Negation in nested character classes doesn’t work for case-insensitive regexes

[piotrrzysko]

It seems that when using Joni as a regex library, negation in nested character classes is not respected in case-insensitive mode. The following examples demonstrate the issue:

-- Returns true
SELECT regexp_like('q', '(?i)[[^Q]]');
-- Returns false
SELECT regexp_like('q', '(?i)[^Q]');
-- Returns true
SELECT regexp_like('q', '(?i)[A-Z&&[^q]]');
-- Returns false
SELECT regexp_like('q', '(?i)[A-Z&&[^Q]]');

In comparison, in Java (which according to the documentation is the regex syntax Trino follows), all of the above cases return false.

This discrepancy appears to occur because Joni applies case folding only to the top-level character class: link. It doesn’t do so when it encounters a nested character class while recursively parsing the top-level class: link.