正则匹配 替换..追加..

这里都是以图片中的元素为例：

匹配出IMG标签中alt的值:

 Regex reg = new Regex(@"(?is)(?<=<img[^>]*alt="")[^""]*(?=""[^>]*>)"); 

替换alt 内容：

var replace = "alt='要替换的内容' ";

source = Regex.Replace(source, "alt=""", replace);

上面那个并不严谨

var replace = " 替换内容 ";

source = Regex.Replace(source, @"(?is)(?<=<img[^>]*alt="")[^""]*(?=""[^>]*>)", replace);

自己先定义规则，然后替换

Regex r = new Regex(@"(?is)(?<=<img[^>]*alt="")");

var replace = " 替换内容 ";

source = r.Replace(source, new MatchEvaluator(x => replace));  

参考： http://bbs.csdn.net/topics/390320097

　　　 https://q.cnblogs.com/q/60920/