-- a, b is our "buffer" for last 2 chars
pairs = {}
function p(m, s)
  if s ~= nil then
    -- fill buffer
    for i = 1, s:len() do
      a = b
      b = s:sub(i)
      if a ~= nil then
        pairs[a] = b
      end
    end
  end
  return pairs[b]
end

m = max chars to return
s = fed "real" input