bug#39678: POSIXLY_CORRECT removal, and oddball regex doc

[Jim Meyering]

On Sat, May 21, 2022 at 3:05 AM Paul Eggert <eggert@cs.ucla.edu> wrote:
> Looking again at grep bug 39678 <https://bugs.gnu.org/39678> I noticed
> that the bug occurs even when grep is not coloring:
>
> echo a | grep -oi --color=never '\a'
>
> This outputs nothing and exits with status 0, which is clearly wrong.

Wrong, indeed!

> I tracked this down to a bug buried deep in the bowels of glibc regex, a
> bug that Tomasz also spotted. It's not trivial to fix (the fix that
> Tomasz sent in doesn't feel right, at least for \X where X is a
> multibyte letter) and any fix would be low priority since the bug occurs
> only in regular expressions like '\a' that have unspecified behavior -
> which means the behavior though wrong nevertheless conforms to POSIX.
>
> I'm inclined to address this by having GNU 'grep' diagnose unspecified
> regexps like '\a' and exit with status 2, much as it already diagnoses
> unspecified regexps like '[:alpha:]'. If this approach sounds too
> drastic, a gentler approach would be for 'grep' to warn about '\a'
> without changing the exit status for now, and escalate the warning to
> exit with status 2 in a later 'grep' release.

In my experience, there are many lurking uses of things like '\a', and
would like to ease into this gently, so I much prefer your latter
approach: warn now, and change grep's exit status later -- at least 6
months to a year later, to give people
at least a chance to fix their offending uses before they break.

Thanks for all the improvements!