Como fazer um web crawler

import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import org.apache.commons.httpclient.HttpClient;
import org.apache.commons.httpclient.NameValuePair;
import org.apache.commons.httpclient.methods.PostMethod;

public class Combo {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		try {
			
			String card = "Torture";
			
			String url = "http://www.essentialmagic.com//combos/search.asp";
			HttpClient client = new HttpClient();
			PostMethod method = new PostMethod(url);

			List params = new ArrayList();

			params.add(new NameValuePair("txtName", ""));
			params.add(new NameValuePair("txtCreator", ""));
			params.add(new NameValuePair("cnbtxtCard", card));
			params.add(new NameValuePair("cnbhdnCard", "5"));
			params.add(new NameValuePair("selDateCompare", "BETWEEN"));
			params.add(new NameValuePair("txtDate", ""));
			params.add(new NameValuePair("selFormat", "3"));
			params.add(new NameValuePair("selThemes", "0"));
			params.add(new NameValuePair("selRatingCompare", ">="));
			params.add(new NameValuePair("selRatings", "0"));
			params.add(new NameValuePair("selNumRatingsCompare", ">="));
			params.add(new NameValuePair("selNumRatings", "-1"));
			params.add(new NameValuePair("selRarities", "0"));
			params.add(new NameValuePair("selInvalid", "0"));
			params.add(new NameValuePair("chkColor0", "on"));
			params.add(new NameValuePair("chkColor1", "on"));
			params.add(new NameValuePair("chkColor2", "on"));
			params.add(new NameValuePair("chkColor3", "on"));
			params.add(new NameValuePair("chkColor4", "on"));
			params.add(new NameValuePair("chkColor5", "on"));
			params.add(new NameValuePair("selColorCompare", "OR"));
			params.add(new NameValuePair("btnSearch", "Begin+Search"));

			method.setRequestBody((NameValuePair[]) params.toArray(new NameValuePair[params.size()]));

			client.executeMethod(method);

			method = new PostMethod("http://www.essentialmagic.com/combos/SearchResults.asp");
			client.executeMethod(method);

			String page = method.getResponseBodyAsString();

			//<div class="Label" style="font-size: 11pt; font-style: italic;">Grim Harvest</div>
			
			Pattern comboLinkpattern = Pattern.compile("(<[^>]*?div[^>]*?(?:Label)[^>]*>)((?:.*?(?:<[ \r\t]*div[^>]*>?.*?(?:<.*?/.*?div.*?>)?)*)*)(<[^>]*?/[^>]*?div[^>]*?>)");
			Matcher comboLinkmatcher = comboLinkpattern.matcher(page);
			
			List comboList = new ArrayList();
			List comboIndexList = new ArrayList();
			
			while(comboLinkmatcher.find()){
				Pattern comboStartLinkPattern = Pattern.compile("<[^>]*?div[^>]*?(?:Label)[^>]*>");
				Pattern comboEndLinkPattern = Pattern.compile("</div>");
				Matcher comboStartLinkmatcher = comboStartLinkPattern.matcher(page.substring(comboLinkmatcher.start(), comboLinkmatcher.end()));
				comboStartLinkmatcher.find();
				int start = comboStartLinkmatcher.end();
				comboStartLinkmatcher = comboEndLinkPattern.matcher(page.substring(comboLinkmatcher.start(), comboLinkmatcher.end()));
				comboStartLinkmatcher.find();
				int end = comboStartLinkmatcher.start();
				
				comboList.add(page.substring(comboLinkmatcher.start(), comboLinkmatcher.end()).substring(start,end));
				comboIndexList.add(new Integer(comboLinkmatcher.start()));
				
				//System.out.println(page.substring(comboLinkmatcher.start(), comboLinkmatcher.end()).substring(start,end));
			}
			
			
			Pattern cardLinkPattern = Pattern.compile("(<[^>]*?a[^>]*?(?:hidePopup)[^>]*>)((?:.*?(?:<[ \r\t]*a[^>]*>?.*?(?:<.*?/.*?a.*?>)?)*)*)(<[^>]*?/[^>]*?a[^>]*?>)");
			Matcher cardLinkmatcher = cardLinkPattern.matcher(page);

			int combo = 0;
			
			while (cardLinkmatcher.find()) {
				
				if((combo < comboIndexList.size()) && (combo==0 || cardLinkmatcher.start()>((Integer)comboIndexList.get(combo)).intValue())){
					
					System.out.println("\n\n>>"+comboList.get(combo));
					combo++;
				}
				
				Pattern cardStartLinkPattern = Pattern.compile("<[^>]*?a[^>]*?(?:hidePopup)[^>]*>");
				Pattern cardEndLinkPattern = Pattern.compile("</a>");
				Matcher cardStartLinkmatcher = cardStartLinkPattern.matcher(page.substring(cardLinkmatcher.start(), cardLinkmatcher.end()));
				cardStartLinkmatcher.find();
				int start = cardStartLinkmatcher.end();
				cardStartLinkmatcher = cardEndLinkPattern.matcher(page.substring(cardLinkmatcher.start(), cardLinkmatcher.end()));
				cardStartLinkmatcher.find();
				int end = cardStartLinkmatcher.start();
				
				System.out.println(page.substring(cardLinkmatcher.start(), cardLinkmatcher.end()).substring(start,end));
			}

		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}