Returns the integer position of the _last_ substring that matches the given argument +pattern+,
or +nil+ if none found.

When +pattern+ is a string, returns the index of the last matching substring in self:

  'foo'.rindex('f')       # => 0
  'foo'.rindex('o')       # => 2
  'foo'.rindex('oo'       # => 1
  'foo'.rindex('ooo')     # => nil
  'тест'.rindex('т')      # => 3
  'こんにちは'.rindex('ち') # => 3

When +pattern+ is a Regexp, returns the index of the last match in self:

  'foo'.rindex(/f/)   # => 0
  'foo'.rindex(/o/)   # => 2
  'foo'.rindex(/oo/)  # => 1
  'foo'.rindex(/ooo/) # => nil

When +offset+ is non-negative, it specifies the maximum starting position in the
string to end the search:

  'foo'.rindex('o', 0) # => nil
  'foo'.rindex('o', 1) # => 1
  'foo'.rindex('o', 2) # => 2
  'foo'.rindex('o', 3) # => 2

With negative integer argument +offset+,
selects the search position by counting backward from the end of +self+:

  'foo'.rindex('o', -1) # => 2
  'foo'.rindex('o', -2) # => 1
  'foo'.rindex('o', -3) # => nil
  'foo'.rindex('o', -4) # => nil

The last match means starting at the possible last position, not
the last of longest matches:

  'foo'.rindex(/o+/) # => 2
  $~                 # => #<MatchData "o">

To get the last longest match, combine with negative lookbehind:

  'foo'.rindex(/(?<!o)o+/) # => 1
  $~                       # => #<MatchData "oo">

Or String#index with negative lookforward.

  'foo'.index(/o+(?!.*o)/) # => 1
  $~                       # => #<MatchData "oo">

Related: see {Querying}[rdoc-ref:String@Querying].