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, @"<script[^>]*?>.*?</script>", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);
output2 = Regex.Replace(ouput , @"<noscript[^>]*?>.*?</noscript>", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);
3.去style,正则为:
output = Regex.Replace(input, @"<style[^>]*?>.*?</style>", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);
4.去其他HTML标记
result = result.Replace("&nbsp;", " ");
result = result.Replace("&quot;", "\"");
result = result.Replace("&lt;", "<");
result = result.Replace("&gt;", ">");
result = result.Replace("&amp", "&");
result = result.Replace("<br>", "\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, @"<style[^>]*?>.*?</style>", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);
return result;
}
private string RemoveScript(string input)
{
string result = input;
result = Regex.Replace(result, @"<script[^>]*?>.*?</script>", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);
result = Regex.Replace(result, @"<noscript[^>]*?>.*?</noscript>", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);
return result;
}
private string RemoveTags(string input)
{
string result = input;
result = result.Replace("&nbsp;", " ");
result = result.Replace("&quot;", "\"");
result = result.Replace("&lt;", "<");
result = result.Replace("&gt;", ">");
result = result.Replace("&amp", "&");
result = result.Replace("<br>", "\r\n");
result = Regex.Replace(result, @"<[\s\S]*?>", string.Empty, RegexOptions.IgnoreCase);
return result;
}
#endregion