reverse find

["Brian Weed" <bw@...>]

Is there a library function for a reverse find, or do I have to do it 
manually as in:

-- untested
i = 0
do
   i = i - 1
   p = string.find(s, ".", i, true)
while (p == nil and i ~= -string.len(s))

(I'm searching for the file extension, and there may be other dots in 
the filename)

Brian