GNU bug report logs - #21865
Parenthesis subexpressions

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Package: grep; Severity: wishlist; Reported by: Valerio Bozzolan <bozzolan.valerio@HIDDEN>; dated Sun, 8 Nov 2015 21:58:02 UTC; Maintainer for grep is bug-grep@HIDDEN.
Severity set to 'wishlist' from 'normal' Request was from Paul Eggert <eggert@HIDDEN> to control <at> debbugs.gnu.org. Full text available.

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From: Stephane Chazelas <stephane.chazelas@HIDDEN>
To: Valerio Bozzolan <bozzolan.valerio@HIDDEN>
Subject: Re: bug#21865: Parenthesis subexpressions
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2015-11-09 17:00:36 +0100, Valerio Bozzolan:
> Thanks for agreeing with the evolution of the meaning of "-o".
> 
> Just to make you a laugh: I was reproducing egrep with $BASH_REMATCH:
> https://gist.github.com/valerio-bozzolan/6787675e931dce1ba7e9
> 
> Definitely not beautiful... but really effective for me.

You may want to read:

https://unix.stackexchange.com/questions/169716/why-is-using-a-shell-loop-to-process-text-considered-bad-practice
https://unix.stackexchange.com/questions/209123/understand-ifs-read-r-line
https://unix.stackexchange.com/questions/65803/why-is-printf-better-than-echo

Here, if there wasn't a pcregrep already, I'd rather do it in
perl or GNU sed than bash.

Like:

perl -lne 'print for /a([a-z])c/g'

Also note that:

echo abac | pcregrep -o1 'a(.)'
b
c

> So something like "egrep -o $n regex" also can save the world from code similar to mine.

GNU grep can't add it like that as that would break backward
compatibility.

grep -o 1 regex file

is currently meant to print the occurrences of "1" in the
"regex" and "file" files.

Even adding it as:

grep -o1 regex file

would probably not be a good idea as that would mean some
ad-hoc parsing of the options (in "grep -o1 regexp", "1" would
be an argument to "-o" while in "grep -oi regexp", "i" currently
is a separate "-i" option).

So reasonably, it should probably be a separate option like -O 1.

-- 
Stephane




Information forwarded to bug-grep@HIDDEN:
bug#21865; Package grep. Full text available.

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Subject: Re: bug#21865: Parenthesis subexpressions
From: Valerio Bozzolan <bozzolan.valerio@HIDDEN>
Date: Mon, 09 Nov 2015 17:03:39 +0100
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Thanks for agreeing with the evolution of the meaning of "-o".

Just to make you a laugh: I was reproducing egrep with $BASH_REMATCH:
https://gist.github.com/valerio-bozzolan/6787675e931dce1ba7e9

Definitely not beautiful... but really effective for me.

So something like "egrep -o $n regex" also can save the world from code similar to mine.

On 9 November 2015 14:50:46 CET, Stephane Chazelas <stephane.chazelas@HIDDEN> wrote:
>2015-11-08 21:49:03 +0100, Valerio Bozzolan:
>> Sorry... typo...
>> 
>>     echo abcde | grep -o -E 'b([a-z])d'
>>     => "bcd"
>> 
>> Can't I choose to have only "c"?
>[...]
>
>That's correct, GNU grep doesn't have that capability (yet).
>Recent versions of pcregrep do:
>
>$ echo abc | pcregrep -o1 '.(.).'
>b
>
>Now, I'm not a GNU grep maintainer but I suppose the question is
>how far do we want to take grep away from its original purpose
>(print the lines that match a pattern which is what g/re/p
>stands for).
>
>GNU grep is already doing find's job with -r, part of sed's job
>with -o/--colour.
>
>Having said that, I do agree it's the logical continuation after
>-o.
>
>Note that for now, you can already do:
>
>$ echo abcde | grep -o -P 'b\K[a-z](?=d)'
>c
>
>
>-- 
>Stephane

-- 
Valerio Bozzolan
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<html><head></head><body>Thanks for agreeing with the evolution of the meaning of &quot;-o&quot;.<br>
<br>
Just to make you a laugh: I was reproducing egrep with $BASH_REMATCH:<br>
<a href="https://gist.github.com/valerio-bozzolan/6787675e931dce1ba7e9">https://gist.github.com/valerio-bozzolan/6787675e931dce1ba7e9</a><br>
<br>
Definitely not beautiful... but really effective for me.<br>
<br>
So something like &quot;egrep -o $n regex&quot; also can save the world from code similar to mine.<br><br><div class="gmail_quote">On 9 November 2015 14:50:46 CET, Stephane Chazelas &lt;stephane.chazelas@HIDDEN&gt; wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<pre class="k9mail">2015-11-08 21:49:03 +0100, Valerio Bozzolan:<br /><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;"> Sorry... typo...<br /> <br />     echo abcde | grep -o -E 'b([a-z])d'<br />     =&gt; "bcd"<br /> <br /> Can't I choose to have only "c"?<br /></blockquote>[...]<br /><br />That's correct, GNU grep doesn't have that capability (yet).<br />Recent versions of pcregrep do:<br /><br />$ echo abc | pcregrep -o1 '.(.).'<br />b<br /><br />Now, I'm not a GNU grep maintainer but I suppose the question is<br />how far do we want to take grep away from its original purpose<br />(print the lines that match a pattern which is what g/re/p<br />stands for).<br /><br />GNU grep is already doing find's job with -r, part of sed's job<br />with -o/--colour.<br /><br />Having said that, I do agree it's the logical continuation after<br />-o.<br /><br />Note that for now, you can already do:<br /><br />$ echo!
  abcde |
grep -o -P 'b\K[a-z](?=d)'<br />c<br /><br /></pre></blockquote></div><br>
-- <br>
Valerio Bozzolan<br>
Email sent from Android (CyanogenMod) using K-9 Mail.</body></html>
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From: Stephane Chazelas <stephane.chazelas@HIDDEN>
To: Valerio Bozzolan <bozzolan.valerio@HIDDEN>
Subject: Re: bug#21865: Parenthesis subexpressions
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2015-11-08 21:49:03 +0100, Valerio Bozzolan:
> Sorry... typo...
> 
>     echo abcde | grep -o -E 'b([a-z])d'
>     => "bcd"
> 
> Can't I choose to have only "c"?
[...]

That's correct, GNU grep doesn't have that capability (yet).
Recent versions of pcregrep do:

$†echo abc | pcregrep -o1 '.(.).'
b

Now, I'm not a GNU grep maintainer but I suppose the question is
how far do we want to take grep away from its original purpose
(print the lines that match a pattern which is what g/re/p
stands for).

GNU grep is already doing find's job with -r, part of sed's job
with -o/--colour.

Having said that, I do agree it's the logical continuation after
-o.

Note that for now, you can already do:

$ echo abcde | grep -o -P 'b\K[a-z](?=d)'
c


-- 
Stephane




Information forwarded to bug-grep@HIDDEN:
bug#21865; Package grep. Full text available.

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Subject: Re: Parenthesis subexpressions
From: Valerio Bozzolan <bozzolan.valerio@HIDDEN>
Date: Sun, 08 Nov 2015 21:49:03 +0100
To: bug-grep@HIDDEN
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Sorry... typo...

    echo abcde | grep -o -E 'b([a-z])d'
    => "bcd"

Can't I choose to have only "c"?

Thanks again!

On 8 November 2015 21:42:44 CET, Valerio Bozzolan <bozzolan.valerio@HIDDEN> wrote:
>Hi,
>
>(First time in a GNU mailing list!)
>
>I've already asked this question to my local GNU/Linux user group and
>in #grep@HIDDEN I'm still confused.
>
>GNU Grep don't have an arg to choose the subexpression. Right?
>
>Stupid e.g.:
>    echo abcde | grep -o -E 'b([a-z])d'
>    => "bcd"
>
>What if I want the first subexpression? ("b")? GNU Grep can't do it.
>Isn't it? (Why?)
>
>I actually use GNU Awk, or GNU Bash with $BASH_REMATCH[$n_sub].
>
>Thank you for the clarification!
>--
>Valerio Bozzolan
>Email sent from Android (CyanogenMod) using K-9 Mail.

-- 
Valerio Bozzolan
Email sent from Android (CyanogenMod) using K-9 Mail.

http://boz.reyboz.it
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<html><head></head><body>Sorry... typo...<br>
<br>
    echo abcde | grep -o -E &#39;b([a-z])d&#39;<br>
    =&gt; &quot;bcd&quot;<br>
<br>
Can&#39;t I choose to have only &quot;c&quot;?<br>
<br>
Thanks again!<br><br><div class="gmail_quote">On 8 November 2015 21:42:44 CET, Valerio Bozzolan &lt;bozzolan.valerio@HIDDEN&gt; wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
Hi,<br />
<br />
(First time in a GNU mailing list!)<br />
<br />
I&#39;ve already asked this question to my local GNU/Linux user group and in #grep@HIDDEN I&#39;m still confused.<br />
<br />
GNU Grep don&#39;t have an arg to choose the subexpression. Right?<br />
<br />
Stupid e.g.:<br />
    echo abcde | grep -o -E &#39;b([a-z])d&#39;<br />
    =&gt; &quot;bcd&quot;<br />
<br />
What if I want the first subexpression? (&quot;b&quot;)? GNU Grep can&#39;t do it. Isn&#39;t it? (Why?)<br />
<br />
I actually use GNU Awk, or GNU Bash with $BASH_REMATCH[$n_sub].<br />
<br />
Thank you for the clarification!<br />
--<br />
Valerio Bozzolan<br />
Email sent from Android (CyanogenMod) using K-9 Mail.</blockquote></div><br>
-- <br>
Valerio Bozzolan<br>
Email sent from Android (CyanogenMod) using K-9 Mail.<br>
<br>
<a href="http://boz.reyboz.it">http://boz.reyboz.it</a></body></html>
------20WOICGV4HQVLHDL12S12HCQDMPG8T--





Information forwarded to bug-grep@HIDDEN:
bug#21865; Package grep. Full text available.

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Subject: Parenthesis subexpressions
From: Valerio Bozzolan <bozzolan.valerio@HIDDEN>
Date: Sun, 08 Nov 2015 21:42:44 +0100
To: bug-grep@HIDDEN
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Hi,

(First time in a GNU mailing list!)

I've already asked this question to my local GNU/Linux user group and in #grep@HIDDEN I'm still confused.

GNU Grep don't have an arg to choose the subexpression. Right?

Stupid e.g.:
    echo abcde | grep -o -E 'b([a-z])d'
    => "bcd"

What if I want the first subexpression? ("b")? GNU Grep can't do it. Isn't it? (Why?)

I actually use GNU Awk, or GNU Bash with $BASH_REMATCH[$n_sub].

Thank you for the clarification!
--
Valerio Bozzolan
Email sent from Android (CyanogenMod) using K-9 Mail.
------NX6JLVEXP41ZA2PIJXJH49U54QQYBR
Content-Type: text/html;
 charset=utf-8
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Hi,<br>
<br>
(First time in a GNU mailing list!)<br>
<br>
I&#39;ve already asked this question to my local GNU/Linux user group and in #grep@HIDDEN I&#39;m still confused.<br>
<br>
GNU Grep don&#39;t have an arg to choose the subexpression. Right?<br>
<br>
Stupid e.g.:<br>
    echo abcde | grep -o -E &#39;b([a-z])d&#39;<br>
    =&gt; &quot;bcd&quot;<br>
<br>
What if I want the first subexpression? (&quot;b&quot;)? GNU Grep can&#39;t do it. Isn&#39;t it? (Why?)<br>
<br>
I actually use GNU Awk, or GNU Bash with $BASH_REMATCH[$n_sub].<br>
<br>
Thank you for the clarification!<br>
--<br>
Valerio Bozzolan<br>
Email sent from Android (CyanogenMod) using K-9 Mail.
------NX6JLVEXP41ZA2PIJXJH49U54QQYBR--





Acknowledgement sent to Valerio Bozzolan <bozzolan.valerio@HIDDEN>:
New bug report received and forwarded. Copy sent to bug-grep@HIDDEN. Full text available.
Report forwarded to bug-grep@HIDDEN:
bug#21865; Package grep. Full text available.
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