A question about a single-line calculator in java?

so, I was trying to solve a question about creating a single line calculator on code forces that takes the input in just one line as following "1+2" and outputs the answer 3
my code works fine with single digits
but when it comes to double digits as the following :
"7+54" gives "12" and that indicates that the compiler does not recognize "4" so why is that happening ?

also, I want to note that the accepted format of the calculator on code forces is for example ‘1+2’ not "1 + 2" that is why I used this line of codeString inputWithSpaces = input.replaceAll(".(?=.)", "$0 "); that separates between the string characters with spaces. I did that because if i wrote "1+2"no spaces
this gave me error

import java.util.*;
 
public class Main {
  public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
      
 String input = in.nextLine();
        String inputWithSpaces = input.replaceAll(".(?=.)", "$0 "); // this code gives spaces to string input to be accepted in parseINT
            String sum[] =inputWithSpaces .split(" ");
            /*  i took the whole expression in one single string line then i splitted it and
stored it in an array called sum , then by parseInt i was able to identify the type of each element in the arrays indexes
i couldnot identify the operator as char as i cant convert string into char so , i used char O=operator.charAt(0) to conv the string into
char
 
             */
 
            long num1 = Integer.parseInt(sum[0]);
            String operator = sum[1];
            long num2 = Integer.parseInt(sum[2]);
 
            char O = operator.charAt(0);  
            switch (O) {
                case '+':
                    System.out.println(num1 + num2);
                    break;
                case '-':
                    System.out.println(num1 - num2);
                    break;
                case '*':
                    System.out.println(num1 * num2);
                    break;
                default:
                    System.out.println(num1 / num2);
                    break;
            }
  }
} 

>Solution :

Issue

the compiler does not recognize "4"

Not at all, only your code is a fault sum[2] takes one char, the 5, not 54. If you have given 12+15 it wouldn’t have worked at all.

Solution

The minimal change would be the regex \D for non-digit. It would put spaces around the operator

input.replaceAll("\\D", " $0 ")

"15+50"   >>   ["15", "+", "50"]

The best would be to directly split the 3 parts

String input = "15+50";
String[] sum = input.split("(?=\\D)|(?<=\\D)");

long num1 = Integer.parseInt(sum[0]);
String operator = sum[1];
long num2 = Integer.parseInt(sum[2]);

switch (operator.charAt(0)) {
    case '+' -> System.out.println(num1 + num2);
    case '-' -> System.out.println(num1 - num2);
    case '*' -> System.out.println(num1 * num2);
    default -> System.out.println(num1 / num2);
}