GNU bug report logs - #21159
Fails To Match Empty String

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Squirrely <squirrely <at> gmx.com>
Subject: bug#21159: closed (Re: bug#21159: Fails To Match Empty String)
Date: Thu, 30 Jul 2015 00:54:02 +0000

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Your bug report

#21159: Fails To Match Empty String

which was filed against the grep package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 21159 <at> debbugs.gnu.org.

-- 
21159: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=21159
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Squirrely <squirrely <at> gmx.com>, 21159-done <at> debbugs.gnu.org
Subject: Re: bug#21159: Fails To Match Empty String
Date: Wed, 29 Jul 2015 17:53:32 -0700

Squirrely wrote:
> bash$ rm empty
>>bash$ touch empty
>>bash$ # I am expecting a match, so grep should return 0.
>>bash$ grep '^$' empty
>>bash$ echo $?

Grep looks for lines that contain matches.  An empty file has no lines, so it 
cannot possibly contain any matches for any regular expression.

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From: Squirrely <squirrely <at> gmx.com>
To: bug-grep <at> gnu.org
Subject: Fails To Match Empty String
Date: Wed, 29 Jul 2015 23:38:47 +0100

Hi

I'm a bit of a regular expression noob, so I'm not sure if this is a bug
or if I'm just missing something about how grep works.

Here's a demo of the issue I have encountered:

> bash$ rm empty
> bash$ touch empty
> bash$ # I am expecting a match, so grep should return 0.
> bash$ grep '^$' empty
> bash$ echo $?
> 1
> bash$ # Hmmm... weird.
> bash$ # Same example but using STDIN instead...
> bash$ echo -n ""| grep '^$'
> bash$ echo $?
> 1
> bash$ # Same result.  How does the python re module treat this?
> bash$ python3
> >>> import re
> >>> m = re.search("^$", "")
> >>> type(m)
> <class '_sre.SRE_Match'>
> >>> # A match was found.  Python returns 'None' if it's not a match,
> >>> # like this...
> >>> m = re.search("fo?", "bar")
> >>> type(m)
> <class 'NoneType'>

I know that the Python re module and grep use a different regex syntax,
but I'm pretty sure "^$" has the same meaning for both.

I discounted the idea that grep only checks lines that end with a
newline character because of this:

> bash$ echo -en "foo\nfoo\nfoo"|grep foo
> foo
> foo
> foo

As you can see, the third foo is checked and matched despite not being
terminated with a newline character (observe the echo "-n" switch).

So... why does ^$ match the empty string with python but not with grep?

-Squirrely

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