Re: BUG #19474: LIKE with nondeterministic collations mis-handle literal backslashes in patterns containing escape

Hi,

I have proposed a fix for this on pgsql-hackers[1]. Please take a look
and let me know what you think.

Thanks & Regards,
Nitin Motiani
Google

[1] https://www.postgresql.org/message-id/CAH5HC94yU%2BK8Gcdy12M5BS8gwD_SXLSHzc9k5tNk7JDnpBiFMA%40mail.gmail.com

On Sat, May 9, 2026 at 8:02 AM PG Bug reporting form
<noreply(at)postgresql(dot)org> wrote:
>
> The following bug has been logged on the website:
>
> Bug reference: 19474
> Logged by: Bowen Shi
> Email address: zxwsbg12138(at)gmail(dot)com
> PostgreSQL version: 18.3
> Operating system: centos
> Description:
>
> After commit 85b7efa1cdd63c2fe2b70b725b8285743ee5787f ("Support LIKE with
> nondeterministic collations"), LIKE on a nondeterministic collation can
> return an incorrect result when the pattern contains a literal backslash.
>
> The problem appears to be in MatchText() in
> src/backend/utils/adt/like_match.c. In the nondeterministic-collation path,
> when a pattern substring contains escape processing, the code builds an
> unescaped copy of the substring. In that logic, a backslash that should
> remain as a literal character can be dropped, so the substring compared by
> pg_strncoll() is not the same as the original SQL pattern semantics.
>
> As a result, a LIKE pattern that should match a string containing a literal
> backslash can incorrectly return false.
>
> SQL reproduction:
>
> CREATE COLLATION ignore_accents (
> provider = icu,
> locale = 'und-u-ks-level1',
> deterministic = false
> );
>
> SELECT 'back\slash' COLLATE ignore_accents LIKE 'back\slash%' ESCAPE '#';
>
> Expected result:
> t
>
> Actual result:
> f
>
> The same pattern works as expected without the nondeterministic collation
> semantics.
>
> A table-based reproduction:
>
> CREATE COLLATION ignore_accents (
> provider = icu,
> locale = 'und-u-ks-level1',
> deterministic = false
> );
>
> CREATE TABLE like_test (val text);
> INSERT INTO like_test VALUES ('back\slash');
>
> SELECT val
> FROM like_test
> WHERE val COLLATE ignore_accents LIKE 'back\slash%' ESCAPE '#';
>
> Expected result:
> one row: back\slash
>
> Actual result:
> zero rows
>
> This seems to be caused by the unescape logic in like_match.c for
> nondeterministic collations, where a pattern fragment containing backslashes
> is copied incorrectly before calling pg_strncoll().
>
>
>
>