Issue #18013 has been updated by jirkamarsik (Jirka Marsik).


This is a simpler reproducer.
```
irb(main):003:0> /[^a]/i.match("a")
=> nil
irb(main):004:0> /[[^a]]/i.match("a")
=> #<MatchData "a">
```

----------------------------------------
Bug #18013: Unexpected results when mxiing negated character classes and case-folding
https://bugs.ruby-lang.org/issues/18013#change-92689

* Author: jirkamarsik (Jirka Marsik)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-linux]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
```
irb(main):001:0> /[^a-c]/i.match("A")
=> nil
irb(main):002:0> /[[^a-c]]/i.match("A")
=> #<MatchData "A">
```

The two regular expressions above match different strings, because the character classes denote different sets of characters. In order for `/[^a-c]/i` to produce correct results, Oniguruma provided a fix that can still be easily seen in the code as it is hidden behind an always-on preprocessor flag (`CASE_FOLD_IS_APPLIED_INSIDE_NEGATIVE_CCLASS`, https://github.com/ruby/ruby/blob/9eae8cdefba61e9e51feb30a4b98525593169666/regparse.c#L5528). The idea of the fix is to first case-fold a character class and only then apply the negation (essentially moving the case-fold operator *inside* the negation).

In the case of our first regular expression, `[a-c]` is case-folded into `[a-cA-C]` and that is then inverted into `[^a-cA-C]`, which is the expected result. However, this case-folding logic is currently only being applied to the top-most character class and so if we use a nested negated character class, the order of the operations will be switched.

With our second regular expression, `[a-c]` will first be negated to yield `[^a-c]`, which will then be case-folded into `.`, the set of all characters (since `[^a-c]` contains `A-C`, which case-fold into `a-c`).

A way to fix this would be to apply case-folding for nested character classes as well, so that the nested character classes behave the same as the top-most character class. Then, we would get the same semantics for both expressions.



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