Making a staring eye with limited rotation?

Here's the illustration

So I used CFrame.new(the eye's position, the torso's position) to make the eye 'stare' at the player but I want the eyes to have limits, if you stand behind the eyes they won't be able to look at you. So how?

You could try something like this:

local min = -90
local max = 90

while wait() do
	local cf = CFrame.new(eye.Position,torso.Position)
	local angles = {cf:toEulerAnglesXYZ()}
	local newAngles = {}
	for i,v in pairs(angles) do
		if math.deg(v) < min or math.deg(v) > max then
			newAngles = {0,0,0}
			break
		else
			newAngles[i] = v
		end
	end
	eye.CFrame = CFrame.new(eye.Position) * CFrame.Angles(unpack(newAngles))
end	

Not sure if toEulerAngles returns radians or degrees, so you might have to use math.rad on the new angles, but something like this might work.