NumberCheck.java

package com;

import java.awt.*;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import javax.swing.*;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;

public class NumberCheck extends JPanel {
    private static final Pattern PATTERN = Pattern.compile("^(?:\\d\\s?){10}$|^(?:\\d\\s?){14}$");

    private JTextField txtNumberInput;
    private JLabel errorMsg;

    public NumberCheck() {
        // construct the components
        errorMsg = new JLabel("Invalid input");
        errorMsg.setForeground(Color.RED);

        txtNumberInput = new JTextField(20);

        // initialize the listeners
        txtNumberInput.addKeyListener(new KeyAdapter() {
            @Override
            public void keyTyped(KeyEvent e) {
                char c = e.getKeyChar();
                if ((!(Character.isDigit(c))) && (c != ' ') && (c != '\b')) {
                    e.consume();
                }
            }
        });

        txtNumberInput.getDocument().addDocumentListener(new DocumentListener() {
            @Override
            public void removeUpdate(DocumentEvent e) {
                validateInput();
            }

            @Override
            public void insertUpdate(DocumentEvent e) {
                validateInput();
            }

            @Override
            public void changedUpdate(DocumentEvent e) {
            } // Not needed for plain-text fields
        });

        // Add the components to the panel
        this.setLayout(new FlowLayout());
        this.add(txtNumberInput);
        this.add(errorMsg);
    }

    private void validateInput() {
        String text = txtNumberInput.getText();
        Matcher m = PATTERN.matcher(text);
        if (m.matches()) {
            errorMsg.setForeground(errorMsg.getBackground());
        }
        else {
            errorMsg.setForeground(Color.RED);
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JFrame frame = new JFrame();
            frame.setContentPane(new NumberCheck());
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.pack();
            frame.setVisible(true);
        });
    }
}