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);
    }

}