Checking if Keyboard input was an integer

Hey guys, I've been trying to find a way to detect if someone inputs an integer from the keyboard or not.

Basically I want the console to only print the integers 0-9 corresponding to the "Alpha" keys.

So far I have gotten to here:

 string userInput;
 bool userValue;
  
 void Update () {
        
        if (Input.anyKeyDown) {
         OnGUI ();
         }
 }
  
 void OnGUI () {
         Event e = Event.current;
         if (e.isKey){
             string key = e.keyCode.ToString();
             Debug.Log(key);
         }
 }

Now I realize you aren't technically supposed to use OnGUI but I have tried literally every other way and the console will print empty strings. Don't know why and no one on the forums really had an answer either.

This is what I have tried to convert the string into an integer and print it to console:

 string userInput;
 int userNumber;
 bool res = true;
 
 void IntegerInput (){
     
         userInput = Input.inputString;
         
         bool res = int.TryParse(userInput, out userNumber);
         
         Debug.Log(res);    
         Debug.Log(userInput);
         Debug.Log(userNumber);    
 }

However it never prints anything when I put it under the anyKeyDown in my if statement under the Update().

If anyone knows a better way to go about this or has some guidance that would be great. I've spent a good number of hours just trying to do this because I am trying to learn how to optimize code rather than rely on tons of if statements to get what I want done.

In the end I am trying to get the user to choose a number by typing it out and if they try to type anything other than an integer the console will warn them that they must give positive integers.