本问主要总结了用正则表达式对网页进行有效内容提取的具体实现方法,并给出了c#代码
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(" ", " ");
result = result.Replace(""", "/"");
result = result.Replace("<", "<");
result = result.Replace(">", ">");
result = result.Replace("&", "&");
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(" ", " ");
result = result.Replace(""", "/"");
result = result.Replace("<", "<");
result = result.Replace(">", ">");
result = result.Replace("&", "&");
result = result.Replace("<br>", "/r/n");
result = Regex.Replace(result, @"<[/s/S]*?>", string.Empty, RegexOptions.IgnoreCase);
return result;
}
#endregion