What's The Simplest Way To Add Commas To Numbers > 999?

So I have this code:

function addCommas(str)
    return #str % 3 == 0 and str:reverse():gsub("(%d%d%d)", "%1,"):reverse():sub(2) or str:reverse():gsub("(%d%d%d)", "%1,"):reverse()
end

But, that's way to complex for me. I need a way to more simply insert commas into numbers above 999.