求个正则表达式。

[kmiaoer]

内容如下：asdfasdfasd<IMG height="339" alt="" src="http://www.baidu.com/uploadfiles/2016-08-10/201608100930114322252.png" width="514" >fasdfa不<IMG alt="" src="http://www.baidu.com/uploadfiles/2016-08-10/201608100930114322252.png" width="514" >让我们小P民。 <img alt="" src="http://www.baidu.com/uploadfiles/2016-08-10/201608101846477975.jpg" width="560" height="359" > <IMG width="514" height="339" alt="" src="http://www.baidu.com/uploadfiles/2016-08-10/201608100930114322252.png">
asdfsafdasfdasf[img src=' src="http://www.baidu.com/uploadfiles/2016-08-10/201608sss322252.png' width='100' height='123']asadfasdfsadf[img src=' src="http://www.baidu.com/uploadfiles/2016-08-10/201608100930114322252.png' width='100' height='123']

只提取<img 里面的图片宽，高，替换为100%

[秋的红果实]

string input="asdfasdfasd<IMG  height='339' alt='' src='http://www.baidu.com/uploadfiles/2016-08-10/201608100930114322252.png' width='514' >fasdfa不<IMG alt='' src='http://www.baidu.com/uploadfiles/2016-08-10/201608100930114322252.png' width='514' >让我们小P民。 <img alt='' src='http://www.baidu.com/uploadfiles/2016-08-";

string Result= Regex.Replace(Regex.Replace(input, @"(?<=height=')\d+(?=')", "100%"), @"(?<=width=')\d+(?=')", "100%");

[秋的红果实]

测试通过，结果： asdfasdfasd<IMG height='100%' alt='' src='http://www.baidu.com/uploadfiles/2016-08-10/201608100930114322252.png' width='100%' >fasdfa不<IMG alt='' src='http://www.baidu.com/uploadfiles/2016-08-10/201608100930114322252.png' width='100%' >让我们小P民。 <img alt='' src='http://www.baidu.com/uploadfiles/2016-08-