(. /), parts: 1000, trim: true)
    |> List.last
    |> parse_id
    |> fetch_id
  end

  defp parse_id(nil), do: :error
  defp parse_id(x) when is_binary(x), do: Integer.parse(x)

  defp fetch_id({number, ""}) when is_integer(number), do: number
  defp fetch_id(:error), do: :error
end

Test it:

iex> urls = ["https://foster.com/death-pancake.1468/", "https://hkd33.net/mr-rogers101.690153/", "whatever_dude", "https://space-force911.gov/sauce-master.13257777/"]

iex> urls |> Enum.map(&Test.extract_id/1)
[1468, 690153, :error, 13257777]

Breaking it down:

• ~w(. /) equals [".", "/"] (so String.split is called with multiple separators).

• parts: 1000 is used to prevent denial-of-service attacks, in case somebody manages to smuggle huge strings to your code. trim: true removes empty strings from the result. Check String.split docs.

• "https://foster.com/death-pancake.1468/" |> String.split(~w(. /), parts: 1000, trim: true) yields this:

["https:", "foster", "com", "death-pancake", "1468"]

…so we are calling List.last on it to give us the desirable piece of data.

• Our internal function parse_id has to also handle invalid data:
  • If String.split returns [], List.last would return nil.
  • If String.split returns ["single_invalid_url"], List.last would return "single_invalid_url".

Both cases would make our internal function parse_id to return :error. (Integer.parse will return :error if you supply it a string that does NOT start with an integer.)

• The fetch_id internal function uses function heads instead of if or case to extract successful integer parsing and return it, or react to an :error return value and just pass it down the line to your consumer code.

• One caveat: notice that fetch_id matches on {number, ""} when is_integer(number) which means the function will be called only if a full integer string is passed, namely “123” or “456” will succeed but “123xyz” will not. If you expect URLs like “https://whatever.man/1234abcd”, this code won’t work.