How to remove all characters from a string, except the integers?

[BoQsc via Digitalmars-d-learn]

I need to check if a string contains integers,
and if it contains integers, remove all the regular string 
characters.
I've looked around and it seems using regex is the only closest 
solution.

```
import std.stdio;

void main(string[] args){

        if (args.length > 1){
        
                write(args[1]); // Needs to print only integers.
                
        } else {
                write("Please write an argument.");
        }
}
```