RINSTR
n = RINSTR ( [start], s1, s2 )
Returns the position of the last occurrence of string s2
in string s1 starting from the position start.
If there is no match, RINSTR returns 0
Example
s = "ab12ab34"
print rinstr(s, "ab")      ' Output: 5
print rinstr(6, s, "ab")   ' Output: 0