Re: pg_collation.collversion for C.UTF-8

	I wrote:

> Consider matching '\d' in a regexp. With C.UTF-8 (glibc-2.35), we
> only match ASCII characters 0-9, or 10 codepoints.  With
> "en-US-u-va-posix-x-icu" we match 660 codepoints comprising all the
> digit characters in all languages, plus a bunch of variants for
> mathematical symbols.

BTW this not specifically a C.UTF-8 versus "en-US-u-va-posix-x-icu"
difference.
If think that any glibc-based locale will consider that \d
in a regexp means [0-9], and that any ICU locale
will make \d match a much larger variety of characters.

While moving to ICU by default, we should expect that 
differences like that will affect apps in a way that might be
more or less disruptive.

Another known difference it that upper() with ICU does not do a
character-by-character conversion, for instance:

WITH words(w) as  (values('muß'),('final'))
 SELECT
  w,
  length(w),
  upper(w collate "C.utf8") as "upper (libc)",
  length(upper(w collate "C.utf8")),
  upper(w collate "en-x-icu") as "upper (ICU)",
  length(upper(w collate "en-x-icu"))
FROM words;

  w   | length | upper libc | length | upper ICU | length 
------+--------+------------+--------+-----------+--------
 muß  |      3 | MUß	    |	   3 | MUSS	 |	4
 final |      4 | fiNAL	    |	   4 | FINAL	 |	5


The fact that the resulting string is larger that the original
might cause problems.

In general, we can't abstract from the fact that ICU semantics
are different.


Best regards,
-- 
Daniel Vérité
https://postgresql.verite.pro/
Twitter: @DanielVerite