Paolo Bonzini wrote:
The correct course of action for grep is to defer range interpretation
to regex, because otherwise you can get mismatches between regexes with
backreferences and those without.
It depends on what one means by "correct". POSIX doesn't say what to do
in this situation, so it's OK as far as POSIX is concerned for grep to
use RRI in the typical case (i.e., without backreferences), and for grep
to use some other interpretation in the rare cases when backreferences
are used.
The documentation for 'grep' attempts to address this issue, perhaps not
as clearly as it could. Maybe the installation instructions should talk
about it as well, and suggest --with-included-regex for people who care
about this sort of thing.