regex match and special characters

Hi,

Here is a simple SQL statement that gives different results on PostgreSQL 9.6 and PostgreSQL 10+. The space character at the end of the string is actually U+2006 SIX-PER-EM SPACE (http://www.fileformat.info/info/unicode/char/2006/index.htm)

test=# select 'abcd ' ~ 'abcd\s';
?column?
----------
t
(1 row)

test=# select version();
version
-------------------------------------------------------------------------------------------------
PostgreSQL 12devel on x86_64-pc-linux-gnu, compiled by gcc (Gentoo 6.4.0-r1 p1.3) 6.4.0, 64-bit
(1 row)

On another server (running on the same system on a different port)

postgres=# select version();
version
-----------------------------------------------------------------------------------------------
PostgreSQL 9.6.9 on x86_64-pc-linux-gnu, compiled by gcc (Gentoo 6.4.0-r1 p1.3) 6.4.0, 64-bit
(1 row)

postgres=# select 'abcd ' ~ 'abcd\s';
?column?
----------
f
(1 row)

For both clusters, the client encoding is UTF8, the database encoding and collation is UTF8 and en_US.utf8 respectively, and the lc_ctype is en_US.utf8. I am accessing the databases running locally by ssh-ing first to the host.

I observed similar issues with other Linux-based servers running Ubuntu, in all cases the regex resulted in true on PostgreSQL 10+ and false on earlier versions (down to 9.3). The query comes from a table check that suddenly stopped accepting rows valid in the older version during the migration. Making it select 'abcd ' ~ E'abcd\\s' doesn't modify the outcome, unsurprisingly.

Is it reproducible for others here as well? Given that it is, Is there a way to make both versions behave the same?

Cheers,
Alex