bug#51733: 27.1; Detect impossible email addresses better

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 51733@debbugs.gnu.org
> Date: Tue, 18 Jan 2022 12:26:30 +0100
> 
> Next stupid question:
> 
> ---
> It must not contain any stateful bidirectional format characters.
> 
>     That is, no [:bidicontrol:] except for the LRM, RLM, and ALM, since the 
> bidirectional controls could influence the ordering of characters outside the 
> quotes.
> ---
> 
> We don't have the :bidicontrol: regexp class.  Do we have another way to
> classify bidi control characters?  The have class Cf, but so does many
> other non-bidi control characters...

I don't think you need any classification: the offending control
characters are very few, so you could just test for them explicitly.