[DOC] Enhanced RDoc for string slices (#5769)

Creates file doc/string/slices.rdoc that the string slicing methods can link to.

1 files changed, 138 insertions, 0 deletions

diff --git a/doc/string/slices.rdoc b/doc/string/slices.rdoc
new file mode 100644
index 0000000000..4689f30cc1
--- /dev/null
+++ b/doc/string/slices.rdoc
@@ -0,0 +1,138 @@
+== \String Slices
+
+A _slice_ of a string is a substring that is selected by certain criteria.
+
+These instance methods make use of slicing:
+
+- String#[] (also aliased as String#slice) returns a slice copied from +self+.
+- String#[]= returns a copy of +self+ with a slice replaced.
+- String#slice! returns +self+ with a slice removed.
+
+Each of the above methods takes arguments that determine the slice
+to be copied or replaced.
+
+The arguments have several forms.
+For string +string+,  the forms are:
+
+- <tt>string[index]</tt>.
+- <tt>string[start, length]</tt>.
+- <tt>string[range]</tt>.
+- <tt>string[regexp, capture = 0]</tt>.
+- <tt>string[substring]</tt>.
+
+<b><tt>string[index]</tt></b>
+
+When non-negative integer argument +index+ is given,
+the slice is the 1-character substring found in +self+ at character offset +index+:
+
+  'bar'[0]       # => "b"
+  'bar'[2]       # => "r"
+  'bar'[20]      # => nil
+  'тест'[2]      # => "с"
+  'こんにちは'[4]  # => "は"
+
+When negative integer +index+ is given,
+the slice begins at the offset given by counting backward from the end of +self+:
+
+  'bar'[-3]         # => "b"
+  'bar'[-1]         # => "r"
+  'bar'[-20]        # => nil
+
+<b><tt>string[start, length]</tt></b>
+
+When non-negative integer arguments +start+ and +length+ are given,
+the slice begins at character offset +start+, if it exists,
+and continues for +length+ characters, if available:
+
+  'foo'[0, 2]       # => "fo"
+  'тест'[1, 2]      # => "ес"
+  'こんにちは'[2, 2]  # => "にち"
+  # Zero length.
+  'foo'[2, 0]       # => ""
+  # Length not entirely available.
+  'foo'[1, 200]     # => "oo"
+  # Start out of range.
+  'foo'[4, 2]      # => nil
+
+Special case: if +start+ is equal to the length of +self+,
+the slice is a new empty string:
+
+  'foo'[3, 2]   # => ""
+  'foo'[3, 200] # => ""
+
+When negative +start+ and non-negative +length+ are given,
+the slice beginning is determined by counting backward from the end of +self+,
+and the slice continues for +length+ characters, if available:
+
+  'foo'[-2, 2]    # => "oo"
+  'foo'[-2, 200]  # => "oo"
+  # Start out of range.
+  'foo'[-4, 2]     # => nil
+
+When negative +length+ is given, there is no slice:
+
+  'foo'[1, -1]  # => nil
+  'foo'[-2, -1] # => nil
+
+<b><tt>string[range]</tt></b>
+
+When Range argument +range+ is given,
+creates a substring of +string+ using the indices in +range+.
+The slice is then determined as above:
+
+  'foo'[0..1]    # => "fo"
+  'foo'[0, 2]    # => "fo"
+
+  'foo'[2...2]   # => ""
+  'foo'[2, 0]    # => ""
+
+  'foo'[1..200]  # => "oo"
+  'foo'[1, 200]  # => "oo"
+
+  'foo'[4..5]    # => nil
+  'foo'[4, 2]    # => nil
+
+  'foo'[-4..-3]  # => nil
+  'foo'[-4, 2]   # => nil
+
+  'foo'[3..4]    # => ""
+  'foo'[3, 2]    # => ""
+
+  'foo'[-2..-1]  # => "oo"
+  'foo'[-2, 2]   # => "oo"
+
+  'foo'[-2..197] # => "oo"
+  'foo'[-2, 200] # => "oo"
+
+<b><tt>string[regexp, capture = 0]</tt></b>
+
+When the \Regexp argument +regexp+ is given,
+and the +capture+ argument is <tt>0</tt>,
+the slice is the first matching substring found in +self+:
+
+  'foo'[/o/] # => "o"
+  'foo'[/x/] # => nil
+  s = 'hello there'
+  s[/[aeiou](.)\1/] # => "ell"
+  s[/[aeiou](.)\1/, 0] # => "ell"
+
+If argument +capture+ is given and not <tt>0</tt>,
+it should be either an capture group index (integer)
+or a capture group name (string or symbol);
+the slice is the specified capture (see Regexp@Capturing):
+
+  s = 'hello there'
+  s[/[aeiou](.)\1/, 1] # => "l"
+  s[/(?<vowel>[aeiou])(?<non_vowel>[^aeiou])/, "non_vowel"] # => "l"
+  s[/(?<vowel>[aeiou])(?<non_vowel>[^aeiou])/, :vowel] # => "e"
+
+If an invalid capture group index is given, there is no slice.
+If an invalid capture group name is given, +IndexError+ is raised.
+
+<b><tt>string[substring]</tt></b>
+
+When the single \String argument +substring+ is given,
+returns the substring from +self+ if found, otherwise +nil+:
+
+  'foo'['oo'] # => "oo"
+  'foo'['xx'] # => nil