From: "andrykonchin (Andrew Konchin)" <noreply@...>
Date: 2022-02-22T23:15:36+00:00
Subject: [ruby-core:107727] [Ruby master Bug#18590] String#downcase and CAPITAL LETTER I WITH DOT ABOVE

Issue #18590 has been updated by andrykonchin (Andrew Konchin).


Thank you for the suggestion.

I am wondering whether `String#downcase` (when called without arguments) follows only Unicode case mapping rules (as stated in the [documentation]). Or also the folding ones.

I would expect that a call of `String#downcase` without arguments uses the one-to-one case mapping rules, that are specified in the [UnicodeData.txt] file.

[documentation]: https://ruby-doc.org/core-3.0.0/String.html#method-i-downcase
[UnicodeData.txt]: https://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt

----------------------------------------
Bug #18590: String#downcase and CAPITAL LETTER I WITH DOT ABOVE
https://bugs.ruby-lang.org/issues/18590#change-96650

* Author: andrykonchin (Andrew Konchin)
* Status: Assigned
* Priority: Normal
* Assignee: duerst (Martin D��rst)
* ruby -v: 3.1.0p0
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
Downcasing for "��" character works in an unexpected way:

```ruby
'��'.downcase
=> "i��"
```

Expected result - downcasing should return "i". Instead, it returns small "i" and additional "dot" character:

```ruby
'��'.downcase.chars
=> ["i", "��"]
```

According to the standard Unicode case mapping character '��'(0130) maps to lowercased 'i' (0069).

```
0130;LATIN CAPITAL LETTER I WITH DOT ABOVE;Lu;0;L;0049 0307;;;;N;LATIN CAPITAL LETTER I DOT;;;0069;
```

https://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt




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