import javax.swing.text.*;import java.util.regex.*;public class RegexConstrainedDocument extends PlainDocument {    Pattern pattern;    Matcher matcher;        public RegexConstrainedDocument () { super(); }    public RegexConstrainedDocument (AbstractDocument.Content c) { super(c); }    public RegexConstrainedDocument (AbstractDocument.Content c, String p) {        super (c);        setPatternByString (p);    }    public RegexConstrainedDocument (String p) {        super();        setPatternByString (p);    }    public void setPatternByString (String p) {        Pattern pattern = Pattern.compile (p);        // check the document against the new pattern        // and removes the content if it no longer matches        try {            matcher = pattern.matcher (getText(0, getLength()));            System.out.println ("matcher reset to " +                                 getText (0, getLength()));            if (! matcher.matches()) {                System.out.println ("does not match");                remove (0, getLength());            }        } catch (BadLocationException ble) {            ble.printStackTrace(); // impossible?        }    }    public Pattern getPattern() { return pattern; }    public void insertString (int offs, String s, AttributeSet a)        throws BadLocationException {        // consider whether this insert will match        String proposedInsert =            getText (0, offs) +            s +            getText (offs, getLength() - offs);        System.out.println ("proposing to change to: " +                            proposedInsert);        if (matcher != null) {            matcher.reset (proposedInsert);            System.out.println ("matcher reset");            if (! matcher.matches()) {                System.out.println ("insert doesn't match");                return;            }        }        super.insertString (offs, s, a);    }}