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

From: Julian Foad
Subject: Re: grep backreference seems to invalidate --ignore-case
Date: Mon, 19 Dec 2005 21:28:21 +0000
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.8) Gecko/20050511

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.


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

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