GNU bug report logs - #47001
Feature request: give a meaning to -A/-B/-C with -o

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

From: Gregory Heytings <gregory <at> heytings.org>
To: bug-grep <at> gnu.org
Subject: Feature request: give a meaning to -A/-B/-C with -o
Date: Mon, 08 Mar 2021 08:21:27 +0000

Hi list,

Grep gives no meaning to the -A/-B/-C flags when they are used together 
with the -o flag (and prints a warning when they are used together).

I suggest to give them a specific meaning when the -o flag is used, namely 
to print NUM _characters_ of trailing/leading/output context around the 
match.

Currently this behavior can be obtained with "grep -oE 
'.{0,A}expose_frame.{0,B}'", where A and B are the number of leading and 
trailing context characters, but it is not efficient, and can become very 
inefficient for "large" values of A and B (say, 40).

Thanks,

Gregory

Message #8 received at 47001 <at> debbugs.gnu.org (full text, mbox):

From: Gregory Heytings <gregory <at> heytings.org>
To: 47001 <at> debbugs.gnu.org
Subject: Re: bug#47001: Feature request: give a meaning to -A/-B/-C with -o
Date: Mon, 08 Mar 2021 17:12:38 +0000

Two further comments:

>
> Grep gives no meaning to the -A/-B/-C flags when they are used together 
> with the -o flag (and prints a warning when they are used together).
>

Both the manual and the man page indicate that "a warning is given" when 
these options are used together, but what actually happens is that a group 
separator is printed at apparently random places.

>
> I suggest to give them a specific meaning when the -o flag is used, 
> namely to print NUM _characters_ of trailing/leading/output context 
> around the match.
>
> Currently this behavior can be obtained with "grep -oE 
> '.{0,A}expose_frame.{0,B}'", where A and B are the number of leading and 
> trailing context characters, but it is not efficient, and can become 
> very inefficient for "large" values of A and B (say, 40).
>

Of course I meant "grep -oE '.{0,A}PATTERN.{0,B}'.

The "very inefficient" case happens in particular for very long lines:

for i in $(seq 1 10)
do
  printf %100000s | tr ' ' 'a'
  printf foobar
  printf %100000s | tr ' ' 'a'
done | grep -oE '.{0,40}foobar.{0,40}'

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