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+Returns +self+ with all, a substring, or none of its contents replaced;
+returns the argument +other_string+.
+
+<b>Form <tt>self[index] = other_string</tt></b>
+
+With non-negative integer argument +index+ given,
+searches for the 1-character substring found in self at character offset index:
+
+  s = 'hello'
+  s[0] = 'foo' # => "foo"
+  s            # => "fooello"
+
+  s = 'hello'
+  s[4] = 'foo' # => "foo"
+  s            # => "hellfoo"
+
+  s = 'hello'
+  s[5] = 'foo' # => "foo"
+  s            # => "hellofoo"
+
+  s = 'hello'
+  s[6] = 'foo' # Raises IndexError: index 6 out of string.
+
+With negative integer argument +index+ given,
+counts backward from the end of +self+:
+
+  s = 'hello'
+  s[-1] = 'foo'  # => "foo"
+  s              # => "hellfoo"
+
+  s = 'hello'
+  s[-5] = 'foo'  # => "foo"
+  s              # => "fooello"
+
+  s = 'hello'
+  s[-6] = 'foo'  # Raises IndexError: index -6 out of string.
+
+<b>Form <tt>self[start, length] = other_string</tt></b>
+
+With integer arguments +start+ and +length+ given,
+searches for a substring of size +length+ characters (as available)
+beginning at character offset specified by +start+.
+
+If argument +start+ is non-negative,
+the offset is +start+:
+
+  s = 'hello'
+  s[0, 1] = 'foo'  # => "foo"
+  s                # => "fooello"
+
+  s = 'hello'
+  s[0, 5] = 'foo'  # => "foo"
+  s                # => "foo"
+
+  s = 'hello'
+  s[0, 9] = 'foo'  # => "foo"
+  s                # => "foo"
+
+  s = 'hello'
+  s[2, 0] = 'foo'  # => "foo"
+  s                # => "hefoollo"
+
+  s = 'hello'
+  s[2, -1] = 'foo' # Raises IndexError: negative length -1.
+
+If argument +start+ is negative,
+counts backward from the end of +self+:
+
+  s = 'hello'
+  s[-1, 1] = 'foo' # => "foo"
+  s                # => "hellfoo"
+
+  s = 'hello'
+  s[-1, 9] = 'foo' # => "foo"
+  s                # => "hellfoo"
+
+  s = 'hello'
+  s[-5, 2] = 'foo' # => "foo"
+  s                # => "foollo"
+
+  s = 'hello'
+  s[-3, 0] = 'foo' # => "foo"
+  s                # => "hefoollo"
+
+  s = 'hello'
+  s[-6, 2] = 'foo' # Raises IndexError: index -6 out of string.
+
+Special case: if +start+ equals the length of +self+,
+the argument is appended to +self+:
+
+  s = 'hello'
+  s[5, 3] = 'foo' # => "foo"
+  s               # => "hellofoo"
+
+<b>Form <tt>self[range] = other_string</tt></b>
+
+With Range argument +range+ given,
+equivalent to <tt>self[range.start, range.size] = other_string</tt>:
+
+  s0 = 'hello'
+  s1 = 'hello'
+  s0[0..2] = 'foo' # => "foo"
+  s1[0, 3] = 'foo' # => "foo"
+  s0               # => "foolo"
+  s1               # => "foolo"
+
+  s = 'hello'
+  s[0...2] = 'foo' # => "foo"
+  s                # => "foollo"
+
+  s = 'hello'
+  s[0...0] = 'foo' # => "foo"
+  s                # => "foohello"
+
+  s = 'hello'
+  s[9..10] = 'foo' # Raises RangeError: 9..10 out of range
+
+<b>Form <tt>self[regexp, capture = 0] = other_string</tt></b>
+
+With Regexp argument +regexp+ given and +capture+ as zero,
+searches for a matching substring in +self+;
+updates {Regexp-related global variables}[rdoc-ref:Regexp@Global+Variables]:
+
+  s = 'hello'
+  s[/l/] = 'L'       # => "L"
+  [$`, $&, $']       # => ["he", "l", "lo"]
+  s[/eLlo/] = 'owdy' # => "owdy"
+  [$`, $&, $']       # => ["h", "eLlo", ""]
+  s[/eLlo/] = 'owdy' # Raises IndexError: regexp not matched.
+  [$`, $&, $']       # => [nil, nil, nil]
+
+With +capture+ as a positive integer +n+,
+searches for the +n+th matched group:
+
+  s = 'hello'
+  s[/(h)(e)(l+)(o)/] = 'foo'    # => "foo"
+  [$`, $&, $']                  # => ["", "hello", ""]
+
+  s = 'hello'
+  s[/(h)(e)(l+)(o)/, 1] = 'foo' # => "foo"
+  s                             # => "fooello"
+  [$`, $&, $']                  # => ["", "hello", ""]
+
+  s = 'hello'
+  s[/(h)(e)(l+)(o)/, 2] = 'foo' # => "foo"
+  s                             # => "hfoollo"
+  [$`, $&, $']                  # => ["", "hello", ""]
+
+  s = 'hello'
+  s[/(h)(e)(l+)(o)/, 4] = 'foo' # => "foo"
+  s                             # => "hellfoo"
+  [$`, $&, $']                  # => ["", "hello", ""]
+
+  s = 'hello'
+  # => "hello"
+  s[/(h)(e)(l+)(o)/, 5] = 'foo  # Raises IndexError: index 5 out of regexp.
+
+  s = 'hello'
+  s[/nosuch/] = 'foo'           # Raises IndexError: regexp not matched.
+
+<b>Form <tt>self[substring] = other_string</tt></b>
+
+With string argument +substring+ given:
+
+  s = 'hello'
+  s['l'] = 'foo'  # => "foo"
+  s  # => "hefoolo"
+
+  s = 'hello'
+  s['ll'] = 'foo'  # => "foo"
+  s  # => "hefooo"
+
+  s = 'こんにちは'
+  s['んにち'] = 'foo'  # => "foo"
+  s  # => "こfooは"
+
+  s['nosuch'] = 'foo' # Raises IndexError: string not matched.
+
+Related: see {Modifying}[rdoc-ref:String@Modifying].