--- rindex(pattern[, pos])

    ʬʸõü麸ü˸äƹԤޤĤäʬʸ
    κüΰ֤֤ޤĤʤ nil ֤ޤ

     pattern ˤõʬʸλʸʸ
    ɤ򼨤 0  255 ɽΤ줫ǻꤷޤ

    pos Ϳ줿ˤϤΰ֤õޤpos ξ
    άͤ self.size (ü)Ǥ

        p "astrochemistry".rindex("str")        # => 10
        p "character".rindex(?c)                # => 5
        p "regexprindex".rindex(/e.*x/, 2)      # => 1

    pos ξ硢ʸ֤õޤ

        p "foobarfoobar".rindex("bar", 6)       # => 3
        p "foobarfoobar".rindex("bar", -6)      # => 3

    index ȴ˺ȿžư򤹤櫓ǤϤ
    õϤγϰ֤򱦤麸ˤ餷ʤԤޤʬʸ
    ξȹϺ鱦˸äƹԤޤʲ򻲾ȤƤ

        # String#index ξ
        p "foobar".index("bar", 2)    # => 3
        #    bar   <- õԤ
        #     bar  <- ˤ餷ƤǸĤ

        # String#rindex ξ
        p "foobar".rindex("bar", -2)  # => 3
        #    bar <- (ü 2 )ǤϤʤ
        #      bar <- õԤ(ü2)
        #     bar  <- ˤ餷ƤǸĤ

