Simple javascript prompt calculator

Question

I am trying to make a simple calculator which calculates two numbers on js. It has 4 operations - "+","-","*" and "/". But the problem is that in current implementation it can operates only with one sign numbers))) What's the best way to solve this?

var express = prompt("Enter your operation, for example 2+2",""); 
if(!express) alert("Ok, it's over!");


express = express.replace(/\s/g, '');
var number1 = +express.charAt(0);
var sign = express.charAt(1);
var number2 = +express.charAt(2);

switch(sign){
case "+" : alert(number1 + number2);  break;
case "-" : alert(number1 - number2);  break;
case "*" : alert(number1 * number2);  break;
case "/" : alert(number1 / number2);  break;
 default : alert("Not a correct operation");
}

Answer 1

This works for integers. I think you can make it work for floats as well:

var expression = "-30--20";

match = expression.match(/^(-?\d+)([-+*/])(-?\d+)$/);

if (match === null) {
    alert("invalid expression");
    return;
}

var number1 = match[1];
var operand = match[2];
var number2 = match[3];

switch(operand){
    case "+" : alert(number1 + number2);  break;
    case "-" : alert(number1 - number2);  break;
    case "*" : alert(number1 * number2);  break;
    case "/" : alert(number1 / number2);  break;
    default : alert("Not a correct operation");
}

Gives -10.