Paul Eggert via Bug reports for GNU grep <bug-grep@gnu.org> writes:

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

/Simon