how to count occurence of one character in in a string

assume word is " aaeehjsfjhk" now i want to count the total occurence of a in this word how i will do
i tried:
occurence=0
for i=0 to len(word)
if instr(1,word,"a") then
occurence=occurence+1
end if
next

its not working, help me
thanks in adavance:)