GNU bug report logs - #79639
grep: uwaga: stray \ before +

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Michal Ligus <maveilthain <at> gmail.com>
To: bug-grep <at> gnu.org
Subject: grep: uwaga: stray \ before +
Date: Thu, 16 Oct 2025 12:58:59 +0200

[Message part 1 (text/plain, inline)]

How to annoying disable message from output?
the  `| grep -v` not working

Please fix issue (by passing additional argument in example --no-warnings)
to GREP
To reproduce:

cat {your_file} | grep '^\+'
cat {your_file} | grep '^\-'

Thanks, please fix this warning

-- 
mgr inż. Michał Ligus

[Message part 2 (text/html, inline)]

Message #8 received at 79639 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Michal Ligus <maveilthain <at> gmail.com>, 79639 <at> debbugs.gnu.org
Subject: Re: bug#79639: grep: uwaga: stray \ before +
Date: Thu, 16 Oct 2025 08:20:45 -0700

On 2025-10-16 03:58, Michal Ligus wrote:
> How to annoying disable message from output?

You can use ">&2 /dev/null" for that.

Warning about misuse of "\" helps programmers avoid unportable regular 
expressions, which produce undefined results.

Message #11 received at 79639 <at> debbugs.gnu.org (full text, mbox):

From: "Dale R. Worley" <Dale.Worley <at> comcast.net>
To: Michal Ligus <maveilthain <at> gmail.com>
Cc: 79639 <at> debbugs.gnu.org
Subject: Re: bug#79639: grep: uwaga: stray \ before +
Date: Thu, 16 Oct 2025 12:04:36 -0400

Michal Ligus <maveilthain <at> gmail.com> writes:
> To reproduce:
>
> cat {your_file} | grep '^\+'

The message in question is:

    $ grep '^\+' </dev/null
    grep: warning: stray \ before +

The message is correct, in the "basic regular expression" mode, "\+" is
not a legitimate regular expression.

However if you enable "extended regular expression mode", "\+" *is* a
legitimate regular expression, it matches the character "+".  (In this
mode, "+" alone is a regexp operator.)  And grep doesn't give you a
warning:

    $ grep -E '^\+' </dev/null

My recommendation is that you either give grep the -E option, so that
"^\+" correctly denotes what you want to search for, or that you give it
the argument "^+", which correctly denotes what you want to search for
in basic mode.

Dale

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