GNU bug report logs - #78940
REGEX wrongfully matches pattern

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

From: "Duchesne, Alexandre" <duchesne.alexandre.3 <at> courrier.uqam.ca>
To: "bug-sed <at> gnu.org" <bug-sed <at> gnu.org>
Subject: REGEX wrongfully matches pattern 
Date: Tue, 1 Jul 2025 20:26:16 +0000

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

Hi,

I wanted to show to my classmate the comportment of a RegEX using sed to capture the subgroup in a pattern match.

But, when trying with sed -E, the pattern matched even though it should not have.

Can someone explain to me why if this comportment turns out to be normal ?

View snippet :

$ echo abc | grep -E '(...).+' -c
0
$ echo abc | sed -E 's/(...).+/\1/'
abc
$ sed --version
sed (GNU sed) 4.9
Packaged by Debian
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Jay Fenlason, Tom Lord, Ken Pizzini,
Paolo Bonzini, Jim Meyering, and Assaf Gordon.

This sed program was built with SELinux support.
SELinux is disabled on this system.

GNU sed home page: <https://www.gnu.org/software/sed/>.
General help using GNU software: <https://www.gnu.org/gethelp/>.
E-mail bug reports to: <bug-sed <at> gnu.org>.


Regards,

-Alexandre.

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

Message #8 received at 78940 <at> debbugs.gnu.org (full text, mbox):

From: Davide Brini <dave_br <at> gmx.com>
To: 78940 <at> debbugs.gnu.org
Subject: Re: bug#78940: REGEX wrongfully matches pattern
Date: Wed, 2 Jul 2025 15:26:19 +0200

On Tue, 1 Jul 2025 20:26:16 +0000, "Duchesne, Alexandre"
<duchesne.alexandre.3 <at> courrier.uqam.ca> wrote:

> Hi,
> 
> I wanted to show to my classmate the comportment of a RegEX using sed to
> capture the subgroup in a pattern match.
> 
> But, when trying with sed -E, the pattern matched even though it should
> not have.
> 
> Can someone explain to me why if this comportment turns out to be normal ?
> 
> View snippet :
> 
> $ echo abc | grep -E '(...).+' -c
> 0
> $ echo abc | sed -E 's/(...).+/\1/'
> abc

This is all expected behavior. Your regexp does not match, no sostitution
is performed, and sed prints the input line unchanged, which is the default
behavior (disable with sed -n).

-- 
D.

Message #13 received at 78940-done <at> debbugs.gnu.org (full text, mbox):

From: Jim Meyering <jim <at> meyering.net>
To: Davide Brini <dave_br <at> gmx.com>
Cc: 78940-done <at> debbugs.gnu.org
Subject: Re: bug#78940: REGEX wrongfully matches pattern
Date: Thu, 3 Jul 2025 10:03:54 -0700

tags 78940 notabug
close 78940
stop

On Wed, Jul 2, 2025 at 6:27 AM Davide Brini via <bug-sed <at> gnu.org> wrote:
> On Tue, 1 Jul 2025 20:26:16 +0000, "Duchesne, Alexandre"
> <duchesne.alexandre.3 <at> courrier.uqam.ca> wrote:
>
> > Hi,
> >
> > I wanted to show to my classmate the comportment of a RegEX using sed to
> > capture the subgroup in a pattern match.
> >
> > But, when trying with sed -E, the pattern matched even though it should
> > not have.
> >
> > Can someone explain to me why if this comportment turns out to be normal ?
> >
> > View snippet :
> >
> > $ echo abc | grep -E '(...).+' -c
> > 0
> > $ echo abc | sed -E 's/(...).+/\1/'
> > abc
>
> This is all expected behavior. Your regexp does not match, no sostitution
> is performed, and sed prints the input line unchanged, which is the default
> behavior (disable with sed -n).

Thanks for replying. I've marked this issue as resolved and "not a bug".

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