bug#57604: [ef]grep usage -> POSIXLY_CORRECT?

[Paul Eggert]

On 9/7/22 03:02, Simon Josefsson via Bug reports for GNU grep wrote:
> > $ grep '\Q' /dev/null
> > grep: warning: stray \ before Q
> > $ grep '[:alpha:]' /dev/null
> > grep: character class syntax is [[:space:]], not [:space:]
> Is the use of diagnostic warnings like this supported by POSIX?

Yes, POSIX says that \Q produces undefined results, and that [:alpha:] 
can be treated as either [[:alpha:]], [:alph], or an error (the last is 
the GNU behavior).

> I'd rather have tools exit with an error code on invalid
> uses rather than issuing warning messages.

GNU grep does that for [:alpha:]. At some point it should do the same 
for \Q, as \Q's behavior is squirrelly anyway. I thought it might be too 
disruptive to do it for \Q right away so in 3.8 \Q merely gets warned about.