之前做聊天室时,由于在聊天室中提供了新闻阅读的功能,写了一个从网页中抓取信息(如最新的头条新闻,新闻的来源,标题,内容等)的类,本文将介绍如何使用这个类来抓取网页中需要的信息。
上图显示的是博客园首页的DOM树,显然只需提取出class为post_item的div,再重中提取出class为titlelnk的a标志即可。这样的功能可以通过以下函数来实现:
/// <summary>
/// 在文本html的文本查找标志名为tagName,并且属性attrName的值为attrValue的所有标志
/// 例如:FindTagByAttr(html, "div", "class", "demo")
/// 返回所有class为demo的div标志
/// </summary>
public static List<HtmlTag> FindTagByAttr(String html, String tagName, String attrName, String attrValue)
{
String format = String.Format(@"<{0}\s[^<>]*{1}\s*=\s*(\x27|\x22){2}(\x27|\x22)[^<>]*>", tagName, attrName, attrValue);
return FindTag(html, tagName, format);
}
public static List<HtmlTag> FindTag(String html, String name, String format)
{
Regex reg = new Regex(format, RegexOptions.IgnoreCase);
Regex tagReg = new Regex(String.Format(@"<(\/|)({0})(\s[^<>]*|)>", name), RegexOptions.IgnoreCase);
List<HtmlTag> tags = new List<HtmlTag>();
int start = 0;
while (true)
{
Match match = reg.Match(html, start);
if (match.Success)
{
start = match.Index + match.Length;
Match tagMatch = null;
int beginTagCount = 1;
while (true)
{
tagMatch = tagReg.Match(html, start);
if (!tagMatch.Success)
{
tagMatch = null;
break;
}
start = tagMatch.Index + tagMatch.Length;
if (tagMatch.Groups[1].Value == "/") beginTagCount--;
else beginTagCount++;
if (beginTagCount == 0) break;
}
if (tagMatch != null)
{
HtmlTag tag = new HtmlTag(name, match.Value, html.Substring(match.Index + match.Length, tagMatch.Index - match.Index - match.Length));
tags.Add(tag);
}
else
{
break;
}
}
else
{
break;
}
}
return tags;
}
有了以上函数,就可以提取需要的HTML标志了,要实现抓取,还需要一个下载网页的函数:
public static String GetHtml(string url)
{
try
{
HttpWebRequest req = HttpWebRequest.Create(url) as HttpWebRequest;
req.Timeout = 30 * 1000;
HttpWebResponse response = req.GetResponse() as HttpWebResponse;
Stream stream = response.GetResponseStream();
MemoryStream buffer = new MemoryStream();
Byte[] temp = new Byte[4096];
int count = 0;
while ((count = stream.Read(temp, 0, 4096)) > 0)
{
buffer.Write(temp, 0, count);
}
return Encoding.GetEncoding(response.CharacterSet).GetString(buffer.GetBuffer());
}
catch
{
return String.Empty;
}
}
以下以抓取博客园首页的文章标题和链接为例,介绍如何使用HtmlTag类来抓取网页信息:
class Program
{
static void Main(string[] args)
{
String html = HtmlTag.GetHtml("http://www.cnblogs.com");
List<HtmlTag> tags = HtmlTag.FindTagByAttr(html, "div", "id", "post_list");
if (tags.Count > 0)
{
List<HtmlTag> item_tags = tags[0].FindTagByAttr("div", "class", "post_item");
foreach (HtmlTag item_tag in item_tags)
{
List<HtmlTag> a_tags = item_tag.FindTagByAttr("a", "class", "titlelnk");
if (a_tags.Count > 0)
{
Console.WriteLine("标题:{0}", a_tags[0].InnerHTML);
Console.WriteLine("链接:{0}", a_tags[0].GetAttribute("href"));
Console.WriteLine("");
}
}
}
}
}
运行结果如下: