Re: master ce63f91025: Add textsec functions for verifying email addresses

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: luangruo@yahoo.com,  emacs-devel@gnu.org
> Date: Thu, 20 Jan 2022 12:08:35 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> (insert "Lars Ingebrigtsen\N{RIGHT-TO-LEFT OVERRIDE}" "larsi@gnus.org")
> >> 
> >> Boom.
> >
> > Then you must pass the entire concatenated string to the function.  Or
> > call it on buffer text after inserting the string there.  This
> > function must see the characters affected by the bidi controls, to
> > tell whether the control do any harm.
> 
> I see.  Perhaps we should have another function in addition -- one that
> says "does this string (if inserted into a buffer) possibly affect other
> text"?  I.e., "does it have dangling directional modifiers"?  I think
> that's really what we want here (and why the Unicode recommendations are
> like they are in this area).

The problem is that the answer to that question depends on the
following text.  E.g., if RIGHT-TO-LEFT OVERRIDE is followed by R2L
characters, they will not be affected.

We could try appending some representative text to the string being
tested, of course.  For example, append a fixed string like this:

   a1א:!

and see if the function returns non-nil position that points to one of
those characters; if so, consider the original string "unsafe".

Would that be good enough for textsec purposes?