Re: Character expansion with ICU collations

Re:
>> Can a CI collation be ordered upper case first, or is this a limitation of ICU?

> I don't know the authoritative answer to that, but to me it doesn't make
> sense, since the effect of a case-insensitive collation is to throw away
> the third-level weights, so there is nothing left for "upper case first"
> to operate on.

It wouldn't make sense for the ICU sort key of a CI collation itself because the sort keys need to be binary equal, but what the collation of interest does is equivalent to adding a secondary "C"-collated expression to the ORDER BY clause. For example:

SELECT ... ORDER BY expr COLLATE ci_as;

Is ordered as if the query had been written:

SELECT ... ORDER BY expr COLLATE ci_as, expr COLLATE "C";

Re:
> tailoring rules
>> yes

It looks like the relevant API call is ucol_openRules(),
Interface documented here: https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/ucol_8h.html
example usage from C here: https://android.googlesource.com/platform/external/icu/+/db20b09/source/test/cintltst/citertst.c

for example:

/* Test with an expanding character sequence */
u_uastrcpy(rule, "&a < b < c/abd < d");
c2 = ucol_openRules(rule, u_strlen(rule), UCOL_OFF, UCOL_DEFAULT_STRENGTH, NULL, &status);

and a reordering rule test:

u_uastrcpy(rule, "&z < AB");
coll = ucol_openRules(rule, u_strlen(rule), UCOL_OFF, UCOL_DEFAULT_STRENGTH, NULL, &status);

that looks encouraging. It returns a UCollator object, like ucol_open(const char *localeString, ...), so it's an alternative to ucol_open(). One of the parameters is the equivalent of colStrength, so then the question would be, how are the other keyword/value pairs like colCaseFirst, colAlternate, etc. specified via the rules argument? In the same way with the exception of colStrength?

e.g. is "colAlternate=shifted;&z < AB" a valid rules string?

The ICU documentation says simply:

" rules A string describing the collation rules. For the syntax of the rules please see users guide."

Transform rules are documented here: http://userguide.icu-project.org/transforms/general/rules

But there are no examples of using the keyword/value pairs that may appear in a locale string with the transform rules, and there's no locale argument on ucol_openRules. How can the keyword/value pairs that may appear in the locale string be applied in combination with tailoring rules (with the exception of colStrength)?