I do not get your question…

The typespec for Regex.replace/4 is replace(Regex.t, String.t, String.t | (... -> String.t), [term]) :: String.t, which has nothing to do with Regex.run/3.

The typespec for String.replace/4 is replace(String.t, pattern | Regex.t, String.t, keyword) :: String.t, and this is what you actually used in iex.

String.replace/3 is not the same as Regex.replace/3.
While String.replace/3 takes a string first, Regex.replace/3 takes a regular expression instead.

Take a look at the differences in the documentation of the two functions:
https://hexdocs.pm/elixir/String.html#replace/4
https://hexdocs.pm/elixir/Regex.html#replace/4

The numbered pipe has been discussed before, I do consider it as unreadable.

But there is param_pipe available on hex. Announcement and discussion is available on the forum as well: