How do I limit the number of digits in a user input?

Pattern p = Pattern.compile("^[0-9]{0,3}$");
Matcher m = p.matcher(in);
if (m.find()) {
    JOptionPane.showMessageDialog(null, " 4 integers please");
}

It is a button that adds numbers

Trying to create exception that limits amount of numbers in dialog , it detects if the number is within the limit but does not stop the program.

You can simply use in.matches("\\d{4}") as the condition and add to the textarea only if this condition is true.

import javax.swing.JOptionPane;

public class Main {
    public static void main(String[] args) {
        String in = JOptionPane.showInputDialog("Number: ");
        if (in.matches("\\d{4}")) {
            // ...Code to add the value to the textarea
        } else {
            JOptionPane.showMessageDialog(null, "Only 4 digits please.");
        }
    }
}