According to the docs, must be all upper case: > Custom sigils may be either a single lowercase character, or an uppercase character followed by more uppercase

According to the docs, must be all upper case: > Custom sigils may be either a single lowercase character, or an uppercase character followed by more uppercase characters and digits. https://hexdocs.pm/elixir/sigils.html https://hexdocs.pm/elixir/sigils.html