GNU bug report logs - #16919
[PATCH] fix mismatch between dfa and regex for treatment of titlecase

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Norihiro Tanaka <noritnk <at> kcn.ne.jp>
Cc: 16919 <at> debbugs.gnu.org
Subject: bug#16919: [PATCH] fix mismatch between dfa and regex for treatment of titlecase
Date: Mon, 03 Mar 2014 23:01:35 -0800

Norihiro Tanaka wrote:
> Final sigma is neither uppwercase nor lowercase of
> sigma, as you have said firstly.  In addition, it isn't even titlecase
> of sigma.

That's what I thought, too.  Certainly an implementation should be free 
to say that stigma and sigma do not match when ignoring case.  That is 
how Solaris 11 /usr/xpg4/bin/grep -i behaves, for example.  But it's not 
clear that POSIX requires such an implementation.

> Even if that behavior of regex is right, it won't work correctly for a
> character that only one lowercase is assigned to two uppercases. (Though,
> I don't know such a character.)

Here's an example: 'İ' (U+0130 LATIN CAPITAL LETTER I WITH DOT ABOVE) 
along with ASCII 'i' and 'I'.  On GNU and Solaris platforms in the 
en_US.UTF-8 locale, towlower (L'İ') == L'i', so 'i' has two uppercase 
counterparts.  Solaris 11 /usr/xpg4/bin/grep -i says that 'i' and 'I' 
match each other but do not match 'İ', and reports a syntax error when 
attempting to match 'İ' to itself:

$ echo 'İ' | /usr/xpg4/bin/grep -i 'İ'
grep: input file "(standard input)": line 1: syntax error

so Solaris 'grep' is clearly busted here.  The GNU regex code says that 
'i' and 'I' match only each other, and that 'İ' matches only itself. 
But the dfa code in the current trunk behaves differently: it says that 
the patterns 'i' and 'I' match each other, whereas the pattern 'İ' 
matches itself and 'i'.  So we have inconsistent behavior with the grep 
current master 5aa1413:

$ printf 'İ\ni\nI\n' | grep -i 'İ' # Use the DFA code.
İ
i
$ printf 'İ\ni\nI\n' | grep -i '\(\)\1İ' # Use the regex code.
İ

Here the DFA behavior is more plausible; but the DFA code is supposed to 
accelerate the regex code without changing behavior, so there is a problem.

After writing the above, I discovered a web page that talks about this 
issue, written about 10 years ago and committed by Charles Levert:

http://www.gnu.org/software/grep/devel.html

Look for the string "POSIX and --ignore-case".  The regex code takes the 
"uc(input_wchar) == uc(pattern_wchar)" approach, whereas the DFA code 
takes an approach not mentioned in that web page, namely "input_wchar == 
pattern_wchar || input_wchar == lc(pattern_wchar) || input_wchar == 
uc(pattern_wchar)".

I'm inclined to fix the DFA code so that it behaves like the regex code 
even if the regex code isn't that nice, because arguably the regex code 
does conform to POSIX and anyway its behavior is longstanding and I 
doubt whether people care all that much about the details so long as the 
two are consistent.

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