public  void init() 
{
          // Results of matching operation
        // Add components using the dreaded GridBagLayout
        GridBagLayout gb = new GridBagLayout();
        setLayout(gb);
        GridBagConstraints c = new GridBagConstraints();
        c.insets = new Insets(5, 5, 5, 5);
        c.anchor = GridBagConstraints.EAST;
        gb.setConstraints(add(new Label("Regular expression:", Label.RIGHT)), c);
        c.gridy = 0;
        c.anchor = GridBagConstraints.WEST;
        gb.setConstraints(add(fieldRE = new TextField("\\[([:javastart:][:javapart:]*)\\]", 40)), c);
        c.gridx = 0;
        c.gridy = GridBagConstraints.RELATIVE;
        c.anchor = GridBagConstraints.EAST;
        gb.setConstraints(add(new Label("String:", Label.RIGHT)), c);
        c.gridy = 1;
        c.gridx = GridBagConstraints.RELATIVE;
        c.anchor = GridBagConstraints.WEST;
        gb.setConstraints(add(fieldMatch = new TextField("aaa([foo])aaa", 40)), c);
        c.gridy = 2;
        c.gridx = GridBagConstraints.RELATIVE;
        c.fill = GridBagConstraints.BOTH;
        c.weighty = 1.0;
        c.weightx = 1.0;
        gb.setConstraints(add(outRE = new TextArea()), c);
        c.gridy = 2;
        c.gridx = GridBagConstraints.RELATIVE;
        gb.setConstraints(add(outMatch = new TextArea()), c);
        // Listen to text changes
        fieldRE.addTextListener(this);
        fieldMatch.addTextListener(this);
        // Initial UI update
        textValueChanged(null);
}

Add controls and init applet

 public static  void main(String[] arg) 
{
        Frame f = new Frame("RE Demo");
        // f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.addWindowListener(new WindowAdapter()
        {
            public void windowClosing(WindowEvent e)
            {
                System.exit(0);
            }
        });
        REDemo demo = new REDemo();
        f.add(demo);
        demo.init();
        f.pack();
        f.setVisible(true);
}

Main application entrypoint.

  void sayMatch(String s) 
{
        outMatch.setText(s);
}

Say something into match text area

  void sayRE(String s) 
{
        outRE.setText(s);
}

Say something into RE text area

 public  void textValueChanged(TextEvent e) 
{
        // If it's a generic update or the regexp changed...
        if (e == null || e.getSource() == fieldRE)
        {
            // Update regexp
            updateRE(fieldRE.getText());
        }
        // We always need to update the match results
        updateMatch(fieldMatch.getText());
}

Called when text values change

 String throwableToString(Throwable t) 
{
        String s = t.getClass().getName();
        String m;
        if ((m = t.getMessage()) != null)
        {
            s += "\n" + m;
        }
        return s;
}

Convert throwable to string

  void updateMatch(String match) 
{
        try
        {
            // If the string matches the regexp
            if (r.match(match))
            {
                // Say that it matches
                String out = "Matches.\n\n";
                // Show contents of parenthesized subexpressions
                for (int i = 0; i <  r.getParenCount(); i++)
                {
                    out += "$" + i + " = " + r.getParen(i) + "\n";
                }
                sayMatch(out);
            }
            else
            {
                // Didn't match!
                sayMatch("Does not match");
            }
        }
        catch (Throwable t)
        {
            sayMatch(throwableToString(t));
        }
}

Update matching info by matching the string against the current compiled regular expression.

  void updateRE(String expr) 
{
        try
        {
            // Compile program
            r.setProgram(compiler.compile(expr));
            // Dump program into RE feedback area
            CharArrayWriter w = new CharArrayWriter();
            compiler.dumpProgram(new PrintWriter(w));
            sayRE(w.toString());
            System.out.println(w);
        }
        catch (Exception e)
        {
            r.setProgram(null);
            sayRE(throwableToString(e));
        }
        catch (Throwable t)
        {
            r.setProgram(null);
            sayRE(throwableToString(t));
        }
}

Change regular expression