Case Mapping

Some string-oriented methods use case mapping.

In String:

In Symbol:

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.

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 is currently not supported.

In most cases, the case conversion of a string has the same number of characters as before. 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.

Case Mappings

Except for casecmp and casecmp?, each of the case-mapping methods listed above accepts an optional argument, mapping.

The argument is one of: