Re: [PATCH 1/3] grep: add --warnings={always,never,auto}.

[Paul Eggert]

On 08/30/10 00:47, Paolo Bonzini wrote:
> It's true that the GNU coding standards say "please don't make the
> behavior of the program depend on the type of output device it is used
> with".  On the other hand they also say "Compatibility requires certain
> programs to depend on the type of output device."

But in this case we're talking about new behavior.  No other grep
implementation outputs warnings for dubious-but-POSIXLY-correct
patterns, right?  So there's no issue about compatibility with
other greps.

How about this idea instead?  If POSIXLY_CORRECT is set, output
no warnings and behave as grep has done for ages.  Otherwise,
if a dubious regular expression is found, output a diagnostic and
exit with status 2.

The documentation should explicitly state that dubious regular
expressions are errors unless POSIXLY_CORRECT is set.  This
satisfies the the GNU coding standards and conforms to POSIX,
and it should work well in practice.