From: "mame (Yusuke Endoh)" <noreply@...>
Date: 2022-02-21T03:34:03+00:00
Subject: [ruby-core:107677] [Ruby master Bug#18590] String#downcase and CAPITAL LETTER I WITH DOT ABOVE

Issue #18590 has been updated by mame (Yusuke Endoh).

Assignee set to duerst (Martin D��rst)
Status changed from Open to Assigned

The document of Unicode case folding (http://www.unicode.org/Public/UCD/latest/ucd/CaseFolding.txt) says:

```
0130; F; 0069 0307; # LATIN CAPITAL LETTER I WITH DOT ABOVE
0130; T; 0069; # LATIN CAPITAL LETTER I WITH DOT ABOVE
```

"F" is for "full case folding", and "T" is for "Turkic languages".

String#downcase uses full Unicode case mapping by default (See https://docs.ruby-lang.org/en/3.0/String.html#method-i-downcase). You can get the result you expected by `:turkic` option.

```
'��'.downcase(:turkic).chars
=> ["i"]
```



----------------------------------------
Bug #18590: String#downcase and CAPITAL LETTER I WITH DOT ABOVE
https://bugs.ruby-lang.org/issues/18590#change-96596

* 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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