Identify Integers and replace digits with X

So, I am working on a program that replaces every digit entered into cin with an x but if it's something like john17 then don't (so basically, if the word is an integer, replace the digits, else, keep the word as-is). I am struggling to find a solution here. How do I break up the words in the line entered and test whether it's an integer?

So to add some clarification...it's a line of text like: "My userID is john17 and my 4 digit pin is 1234 which is secret", so I want to keep john17 there but replace 4 with x and 1234 with all x's

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

// Our variables
string str;
string result;

//Function Prototypes
void replace_num_with_x(string str);

int main() {

    cout<<"Please enter a line of text:";
    getline(cin, str);
    replace_num_with_x(str);
    cout<<result;

}

//Function Definition
void replace_num_with_x(string str) {
    stringstream str_strm;
    str_strm << str; //Convert string to stringstream

    for(int i = 0; i <= str.length() ; i++) {
        if((char)str[i] >=48 && (char)str[i] <= 57) {
            result +='x';
        }
        else {
            result +=str[i];
        }
    }
}