LASTPOS(needle,haystack[,position])

This function finds the last occurrence, if any, of the needle within
haystack. The answer is the position of the last occurrence if there is
one (numbering from 1 to the length of haystack), or zero if needle does
not occur in haystack. If the optional position is supplied, the search
starts from this position instead of the end of haystack.  If the
needle is the empty string, zero is returned.