The Swing tutorial for the JTextPane and demo code is good and comprehensive, covering multiple ways to interact with the the StyledDocument and add Styles. So, without repeating all of that, here’s a minimal example including only simple style changes on the insert update and selection events.

In the first snippet below, the implementation for the insertUpdate event is shown. For each typed character or pasted text, the entire text of the document will be searched for the word “foo”. For each occurrence, the bold attribute is set for that word. Notice that the update of the attribute happens on a separate AWT thread.

  @Override
  public void insertUpdate(DocumentEvent event) {
    try {
      String text = doc.getText(0, doc.getLength());

      Pattern p = Pattern.compile(FOO);
      Matcher matcher = p.matcher(text);
      while (matcher.find()) {
        updateAttribute(matcher.start(), FOO.length(), Style.BOLD);
      }
    } catch (BadLocationException e) {
      e.printStackTrace();
    }
  }

  private void updateAttribute(int pos, int len, Style style) {
    SwingUtilities.invokeLater(() -> {
      doc.setCharacterAttributes(pos, len, style.get(), true);
    });
  }

The other functionality of this small application is on select. If the selected word is “bar”, it is set to italic. Notice the dot and mark positions, which might be at the start or the end of the selection. Furthermore, notice that here the predefined ItalicAction from the StyledEditorKit is used, since we’re dealing with a selection. We just have to translate the CaretEvent into an ActionEvent, or rather just make sure to forward the source component of the selection. (The alternative would have been to go with the plain update as in the example above).

  @Override
  public void caretUpdate(CaretEvent event) {
    int dot = event.getDot();
    int mark = event.getMark();
    int start;
    int end;

    if (dot == mark) {
      return;
    } else if (dot < mark) {
      start = dot;
      end = mark;
    } else {
      start = mark;
      end = dot;
    }

    System.out.println(start + ", " + end);
    try {
      if (doc.getText(start, BAR.length()).startsWith(BAR)) {
        ItalicAction action = new StyledEditorKit.ItalicAction();
        action.actionPerformed(new ActionEvent(event.getSource(), 0, ""));

        // Alternative custom update:
        // updateAttribute(start, BAR.length(), Style.ITALIC);
      }
    } catch (BadLocationException e) {
      e.printStackTrace();
    }
  }

Here’s the full file listing, as a stand-alone application.

JTextPaneStylesExample.java
GitHub Raw 
/* Copyright rememberjava.com. Licensed under GPL 3. See http://rememberjava.com/license */
package com.rememberjava.ui;

import java.awt.event.ActionEvent;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import javax.swing.JFrame;
import javax.swing.JTextPane;
import javax.swing.SwingUtilities;
import javax.swing.event.CaretEvent;
import javax.swing.event.CaretListener;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.MutableAttributeSet;
import javax.swing.text.SimpleAttributeSet;
import javax.swing.text.StyleConstants;
import javax.swing.text.StyledDocument;
import javax.swing.text.StyledEditorKit;
import javax.swing.text.StyledEditorKit.ItalicAction;

/**
 * Minimal example with JTextPane and StyledDocument. When the string "foo" is
 * typed or pasted it is made bold. When the string "bar" is selected it is made
 * italic.
 */
@SuppressWarnings("serial")
public class JTextPaneStylesExample extends JFrame implements DocumentListener, CaretListener {

  /**
   * Predefined styles.
   */
  enum Style {
    BOLD(StyleConstants.Bold),
    ITALIC(StyleConstants.Italic);

    private MutableAttributeSet attrib;

    private Style(Object style) {
      attrib = new SimpleAttributeSet();
      attrib.addAttribute(style, true);
    }

    AttributeSet get() {
      return attrib;
    }
  }

  private static final String FOO = "foo";

  private static final String BAR = "bar";

  private final StyledDocument doc;

  public static void main(String[] args) {
    new JTextPaneStylesExample();
  }

  public JTextPaneStylesExample() {
    JTextPane editor = new JTextPane();
    doc = editor.getStyledDocument();
    getContentPane().add(editor);

    setSize(500, 500);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setVisible(true);

    editor.getDocument().addDocumentListener(this);
    editor.addCaretListener(this);
  }

  /**
   * Check of a selection of the string "bar".
   */
  @Override
  public void caretUpdate(CaretEvent event) {
    int dot = event.getDot();
    int mark = event.getMark();
    int start;
    int end;

    if (dot == mark) {
      return;
    } else if (dot < mark) {
      start = dot;
      end = mark;
    } else {
      start = mark;
      end = dot;
    }

    System.out.println(start + ", " + end);
    try {
      if (doc.getText(start, BAR.length()).startsWith(BAR)) {
        ItalicAction action = new StyledEditorKit.ItalicAction();
        action.actionPerformed(new ActionEvent(event.getSource(), 0, ""));

        // Alternative custom update:
        // updateAttribute(start, BAR.length(), Style.ITALIC);
      }
    } catch (BadLocationException e) {
      e.printStackTrace();
    }
  }

  /**
   * Check for the occurrence of the string "foo".
   */
  @Override
  public void insertUpdate(DocumentEvent event) {
    try {
      String text = doc.getText(0, doc.getLength());

      Pattern p = Pattern.compile(FOO);
      Matcher matcher = p.matcher(text);
      while (matcher.find()) {
        updateAttribute(matcher.start(), FOO.length(), Style.BOLD);
      }
    } catch (BadLocationException e) {
      e.printStackTrace();
    }
  }

  /**
   * Update the string at the given position and length with the given style.
   * The update happens on a separate AWT thread, to avoid mutations of the
   * Document model while the event is processing.
   */
  private void updateAttribute(int pos, int len, Style style) {
    SwingUtilities.invokeLater(() -> {
      doc.setCharacterAttributes(pos, len, style.get(), true);
    });
  }

  @Override
  public void removeUpdate(DocumentEvent e) {}

  @Override
  public void changedUpdate(DocumentEvent e) {}
}