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 Unicode Latin Case Chart.

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 of the Unicode standard 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, *options.

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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