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== Case Mapping
Some string-oriented methods use case mapping.
In String:
- String#capitalize
- String#capitalize!
- String#casecmp
- String#casecmp?
- String#downcase
- String#downcase!
- String#swapcase
- String#swapcase!
- String#upcase
- String#upcase!
In Symbol:
- Symbol#capitalize
- Symbol#casecmp
- Symbol#casecmp?
- Symbol#downcase
- Symbol#swapcase
- Symbol#upcase
=== Default Case Mapping
By default, all of these methods use full Unicode case mapping,
which is suitable for most languages.
See {Section 3.13 (Default Case Algorithms) of the Unicode standard}[https://www.unicode.org/versions/latest/ch03.pdf].
Non-ASCII case mapping and folding are supported for UTF-8,
UTF-16BE/LE, UTF-32BE/LE, and ISO-8859-1~16 Strings/Symbols.
Context-dependent case mapping as described in
{Table 3-17 (Context Specification for Casing) of the Unicode standard}[https://www.unicode.org/versions/latest/ch03.pdf]
is currently not supported.
In most cases, case conversions of a string have the same number of characters.
There are exceptions (see also +:fold+ below):
s = "\u00DF" # => "ß"
s.upcase # => "SS"
s = "\u0149" # => "ʼn"
s.upcase # => "ʼN"
Case mapping may also depend on locale (see also +:turkic+ below):
s = "\u0049" # => "I"
s.downcase # => "i" # Dot above.
s.downcase(:turkic) # => "ı" # No dot above.
Case changes may not be reversible:
s = 'Hello World!' # => "Hello World!"
s.downcase # => "hello world!"
s.downcase.upcase # => "HELLO WORLD!" # Different from original s.
Case changing methods may not maintain Unicode normalization.
See String#unicode_normalize).
=== Options for Case Mapping
Except for +casecmp+ and +casecmp?+,
each of the case-mapping methods listed above
accepts optional arguments, <tt>*options</tt>.
The arguments may be:
- +:ascii+ only.
- +:fold+ only.
- +:turkic+ or +:lithuanian+ or both.
The options:
- +:ascii+:
ASCII-only mapping:
uppercase letters ('A'..'Z') are mapped to lowercase letters ('a'..'z);
other characters are not changed
s = "Foo \u00D8 \u00F8 Bar" # => "Foo Ø ø Bar"
s.upcase # => "FOO Ø Ø BAR"
s.downcase # => "foo ø ø bar"
s.upcase(:ascii) # => "FOO Ø ø BAR"
s.downcase(:ascii) # => "foo Ø ø bar"
- +:turkic+:
Full Unicode case mapping, adapted for the Turkic languages
that distinguish dotted and dotless I, for example Turkish and Azeri.
s = 'Türkiye' # => "Türkiye"
s.upcase # => "TÜRKIYE"
s.upcase(:turkic) # => "TÜRKİYE" # Dot above.
s = 'TÜRKIYE' # => "TÜRKIYE"
s.downcase # => "türkiye"
s.downcase(:turkic) # => "türkıye" # No dot above.
- +:lithuanian+:
Not yet implemented.
- +:fold+ (available only for String#downcase, String#downcase!,
and Symbol#downcase):
Unicode case folding,
which is more far-reaching than Unicode case mapping.
s = "\u00DF" # => "ß"
s.downcase # => "ß"
s.downcase(:fold) # => "ss"
s.upcase # => "SS"
s = "\uFB04" # => "ffl"
s.downcase # => "ffl"
s.upcase # => "FFL"
s.downcase(:fold) # => "ffl"
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