Actually, it returns the position of the rightmost occurrence of a substring rather than a character. Though, you could use it to check for a substring that is only one character.

       rindex STR,SUBSTR,POSITION
       rindex STR,SUBSTR
               Works just like index() except that it returns the
               position of the LAST occurrence of SUBSTR in STR.
               If POSITION is specified, returns the last occur­
               rence at or before that position.

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