Re: grep backreference seems to invalidate --ignore-case

[Julian Foad]

Mabry Tyson wrote:
> 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.

Thanks for this well-written bug report.

> ftp://ftp.gnu.org/gnu/grep/grep-2.5.1a.tar.gz

> > manresa 183: cat /tmp/test
> > A abcd abcd
> > manresa 184: src/grep  --ignore-case 'a \(abcd\) \1' /tmp/test
(no output)

In the development (CVS) version of GNU grep, lots of problems related to 
case-insensitive matching have been fixed, and this now works (matches the line).

Hopefully a new version will be released one day.


> > manresa 51: cat /tmp/test2
> > a abcd aBcD
> > manresa 52: src/grep --ignore-case 'a \(abcd\) \1' /tmp/test2
(no output)

This case is also fixed: the back-reference matches case-insensitively.


> 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.

I agree with you, and that is what GNU grep (CVS head) currently does.

I suppose it would be good to include a few words to this effect in the 
documentation; I'll make a note to do so.  Suggestions welcome.


Thanks again for the report.

- Julian