GNU bug report logs - #18377
piping output through egrep -- escaping of "+" only needed sometimes. Why?

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

From: terrygalant.lists <at> fastest.cc
To: bug-grep <at> gnu.org
Subject: piping output through egrep -- escaping of "+" only needed sometimes.
 Why?
Date: Sun, 31 Aug 2014 19:07:48 -0700

Hi all,

I'm trying to pipe of output of a command through egrep.

If I do this

	ldconfig -p | egrep "libstdc++"
		libstdc++.so.6 (libc6,x86-64) => /usr/lib64/libstdc++.so.6
		libstdc++.so.6 (libc6) => /usr/lib/libstdc++.so.6

that *IS* the answer I expect.

But if I simply add a ".so" to the egrep target, it returns nothing

	ldconfig -p | egrep "libstdc++.so"
		(empty)

If I *escape* the two "+", it works again.

	ldconfig -p | egrep "libstdc\+\+.so"
		libstdc++.so.6 (libc6,x86-64) => /usr/lib64/libstdc++.so.6
		libstdc++.so.6 (libc6) => /usr/lib/libstdc++.so.6

Why do I need to escape the "+" in the last case, but not in the first?

Terry

Message #10 received at 18377-done <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: terrygalant.lists <at> fastest.cc, 18377-done <at> debbugs.gnu.org
Subject: Re: bug#18377: piping output through egrep -- escaping of "+" only
 needed sometimes. Why?
Date: Mon, 01 Sep 2014 01:05:51 -0700

terrygalant.lists <at> fastest.cc wrote:
> Why do I need to escape the "+" in the last case, but not in the first?

Because "+" is a special character to egrep.  The pattern "libstdc++" is 
equivalent to the pattern "libstdcc*" which matches in the first case, 
whereas the pattern "libstdc++.so" is equivalent to the pattern 
"libstdcc*.so" which does not match in the last case.

Message #13 received at 18377-done <at> debbugs.gnu.org (full text, mbox):

From: Norihiro Tanaka <noritnk <at> kcn.ne.jp>
To: terrygalant.lists <at> fastest.cc
Cc: 18377-done <at> debbugs.gnu.org
Subject: Re: bug#18377: piping output through egrep -- escaping of "+" only
 needed sometimes. Why?
Date: Mon, 01 Sep 2014 20:44:35 +0900

"egrep 'libstdc++'" is taken as "egrep 'libstd(c+)+'", although I don't
see that this behaviour is clearly defined in POSIX.  Therefore, as Paul
says, it's synonymous with 'libstdc(c*)'.


On Sun, 31 Aug 2014 19:07:48 -0700
terrygalant.lists <at> fastest.cc wrote:

> Hi all,
> 
> I'm trying to pipe of output of a command through egrep.
> 
> If I do this
> 
> 	ldconfig -p | egrep "libstdc++"
> 		libstdc++.so.6 (libc6,x86-64) => /usr/lib64/libstdc++.so.6
> 		libstdc++.so.6 (libc6) => /usr/lib/libstdc++.so.6
> 
> that *IS* the answer I expect.
> 
> But if I simply add a ".so" to the egrep target, it returns nothing
> 
> 	ldconfig -p | egrep "libstdc++.so"
> 		(empty)
> 
> If I *escape* the two "+", it works again.
> 
> 	ldconfig -p | egrep "libstdc\+\+.so"
> 		libstdc++.so.6 (libc6,x86-64) => /usr/lib64/libstdc++.so.6
> 		libstdc++.so.6 (libc6) => /usr/lib/libstdc++.so.6
> 
> Why do I need to escape the "+" in the last case, but not in the first?
> 
> Terry
> 
> 

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