grep backreference seems to invalidate --ignore-case

[Mabry Tyson]

I found what I believe to be a failure of grep to match something it 
should.  It appears that the use of a backreference invalidates the 
existence of an --ignore-case switch.   I get the same (unexpected) 
results in GNU grep 2.4.2, 2.5, and 2.5.1 on Solaris 8 and on grep 2.5 
on Mac OSX 10.3.9.   The version of grep from Sun in Solaris 8 does work 
as expected.

To make sure this hasn't been recently fixed, I downloaded 
ftp://ftp.gnu.org/gnu/grep/grep-2.5.1a.tar.gz
and built grep from that.

> manresa 181: uname -a
> SunOS manresa 5.8 Generic_108528-24 sun4u sparc SUNW,Sun-Blade-100
> manresa 182: src/grep --version
> grep (GNU grep) 2.5.1
>
> Copyright 1988, 1992-1999, 2000, 2001 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions. There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR 
> PURPOSE.
>
> manresa 183: cat /tmp/test
> A abcd abcd
> manresa 184: src/grep  --ignore-case 'a \(abcd\) \1' /tmp/test
> manresa 185: src/grep  --ignore-case 'A \(abcd\) \1' /tmp/test
> A abcd abcd
> manresa 186: src/grep  --ignore-case 'A \(ABCD\) \1' /tmp/test
> manresa 187: src/grep  --ignore-case 'A \(ABCD\) ABCD' /tmp/test
> A abcd abcd
> manresa 188: src/grep  --ignore-case 'a \(ABCD\) ABCD' /tmp/test
> A abcd abcd


It is my belief that all of these calls to grep should have returned the 
line from the file.

The grep distributed with Solaris 8 acts as I expect

> manresa 192: /usr/bin/grep -i 'a \(abcd\) \1' /tmp/test 
>                                                                                                                    
>
> A abcd 
> abcd                                                                                                                                                                  
>  


Another test case:

> manresa 51: cat /tmp/test2
> a abcd aBcD
> manresa 52: src/grep --ignore-case 'a \(abcd\) \1' /tmp/test2
> manresa 53: /usr/bin/grep -i 'a \(abcd\) \1' /tmp/test2
> a abcd aBcD

In this case, however, the documentation is somewhat ambiguous.  
--ignore-case is documented as "Ignore case distinctions in both the 
PATTERN and the input files."  A backreference is documented as "matches 
the substring previously matched by the Nth parenthesized subexpression 
of the regular expression."   It isn't clear whether a backreference 
must match the substring exactly, or possibly match it, ignoring 
case.     It appears that at least the grep used in Solaris matched the 
substring, ignoring case if --ignore-case is also given.   I would argue 
that this is the correct behavior as the --ignore-case indicates to 
ignore the case in the input files.    However this is resolved, the 
documentation should clarify what it does.


It appears that GNU emacs 21.12.1 (on Mac OS X) does regular expression 
matching as I expect. 
When case-fold-search = t, the expression

> (search-forward-regexp "a \\(abcd\\) \\1")

will match each of the lines

> a abcd 
> abcd                                                                                                                                                 
>
> A abcd 
> abcd                                                                                                                                                 
>
> a abcd 
> aBcD