Creating an advanced enemy AI and i need to know how to make it check if a player entered range?

I don't know exactly, but I saw it being done. There is a thingy called magnitude that returns the distance from point A to point B, so you could have a script that is similar to the following one:

local target = nil --The AI's current target
local dis = 20 --The max distance

while true do wait(1)
	local cpos = script.Parent.Position --The AI's current position
	local nearest = math.huge() --Nearest distance from player to AI
	local players = game.Players:GetChildren()
	for _, player in ipairs(players) do
		local chara = player.Character
		if chara ~= nil then --Checks if player is alive
			local torso = chara:FindFirstChild('Torso') --Finds R6 torso
			if torso == nil then torso = chara:FindFirstChild('UpperTorso') end --Finds R15 torso
			local mag = (cpos - torso.Position).magnitude --Finds distance
			if mag < nearest and mag < dis  then --Checks if the player is nearer then the last one checked
				nearest = mag
				target = player --Sets target
			end
		end
	end
end

I wrote this really quickly, so I didn't test it. (May not work) Try altering this

Accept if helped