import jregex.*;
import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class Constrained extends Applet{
   Label lText=new Label("Enter a text:");
   Label lPatt=new Label("Enter a pattern:");
   ConstrainedTextField textInput=new ConstrainedTextField(20);
   TextField patternInput=new TextField(20);
   
   public void init(){
      setLayout(new GridLayout(2,2));
      add(lText);
      add(lPatt);
      add(textInput);
      add(patternInput);
      patternInput.addTextListener(new TextListener(){
         public void textValueChanged(TextEvent e){
            setPattern(patternInput.getText());
         }
      });
   }
   
   public void start(){
      patternInput.setText(getParameter("pattern"));
   }
   
   public void setPattern(String pattern){
      try{
         textInput.setPattern(new Pattern(pattern));
         textInput.setEnabled(true);
      }
      catch(PatternSyntaxException e){
         textInput.setEnabled(false);
      }
   }
}

class ConstrainedTextField extends TextField implements TextListener{
   private String lastAccepted;
   private Matcher matcher;
   
   public ConstrainedTextField(int columns){
      super(columns);
      addTextListener(this);
   }
   
   public void setPattern(Pattern p){
      matcher=p.matcher();
      lastAccepted=null;
      applyConstraint();
   }
   
   public void textValueChanged(TextEvent e){
      if(matcher==null) return; //no constraints
      applyConstraint();
   }
   
   private void applyConstraint(){
      String newValue=getText();
      
      matcher.setTarget(newValue);
      if(matcher.matchesPrefix()){
         //accept!
         lastAccepted=newValue;
      }
      else{
         //reject
         getToolkit().beep();
         removeTextListener(this); //avoid looping
         int cp=getCaretPosition();
         setText(lastAccepted);
         setCaretPosition(cp);
         addTextListener(this);
      }
   }
}