string str = Regex.Replace(inputstr,@"(<a\b[^>]*?)target=[""'][^""'>\s]+[""']([^>]*>)","$1$2", RegexOptions.IgnoreCase);

------解决方案--------------------
探讨

使用正则表达式将<a href="www.china104.com" target="_blank">中的target="_blank"过滤掉 怎么写？


------解决方案--------------------
string input=@"<a href="www.china104.com" target="_blank">";
string reslut=Regex.Replace(input,@"<a\s*href=""www.china104.com""\s*(?<target>(.*))>","");

------解决方案--------------------
yourhtml=Regex.Replace(yourhtml,@"(?i)(<a[^>]*?)target=(['""]?)_blank\2([^>]*?>)","$1$3");