How can I do this?

I'm trying to figure out how I can find a certain word in a sentence, and then return the position of the first letter in the string.

I can't figure out how to loop from the first letter to the last in W while still looking at S

S = String W = Word My problem is on line 7 of this script

function GetWordPosition(S,W)
    S=S:lower()
    W=W:lower()
    local Pos=nil
    print(S,W)
    for i=1,#S do
        if S:sub(i,i)==W:sub() then --Right here
            local Found=true
            local b=false
            for k=i,i+#W do
                if not b then
                    if not S:sub(k,k)==W:sub(k,k) then
                        Found=false
                        b=true
                    end
                end
            end
            if Found then
                Pos=i
                break
            end
        end
    end
    return Pos
end