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regexp: wrong match with 'ic' and comparing multi-byte with single byte char #14756

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regexp: wrong match with 'ic' and comparing multi-byte with single byte char #14756

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Commits on Jul 28, 2024

  1. regexp: wrong match with 'ic' and comparing multi-byte with single by… 

    …te char

This is v2 of v9.1.296, but still draft to see if this still breaks.

When the regexp engine compares two utf-8 codepoints case insensitively it may
match an adjacent character, because it assumes it can step over as many bytes
as the pattern contains.

This however is not necessarily true because of case-folding, a multi-byte
UTF-8 character can be considered equal to some single-byte value.

Let's consider the pattern 'ſ' and the string 's'. When comparing and ignoring
case, the single character 's' matches, and since it matches Vim will try to
step over the match (by the amount of bytes of the pattern), assuming that
since it matches, the length of both strings is the same.

However in that case, it should only step over the single byte value 's' by 1
byte and try to start matching after it again. So for the backtracking engine
we need to ensure:
- we try to match the correct length for the pattern and the text
- in case of a match, we step over it correctly

There is one tricky thing for the backtracing engine. We also need to calculate
correctly the number of bytes to compare the 2 different utf-8 strings s1 and
s2. So we will count the number of characters in s1 that the byte len
specified. Then we count the number of bytes to step over the same number of
characters in string s2 and then we can correctly compare the 2 utf-8 strings.

A similar thing can happen for the NFA engine, when skipping to the next
character to test for a match. We are skipping over the regstart pointer,
however we do not consider the case that because of case-folding we may need to
adjust the number of bytes to skip over. So this needs to be adjusted in
find_match_text() as well.

A related issue turned out, when prog->match_text is actually empty. In that
case we should try to find the next match and skip this condition.

Note: this breaks Mail and CSS Syntax highlighting and CI on FreeBSD/MacOS
vim#14487 and
https://groups.google.com/d/msgid/vim_dev/CAJkCKXtui%3DDTWx9eV8Dbs19XoFL9b63ObSNXWCRvLsEZCB6yfw%40mail.gmail.com.

fixes: vim#14294

Signed-off-by: Christian Brabandt <cb@256bit.org>
    @chrisbra
    chrisbra committed Jul 28, 2024
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  2. compare characters in brace expressions using proper case folding 

    Signed-off-by: Christian Brabandt <cb@256bit.org>
    @chrisbra
    chrisbra committed Jul 28, 2024
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