import javax.swing.*;import java.util.regex.*;public class MyDocument extends PlainDocument{ int maxLength; public MyDocument(int maxLength){ this.maxLength = maxLength; } public void insertString(int offset, String text, AttributeSet a){ //Regex, um zu überprüfen, ob der Text eine Zahl ist if(Pattern.matches("[0-9]*", text)){ if(this.getLength() + text.length() <= maxLength){ super.insertString(offset, text, a); } else{ super.insertString(offset, text.substring(0, maxLength - this.getLength()), a); } } }}