From type/Cool

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multi sub rindex(Cool:D $s, Cool:D $needle --> Int:D)
multi sub rindex(Cool:D $s, Cool:D $needle, Cool:D $pos --> Int:D)
multi method rindex(Cool:D: Cool:D $needle --> Int:D)
multi method rindex(Cool:D: Cool:D $needle, Cool:D $pos --> Int:D)

Coerces the first two arguments (including the invocant in method form) to Str and $pos to Int, and returns the last position of $needle in the string not after $pos. Returns Nil if $needle wasn't found.

See the documentation in type Str for examples.

From type/Str

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multi method rindex(Str:D: Str:D $needle --> Int:D)
multi method rindex(Str:D: Str:D $needle, Int:D $pos --> Int:D)
multi method rindex(Str:D: @needles --> Int:D)

Returns the last position of $needle in the string not after $pos. Returns Nil if $needle wasn't found.

Since Rakudo version 2020.05, rindex accepts a list of needles to search the string with, and return the highest index found or Nil.

Examples:

say "aardvark".rindex: "a";       # OUTPUT: «5␤» 
say "aardvark".rindex: "a", 0;    # OUTPUT: «0␤ 
say "aardvark".rindex: "t";       # OUTPUT: «Nil␤» 
say "aardvark".rindex: <d v k>;   # OUTPUT: «7␤»

Other forms of rindex, including subs, are inherited from Cool.