Why are my string variables not being pushed in my stacks?

Why are my string variables not being pushed in my stacks?

import java.util.*;
public class Pemdas {
public static double Express(String str)
{
Stack<Double> num = new Stack<Double>();
Stack<String> op = new Stack<String>();
String number = "[0-9]*"; // any digit from 0-9
for (int i = 0; i < str.length(); i++)
{
if (str.substring(i,i+1).equals(number))
num.push(Double.parseDouble(str.substring(i, i+1)));
else if (str.substring(i, i+1).equals("+"))
op.push(str.substring(i, i +1));
System.out.println(str);
}
double n = num.pop();
if (op.pop().equals("+"))
n = n + num.pop();
return n;
}
public static void main(String[] args)
{
System.out.print("Enter an Expression: ");
String ex = StdIn.readString(); // This is where I enter my string input
System.out.println(Express(ex));
}
}
Let's say that I have an String variable of "5 + 5" as my input. In the
for loop, the 5 is supposed to be pushed into the num stack, but I keep
getting an ESE and I don't understand why.