JS:

	function test() {

        	var str = "<img title=\"\" alt=\"\" align=\"\" src=\"/kindeditor/attached/image/20161214/20161214162554_8001.jpg\" width=\"312\" height=\"312\" />";
        	str += "<img title=\"\" alt=\"\" align=\"\" src=\"/kindeditor/attached/image/20161214/20161214162554_8001.jpg\" width=\"412\" height=\"412\" />";
        	str = str.replace(/ height="\d+"/g, " height=\"90\"");// “/”後面的是要替換的字元，“d\+”是數字,最後""裡是用來填充的字元

        	str = str.replace(/ width="\d+"/g, " width=\"90\"");

        	alert(str);
        }
 

C#：

var str = "<img title=\"\" alt=\"\" align=\"\" src=\"/kindeditor/attached/image/20161214/20161214162554_8001.jpg\" width=\"312\" height=\"312\" />";
			str += "<img title=\"\" alt=\"\" align=\"\" src=\"/kindeditor/attached/image/20161214/20161214162554_8001.jpg\" width=\"412\" height=\"412\" />";

			Regex reg = new Regex("width\\s*=\\s*\\S+ height\\s*=\\s*\\S+");
                        string result = reg.Replace(str, "width=\"90\" height=\"90\"");
                       第二種方法替換
                       //Regex reg1 = new Regex("height\\s*=\\s*\\S+");
			//string result1 = reg1.Replace(result, "height=\"90\"");
 

處理多張圖片中出現沒有width height的情況：

//正則替換圖片的寬高
                                        Regex regImg = new Regex(@"<img\b[^<>]*?\bsrc[\s\t\r\n]*=[\s\t\r\n]*[""']?[\s\t\r\n]*(?<imgUrl>[^\s\t\r\n""'<>]*)[^<>]*?/?[\s\t\r\n]*>", RegexOptions.IgnoreCase);
                                        MatchCollection matches = regImg.Matches(item.AdImg);
                                        var relImg = string.Empty;
                                        foreach (Match m in matches)
                                        {
                                            //進行匹配img是否存在width height
                                            Regex reg = new Regex("width\\s*=\\s*\\S+ height\\s*=\\s*\\S+");
                                            if (!reg.IsMatch(m.Value))//img 中不存在width height
                                            {   
                                                //獲取其他屬性進行替換
                                                Regex reg1 = new Regex("alt\\s*=\\s*\\S+");
                                                relImg += reg1.Replace(m.Value, "style=\"width:90px;height:90px;\"");
                                            }
                                            else
                                            {
                                               relImg+= reg.Replace(m.Value, "style=\"width:90px;height:90px;\"");
                                            }
                                        }