On Elixir 1.19+ we can use ... to match anything until the end of the message, including multiline output. Support ellipsis in doctest exceptions by sabiwara · Pull Request #14233 · elixir-lang/elixir · GitHub

I think you may need to use assert_raise in that case:
iex> assert_raise KeyError, fn -> map.c end

As far as I can tell a “pure” doctest will always assert on the full exception text (with the caveat that multi-line exceptions are not supported: Handling multi-newline exception messages in doctests? (Elixir v1.12.0))

Thanks for the answers.

I’ll mention I also found one other workaround, which is, at least for the doctests, to pare your examples down to have only one key in the error-inspected map:

@doc """
…

    iex> difference!(%{a: 10}, %{a: 11})
    ** (KeyError) key {:a, 11} not found in: %{a: 10}

    iex> difference!(%{a: 10}, %{a: 3, b: 4})
    ** (KeyError) key {:b, 4} not found in: %{a: 10}
"""

Oooh! That’s really exciting