How can I validate the username in a textbook field in Java?

So I wanted to validate the username (length should be between 6 to 30 characters, and numbers should not be allowed) that the user entered before the game proceeds, and I searched the internet and found out about the regex expressions. I am not used to it so I have found some problems. The code should do the following: if the user didn't enter a username or if it contained characters not allowed when he clicked the "start game" button, I wanted a dialog box to pop up and tell him to try again. And if everything was correct the game should proceed as normal, but when I run this code the username is not checked and it enters the game despite what is entered in the text box, and below is what I have so far.

import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import java.awt.Font;
import javax.swing.JTextField;
import javax.swing.KeyStroke;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.ActionEvent;
import java.util.regex.*;
public class StartScreen extends JFrame {
private JPanel contentPane;
private JTextField textbox_name;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
StartScreen frame = new StartScreen();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the frame.
*/
public static boolean isValidUsername(String name) 
{ 
String regex = "^[a-zA-Z]{5,29}$";  //Removed ^[aA-zZ]\\\\w{5,29}$
Pattern p = Pattern.compile(regex);     
if (name == null) { 
return false; 
}   
Matcher m = p.matcher(name);    
return m.matches(); 
} 
public StartScreen() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 455, 191);
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
setContentPane(contentPane);
contentPane.setLayout(null);
JLabel lblEnterName = new JLabel("Enter Name:");
lblEnterName.setFont(new Font("Lucida Grande", Font.PLAIN, 15));
lblEnterName.setBounds(91, 48, 102, 29);
contentPane.add(lblEnterName);
textbox_name = new JTextField();
textbox_name.setBounds(205, 49, 130, 28);
contentPane.add(textbox_name);
textbox_name.setColumns(10);
String field;
field = textbox_name.getText();
JButton btnStartGame = new JButton("Start Game");
btnStartGame.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
if(isValidUsername(field)) {
MainScreen window = new MainScreen();
window.setVisible(true);
dispose();
}
else {
JOptionPane.showMessageDialog(null, "Please enter your username correctly");
}
}
});
btnStartGame.setBounds(150, 102, 117, 29);
contentPane.add(btnStartGame);
}

}

I tried your isValidUsername as a standalone method and it didn't give valid results. It was giving false even for valid names. I changed it to the following and it worked well:

public static boolean isValidUsername(String name) 
{ 
String regex = "^[a-zA-Z]{5,29}$";	//Removed ^[aA-zZ]\\\\w{5,29}$
Pattern p = Pattern.compile(regex); 	
if (name == null) { 
return false; 
} 	
Matcher m = p.matcher(name); 	
return m.matches(); 
} 

Also, since the code is being executed after the button btnStartGame has been clicked, its not therefore possible that the button is disabled (A disabled button doesn't fire events). For that reason, I think the check if(btnStartGame.isEnabled()) is redundant (I may be wrong). You can hence just check:

if(sValidUsername(field)){
//start game here
} else {
//display prompt here
}

These two lines from the code you posted in your question:

String field;
field = textbox_name.getText();

need to be inside method actionPerformed() because you want to get the contents of textbox_name after the user clicks on the button btnStartGame.

Here is my rewrite of the code you posted.

import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JLabel;
import javax.swing.JOptionPane;

import java.awt.Font;
import javax.swing.JTextField;
import javax.swing.KeyStroke;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.ActionEvent;
import java.util.regex.*;

public class StartScreen extends JFrame {

private JPanel contentPane;
private JTextField textbox_name;

/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                StartScreen frame = new StartScreen();
                frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

/**
 * Create the frame.
 */

public static boolean isValidUsername(String name) 
{ 
    String regex = "^[a-zA-Z]{5,29}$";  //Removed ^[aA-zZ]\\\\w{5,29}$
    Pattern p = Pattern.compile(regex);     
    if (name == null) { 
        return false; 
    }   
    Matcher m = p.matcher(name);    
    return m.matches(); 
 } 

public StartScreen() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 455, 191);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    contentPane.setLayout(null);
    
    JLabel lblEnterName = new JLabel("Enter Name:");
    lblEnterName.setFont(new Font("Lucida Grande", Font.PLAIN, 15));
    lblEnterName.setBounds(91, 48, 102, 29);
    contentPane.add(lblEnterName);
    
    textbox_name = new JTextField();
    textbox_name.setBounds(205, 49, 130, 28);
    contentPane.add(textbox_name);
    textbox_name.setColumns(10);
    
    JButton btnStartGame = new JButton("Start Game");
    btnStartGame.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            String field;  // added this line
            field = textbox_name.getText();  // added this line
            
            if(isValidUsername(field)) {
                MainScreen window = new MainScreen();
                window.setVisible(true);
                dispose();
            }
            else {
                JOptionPane.showMessageDialog(null, "Please enter your username correctly");
            }
        }
    });
    
    btnStartGame.setBounds(150, 102, 117, 29);
    contentPane.add(btnStartGame);
}

By the way, you should try to use layout managers.