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+Returns a new string containing the upcased characters in +self+:
+
+  'hello'.upcase        # => "HELLO"
+  'straße'.upcase       # => "STRASSE"
+  'привет'.upcase       # => "ПРИВЕТ"
+  'RubyGems.org'.upcase # => "RUBYGEMS.ORG"
+
+The sizes of +self+ and the upcased result may differ:
+
+   s = 'Straße'
+   s.size        # => 6
+   s.upcase      # => "STRASSE"
+   s.upcase.size # => 7
+
+Some characters (and some character sets) do not have upcase and downcase versions;
+see {Case Mapping}[rdoc-ref:case_mapping.rdoc]:
+
+   s = '1, 2, 3, ...'
+   s.upcase == s # => true
+   s = 'こんにちは'
+   s.upcase == s # => true
+
+The casing is affected by the given +mapping+,
+which may be +:ascii+, +:fold+, or +:turkic+;
+see {Case Mappings}[rdoc-ref:case_mapping.rdoc@Case+Mappings].
+
+Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].