Sunday, 29 May 2011

URL Searching using java

import java.awt.BorderLayout;
import java.awt.Cursor;
import java.awt.Font;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.io.BufferedReader;
import java.io.FileWriter;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JProgressBar;
import javax.swing.JScrollPane;
import javax.swing.JSeparator;
import javax.swing.JTable;
import javax.swing.JTextField;
import javax.swing.table.DefaultTableModel;

// The Search Web Crawler
public class SearchCrawler extends JFrame {
  // Max URLs drop-down values.
  private static final String[] MAX_URLS = { "50", "100", "500", "1000" };

  // Cache of robot disallow lists.
  private HashMap disallowListCache = new HashMap();

  // Search GUI controls.
  private JTextField startTextField;

  private JComboBox maxComboBox;

  private JCheckBox limitCheckBox;

  private JTextField logTextField;

  private JTextField searchTextField;

  private JCheckBox caseCheckBox;

  private JButton searchButton;

  // Search stats GUI controls.
  private JLabel crawlingLabel2;

  private JLabel crawledLabel2;

  private JLabel toCrawlLabel2;

  private JProgressBar progressBar;

  private JLabel matchesLabel2;

  // Table listing search matches.
  private JTable table;

  // Flag for whether or not crawling is underway.
  private boolean crawling;

  // Matches log file print writer.
  private PrintWriter logFileWriter;

  // Constructor for Search Web Crawler.
  public SearchCrawler() {
    // Set application title.
    setTitle("Search Crawler");

    // Set window size.
    setSize(600, 600);

    // Handle window closing events.
    addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent e) {
        actionExit();
      }
    });

    // Set up File menu.
    JMenuBar menuBar = new JMenuBar();
    JMenu fileMenu = new JMenu("File");
    fileMenu.setMnemonic(KeyEvent.VK_F);
    JMenuItem fileExitMenuItem = new JMenuItem("Exit", KeyEvent.VK_X);
    fileExitMenuItem.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        actionExit();
      }
    });
    fileMenu.add(fileExitMenuItem);
    menuBar.add(fileMenu);
    setJMenuBar(menuBar);

    // Set up search panel.
    JPanel searchPanel = new JPanel();
    GridBagConstraints constraints;
    GridBagLayout layout = new GridBagLayout();
    searchPanel.setLayout(layout);

    JLabel startLabel = new JLabel("Start URL:");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.EAST;
    constraints.insets = new Insets(5, 5, 0, 0);
    layout.setConstraints(startLabel, constraints);
    searchPanel.add(startLabel);

    startTextField = new JTextField();
    constraints = new GridBagConstraints();
    constraints.fill = GridBagConstraints.HORIZONTAL;
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    constraints.insets = new Insets(5, 5, 0, 5);
    layout.setConstraints(startTextField, constraints);
    searchPanel.add(startTextField);

    JLabel maxLabel = new JLabel("Max URLs to Crawl:");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.EAST;
    constraints.insets = new Insets(5, 5, 0, 0);
    layout.setConstraints(maxLabel, constraints);
    searchPanel.add(maxLabel);

    maxComboBox = new JComboBox(MAX_URLS);
    maxComboBox.setEditable(true);
    constraints = new GridBagConstraints();
    constraints.insets = new Insets(5, 5, 0, 0);
    layout.setConstraints(maxComboBox, constraints);
    searchPanel.add(maxComboBox);

    limitCheckBox = new JCheckBox("Limit crawling to Start URL site");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.WEST;
    constraints.insets = new Insets(0, 10, 0, 0);
    layout.setConstraints(limitCheckBox, constraints);
    searchPanel.add(limitCheckBox);

    JLabel blankLabel = new JLabel();
    constraints = new GridBagConstraints();
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    layout.setConstraints(blankLabel, constraints);
    searchPanel.add(blankLabel);

    JLabel logLabel = new JLabel("Matches Log File:");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.EAST;
    constraints.insets = new Insets(5, 5, 0, 0);
    layout.setConstraints(logLabel, constraints);
    searchPanel.add(logLabel);

    String file = System.getProperty("user.dir")
        + System.getProperty("file.separator") + "crawler.log";
    logTextField = new JTextField(file);
    constraints = new GridBagConstraints();
    constraints.fill = GridBagConstraints.HORIZONTAL;
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    constraints.insets = new Insets(5, 5, 0, 5);
    layout.setConstraints(logTextField, constraints);
    searchPanel.add(logTextField);

    JLabel searchLabel = new JLabel("Search String:");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.EAST;
    constraints.insets = new Insets(5, 5, 0, 0);
    layout.setConstraints(searchLabel, constraints);
    searchPanel.add(searchLabel);

    searchTextField = new JTextField();
    constraints = new GridBagConstraints();
    constraints.fill = GridBagConstraints.HORIZONTAL;
    constraints.insets = new Insets(5, 5, 0, 0);
    constraints.gridwidth = 2;
    constraints.weightx = 1.0d;
    layout.setConstraints(searchTextField, constraints);
    searchPanel.add(searchTextField);

    caseCheckBox = new JCheckBox("Case Sensitive");
    constraints = new GridBagConstraints();
    constraints.insets = new Insets(5, 5, 0, 5);
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    layout.setConstraints(caseCheckBox, constraints);
    searchPanel.add(caseCheckBox);

    searchButton = new JButton("Search");
    searchButton.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        actionSearch();
      }
    });
    constraints = new GridBagConstraints();
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    constraints.insets = new Insets(5, 5, 5, 5);
    layout.setConstraints(searchButton, constraints);
    searchPanel.add(searchButton);

    JSeparator separator = new JSeparator();
    constraints = new GridBagConstraints();
    constraints.fill = GridBagConstraints.HORIZONTAL;
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    constraints.insets = new Insets(5, 5, 5, 5);
    layout.setConstraints(separator, constraints);
    searchPanel.add(separator);

    JLabel crawlingLabel1 = new JLabel("Crawling:");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.EAST;
    constraints.insets = new Insets(5, 5, 0, 0);
    layout.setConstraints(crawlingLabel1, constraints);
    searchPanel.add(crawlingLabel1);

    crawlingLabel2 = new JLabel();
    crawlingLabel2.setFont(crawlingLabel2.getFont().deriveFont(Font.PLAIN));
    constraints = new GridBagConstraints();
    constraints.fill = GridBagConstraints.HORIZONTAL;
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    constraints.insets = new Insets(5, 5, 0, 5);
    layout.setConstraints(crawlingLabel2, constraints);
    searchPanel.add(crawlingLabel2);

    JLabel crawledLabel1 = new JLabel("Crawled URLs:");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.EAST;
    constraints.insets = new Insets(5, 5, 0, 0);
    layout.setConstraints(crawledLabel1, constraints);
    searchPanel.add(crawledLabel1);

    crawledLabel2 = new JLabel();
    crawledLabel2.setFont(crawledLabel2.getFont().deriveFont(Font.PLAIN));
    constraints = new GridBagConstraints();
    constraints.fill = GridBagConstraints.HORIZONTAL;
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    constraints.insets = new Insets(5, 5, 0, 5);
    layout.setConstraints(crawledLabel2, constraints);
    searchPanel.add(crawledLabel2);

    JLabel toCrawlLabel1 = new JLabel("URLs to Crawl:");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.EAST;
    constraints.insets = new Insets(5, 5, 0, 0);
    layout.setConstraints(toCrawlLabel1, constraints);
    searchPanel.add(toCrawlLabel1);

    toCrawlLabel2 = new JLabel();
    toCrawlLabel2.setFont(toCrawlLabel2.getFont().deriveFont(Font.PLAIN));
    constraints = new GridBagConstraints();
    constraints.fill = GridBagConstraints.HORIZONTAL;
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    constraints.insets = new Insets(5, 5, 0, 5);
    layout.setConstraints(toCrawlLabel2, constraints);
    searchPanel.add(toCrawlLabel2);

    JLabel progressLabel = new JLabel("Crawling Progress:");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.EAST;
    constraints.insets = new Insets(5, 5, 0, 0);
    layout.setConstraints(progressLabel, constraints);
    searchPanel.add(progressLabel);

    progressBar = new JProgressBar();
    progressBar.setMinimum(0);
    progressBar.setStringPainted(true);
    constraints = new GridBagConstraints();
    constraints.fill = GridBagConstraints.HORIZONTAL;
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    constraints.insets = new Insets(5, 5, 0, 5);
    layout.setConstraints(progressBar, constraints);
    searchPanel.add(progressBar);

    JLabel matchesLabel1 = new JLabel("Search Matches:");
    constraints = new GridBagConstraints();
    constraints.anchor = GridBagConstraints.EAST;
    constraints.insets = new Insets(5, 5, 10, 0);
    layout.setConstraints(matchesLabel1, constraints);
    searchPanel.add(matchesLabel1);
    matchesLabel2 = new JLabel();
    matchesLabel2.setFont(matchesLabel2.getFont().deriveFont(Font.PLAIN));
    constraints = new GridBagConstraints();
    constraints.fill = GridBagConstraints.HORIZONTAL;
    constraints.gridwidth = GridBagConstraints.REMAINDER;
    constraints.insets = new Insets(5, 5, 10, 5);
    layout.setConstraints(matchesLabel2, constraints);
    searchPanel.add(matchesLabel2);

    // Set up matches table.
    table = new JTable(new DefaultTableModel(new Object[][] {},
        new String[] { "URL" }) {
      public boolean isCellEditable(int row, int column) {
        return false;
      }
    });

    // Set up Matches panel.
    JPanel matchesPanel = new JPanel();
    matchesPanel.setBorder(BorderFactory.createTitledBorder("Matches"));
    matchesPanel.setLayout(new BorderLayout());
    matchesPanel.add(new JScrollPane(table), BorderLayout.CENTER);

    // Add panels to display.
    getContentPane().setLayout(new BorderLayout());
    getContentPane().add(searchPanel, BorderLayout.NORTH);
    getContentPane().add(matchesPanel, BorderLayout.CENTER);
  }

  // Exit this program.
  private void actionExit() {
    System.exit(0);
  }

  // Handle Search/Stop button being clicked.
  private void actionSearch() {
    // If stop button clicked, turn crawling flag off.
    if (crawling) {
      crawling = false;
      return;
    }

    ArrayList errorList = new ArrayList();

    // Validate that start URL has been entered.
    String startUrl = startTextField.getText().trim();
    if (startUrl.length() < 1) {
      errorList.add("Missing Start URL.");
    }
    // Verify start URL.
    else if (verifyUrl(startUrl) == null) {
      errorList.add("Invalid Start URL.");
    }

    // Validate that Max URLs is either empty or is a number.
    int maxUrls = 0;
    String max = ((String) maxComboBox.getSelectedItem()).trim();
    if (max.length() > 0) {
      try {
        maxUrls = Integer.parseInt(max);
      } catch (NumberFormatException e) {
      }
      if (maxUrls < 1) {
        errorList.add("Invalid Max URLs value.");
      }
    }

    // Validate that matches log file has been entered.
    String logFile = logTextField.getText().trim();
    if (logFile.length() < 1) {
      errorList.add("Missing Matches Log File.");
    }

    // Validate that search string has been entered.
    String searchString = searchTextField.getText().trim();
    if (searchString.length() < 1) {
      errorList.add("Missing Search String.");
    }

    // Show errors, if any, and return.
    if (errorList.size() > 0) {
      StringBuffer message = new StringBuffer();

      // Concatenate errors into single message.
      for (int i = 0; i < errorList.size(); i++) {
        message.append(errorList.get(i));
        if (i + 1 < errorList.size()) {
          message.append("\n");
        }
      }

      showError(message.toString());
      return;
    }

    // Remove "www" from start URL if present.
    startUrl = removeWwwFromUrl(startUrl);

    // Start the Search Crawler.
    search(logFile, startUrl, maxUrls, searchString);
  }

  private void search(final String logFile, final String startUrl,
      final int maxUrls, final String searchString) {
    // Start the search in a new thread.
    Thread thread = new Thread(new Runnable() {
      public void run() {
        // Show hour glass cursor while crawling is under way.
        setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR));

        // Disable search controls.
        startTextField.setEnabled(false);
        maxComboBox.setEnabled(false);
        limitCheckBox.setEnabled(false);
        logTextField.setEnabled(false);
        searchTextField.setEnabled(false);
        caseCheckBox.setEnabled(false);

        // Switch Search button to "Stop."
        searchButton.setText("Stop");

        // Reset stats.
        table.setModel(new DefaultTableModel(new Object[][] {},
            new String[] { "URL" }) {
          public boolean isCellEditable(int row, int column) {
            return false;
          }
        });
        updateStats(startUrl, 0, 0, maxUrls);

        // Open matches log file.
        try {
          logFileWriter = new PrintWriter(new FileWriter(logFile));
        } catch (Exception e) {
          showError("Unable to open matches log file.");
          return;
        }

        // Turn crawling flag on.
        crawling = true;

        // Perform the actual crawling.
        crawl(startUrl, maxUrls, limitCheckBox.isSelected(),
            searchString, caseCheckBox.isSelected());

        // Turn crawling flag off.
        crawling = false;

        // Close matches log file.
        try {
          logFileWriter.close();
        } catch (Exception e) {
          showError("Unable to close matches log file.");
        }

        // Mark search as done.
        crawlingLabel2.setText("Done");

        // Enable search controls.
        startTextField.setEnabled(true);
        maxComboBox.setEnabled(true);
        limitCheckBox.setEnabled(true);
        logTextField.setEnabled(true);
        searchTextField.setEnabled(true);
        caseCheckBox.setEnabled(true);

        // Switch search button back to "Search."
        searchButton.setText("Search");

        // Return to default cursor.
        setCursor(Cursor.getDefaultCursor());

        // Show message if search string not found.
        if (table.getRowCount() == 0) {
          JOptionPane
              .showMessageDialog(
                  SearchCrawler.this,
                  "Your Search String was not found. Please try another.",
                  "Search String Not Found",
                  JOptionPane.WARNING_MESSAGE);
        }
      }
    });
    thread.start();
  }

  // Show dialog box with error message.
  private void showError(String message) {
    JOptionPane.showMessageDialog(this, message, "Error",
        JOptionPane.ERROR_MESSAGE);
  }

  // Update crawling stats.
  private void updateStats(String crawling, int crawled, int toCrawl,
      int maxUrls) {
    crawlingLabel2.setText(crawling);
    crawledLabel2.setText("" + crawled);
    toCrawlLabel2.setText("" + toCrawl);

    // Update progress bar.
    if (maxUrls == -1) {
      progressBar.setMaximum(crawled + toCrawl);
    } else {
      progressBar.setMaximum(maxUrls);
    }
    progressBar.setValue(crawled);

    matchesLabel2.setText("" + table.getRowCount());
  }

  // Add match to matches table and log file.
  private void addMatch(String url) {
    // Add URL to matches table.
    DefaultTableModel model = (DefaultTableModel) table.getModel();
    model.addRow(new Object[] { url });

    // Add URL to matches log file.
    try {
      logFileWriter.println(url);
    } catch (Exception e) {
      showError("Unable to log match.");
    }
  }

  // Verify URL format.
  private URL verifyUrl(String url) {
    // Only allow HTTP URLs.
    if (!url.toLowerCase().startsWith("http://"))
      return null;

    // Verify format of URL.
    URL verifiedUrl = null;
    try {
      verifiedUrl = new URL(url);
    } catch (Exception e) {
      return null;
    }

    return verifiedUrl;
  }

  // Check if robot is allowed to access the given URL.
  private boolean isRobotAllowed(URL urlToCheck) {
    String host = urlToCheck.getHost().toLowerCase();

    // Retrieve host's disallow list from cache.
    ArrayList disallowList = (ArrayList) disallowListCache.get(host);

    // If list is not in the cache, download and cache it.
    if (disallowList == null) {
      disallowList = new ArrayList();

      try {
        URL robotsFileUrl = new URL("http://" + host + "/robots.txt");

        // Open connection to robot file URL for reading.
        BufferedReader reader = new BufferedReader(
            new InputStreamReader(robotsFileUrl.openStream()));

        // Read robot file, creating list of disallowed paths.
        String line;
        while ((line = reader.readLine()) != null) {
          if (line.indexOf("Disallow:") == 0) {
            String disallowPath = line.substring("Disallow:"
                .length());

            // Check disallow path for comments and remove if
            // present.
            int commentIndex = disallowPath.indexOf("#");
            if (commentIndex != -1) {
              disallowPath = disallowPath.substring(0,
                  commentIndex);
            }

            // Remove leading or trailing spaces from disallow path.
            disallowPath = disallowPath.trim();

            // Add disallow path to list.
            disallowList.add(disallowPath);
          }
        }

        // Add new disallow list to cache.
        disallowListCache.put(host, disallowList);
      } catch (Exception e) {
        /*
         * Assume robot is allowed since an exception is thrown if the
         * robot file doesn't exist.
         */
        return true;
      }
    }

    /*
     * Loop through disallow list to see if crawling is allowed for the
     * given URL.
     */
    String file = urlToCheck.getFile();
    for (int i = 0; i < disallowList.size(); i++) {
      String disallow = (String) disallowList.get(i);
      if (file.startsWith(disallow)) {
        return false;
      }
    }

    return true;
  }

  // Download page at given URL.
  private String downloadPage(URL pageUrl) {
    try {
      // Open connection to URL for reading.
      BufferedReader reader = new BufferedReader(new InputStreamReader(
          pageUrl.openStream()));

      // Read page into buffer.
      String line;
      StringBuffer pageBuffer = new StringBuffer();
      while ((line = reader.readLine()) != null) {
        pageBuffer.append(line);
      }

      return pageBuffer.toString();
    } catch (Exception e) {
    }

    return null;
  }

  // Remove leading "www" from a URL's host if present.
  private String removeWwwFromUrl(String url) {
    int index = url.indexOf("://www.");
    if (index != -1) {
      return url.substring(0, index + 3) + url.substring(index + 7);
    }

    return (url);
  }

  // Parse through page contents and retrieve links.
  private ArrayList retrieveLinks(URL pageUrl, String pageContents,
      HashSet crawledList, boolean limitHost) {
    // Compile link matching pattern.
    Pattern p = Pattern.compile("<a\\s+href\\s*=\\s*\"?(.*?)[\" |>]",
        Pattern.CASE_INSENSITIVE);
    Matcher m = p.matcher(pageContents);

    // Create list of link matches.
    ArrayList linkList = new ArrayList();
    while (m.find()) {
      String link = m.group(1).trim();

      // Skip empty links.
      if (link.length() < 1) {
        continue;
      }

      // Skip links that are just page anchors.
      if (link.charAt(0) == '#') {
        continue;
      }

      // Skip mailto links.
      if (link.indexOf("mailto:") != -1) {
        continue;
      }

      // Skip JavaScript links.
      if (link.toLowerCase().indexOf("javascript") != -1) {
        continue;
      }

      // Prefix absolute and relative URLs if necessary.
      if (link.indexOf("://") == -1) {
        // Handle absolute URLs.
        if (link.charAt(0) == '/') {
          link = "http://" + pageUrl.getHost() + link;
          // Handle relative URLs.
        } else {
          String file = pageUrl.getFile();
          if (file.indexOf('/') == -1) {
            link = "http://" + pageUrl.getHost() + "/" + link;
          } else {
            String path = file.substring(0,
                file.lastIndexOf('/') + 1);
            link = "http://" + pageUrl.getHost() + path + link;
          }
        }
      }

      // Remove anchors from link.
      int index = link.indexOf('#');
      if (index != -1) {
        link = link.substring(0, index);
      }

      // Remove leading "www" from URL's host if present.
      link = removeWwwFromUrl(link);

      // Verify link and skip if invalid.
      URL verifiedLink = verifyUrl(link);
      if (verifiedLink == null) {
        continue;
      }

      /*
       * If specified, limit links to those having the same host as the
       * start URL.
       */
      if (limitHost
          && !pageUrl.getHost().toLowerCase().equals(
              verifiedLink.getHost().toLowerCase())) {
        continue;
      }

      // Skip link if it has already been crawled.
      if (crawledList.contains(link)) {
        continue;
      }

      // Add link to list.
      linkList.add(link);
    }

    return (linkList);
  }

  /*
   * Determine whether or not search string is matched in the given page
   * contents.
   */
  private boolean searchStringMatches(String pageContents,
      String searchString, boolean caseSensitive) {
    String searchContents = pageContents;

    /*
     * If case-sensitive search, lowercase page contents for comparison.
     */
    if (!caseSensitive) {
      searchContents = pageContents.toLowerCase();
    }
    // Split search string into individual terms.
    Pattern p = Pattern.compile("[\\s]+");
    String[] terms = p.split(searchString);

    // Check to see if each term matches.
    for (int i = 0; i < terms.length; i++) {
      if (caseSensitive) {
        if (searchContents.indexOf(terms[i]) == -1) {
          return false;
        }
      } else {
        if (searchContents.indexOf(terms[i].toLowerCase()) == -1) {
          return false;
        }
      }
    }

    return true;
  }

  // Perform the actual crawling, searching for the search string.
  public void crawl(String startUrl, int maxUrls, boolean limitHost,
      String searchString, boolean caseSensitive) {
    // Set up crawl lists.
    HashSet crawledList = new HashSet();
    LinkedHashSet toCrawlList = new LinkedHashSet();

    // Add start URL to the to crawl list.
    toCrawlList.add(startUrl);

    /*
     * Perform actual crawling by looping through the To Crawl list.
     */
    while (crawling && toCrawlList.size() > 0) {
      /*
       * Check to see if the max URL count has been reached, if it was
       * specified.
       */
      if (maxUrls != -1) {
        if (crawledList.size() == maxUrls) {
          break;
        }
      }

      // Get URL at bottom of the list.
      String url = (String) toCrawlList.iterator().next();

      // Remove URL from the To Crawl list.
      toCrawlList.remove(url);

      // Convert string url to URL object.
      URL verifiedUrl = verifyUrl(url);

      // Skip URL if robots are not allowed to access it.
      if (!isRobotAllowed(verifiedUrl)) {
        continue;
      }

      // Update crawling stats.
      updateStats(url, crawledList.size(), toCrawlList.size(), maxUrls);

      // Add page to the crawled list.
      crawledList.add(url);

      // Download the page at the given URL.
      String pageContents = downloadPage(verifiedUrl);

      /*
       * If the page was downloaded successfully, retrieve all its links
       * and then see if it contains the search string.
       */
      if (pageContents != null && pageContents.length() > 0) {
        // Retrieve list of valid links from page.
        ArrayList links = retrieveLinks(verifiedUrl, pageContents,
            crawledList, limitHost);

        // Add links to the To Crawl list.
        toCrawlList.addAll(links);

        /*
         * Check if search string is present in page, and if so, record
         * a match.
         */
        if (searchStringMatches(pageContents, searchString,
            caseSensitive)) {
          addMatch(url);
        }
      }

      // Update crawling stats.
      updateStats(url, crawledList.size(), toCrawlList.size(), maxUrls);
    }
  }

  // Run the Search Crawler.
  public static void main(String[] args) {
    SearchCrawler crawler = new SearchCrawler();
    crawler.show();
  }
}
/**
A quantifier determines how many times an expression is matched. The quantifiers are shown here:
+   Match one or more.
*   Match zero or more.
?   Match zero or one.

*/

/*
Character Sequence Explanation

<a Look for the characters "<a".

\\s+ Look for one or more space characters.

href Look for the characters "href".

\\s* Look for zero or more space characters.

=    Look for the character "--".

\\s* Look for zero or more space characters.

\"?  Look for zero or one quote character.

(.*?)Look for zero or more of any character until the next part of the pattern is matched, and place the results in a group.

[\">]Look for quote character or greater than (">") character.

*/
 
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