搜索引擎中一个比较重要的环节就是从网页中抽取出有效内容。简单来说，就是吧HTML文本中的HTML标记去掉,留下我们用IE等浏览器打开HTML文档看到的部分（我们这里不考虑图片）.

将HTML文本中的标记分为:注释,script ,style，以及其他标记分别去掉：

1.去注释,正则为:

output = Regex.Replace(input, @"", string.Empty, RegexOptions.IgnoreCase);

2.去script,正则为:

ouput = Regex.Replace(input, @"]*?>.*?", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);

output2 = Regex.Replace(ouput , @"]*?>.*?", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);

3.去style,正则为:

output = Regex.Replace(input, @"]*?>.*?", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);

4.去其他HTML标记

result = result.Replace(" ", " ");

result = result.Replace(""", "\"");

result = result.Replace("<", "<");

result = result.Replace(">", ">");

result = result.Replace("&", "&");

result = result.Replace("
", "\r\n");

result = Regex.Replace(result, @"<[\s\S]*?>", string.Empty, RegexOptions.IgnoreCase);

以上的代码中大家可以看到,我使用了RegexOptions.Singleline参数，这个参数很重要，他主要是为了让"."(小圆点)可以匹配换行符.如果没有这个参数，大多数情况下，用上面列正则表达式来消除网页HTML标记是无效的.

HTML发展至今，语法已经相当复杂,上面只列出了几种最主要的标记,更多的去HTML标记的正则我将在

Rost WebSpider 的开发过程中补充进来。

下面用c#实现了一个从HTML字符串中提取有效内容的类:

using System;

using System.Collections.Generic;

using System.Text;

using System.Text.RegularExpressions;

class HtmlExtract

{

#region private attributes

private string _strHtml;

#endregion

#region public mehtods

public HtmlExtract(string inStrHtml)

{

_strHtml = inStrHtml

}

public override string ExtractText()

{

string result = _strHtml;

result = RemoveComment(result);

result = RemoveScript(result);

result = RemoveStyle(result);

result = RemoveTags(result);

return result.Trim();

}

#endregion

#region private methods

private string RemoveComment(string input)

{

string result = input;

//remove comment

result = Regex.Replace(result, @"", string.Empty, RegexOptions.IgnoreCase);

return result;

}

private string RemoveStyle(string input)

{

string result = input;

//remove all styles

result = Regex.Replace(result, @"]*?>.*?", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);

return result;

}

private string RemoveScript(string input)

{

string result = input;

result = Regex.Replace(result, @"]*?>.*?", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);

result = Regex.Replace(result, @"]*?>.*?", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);

return result;

}

private string RemoveTags(string input)

{

string result = input;

result = result.Replace(" ", " ");

result = result.Replace(""", "\"");

result = result.Replace("<", "<");

result = result.Replace(">", ">");

result = result.Replace("&", "&");

result = result.Replace("
", "\r\n");

result = Regex.Replace(result, @"<[\s\S]*?>", string.Empty, RegexOptions.IgnoreCase);

return result;

}

#endregion

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