class Program
    {
        static void Main(string[] args)
        {
            string a = "xxxx <a>abcdxxxxyyyxxx</a> <a>xxxxyyy</a> xxxyyyxx <img alt=xxx/> yyy";

            Console.WriteLine(RegReplace(a, "yyy", "DDD"));
            Console.Read();
        }


        public static string RegReplace(string content, string oldstr, string newstr)
        {

            //这个正则有问题，第一次出现在a标签中的yyy仍被替换为DDD，与预期不符。求帮忙修改！
            return Regex.Replace(content, @"(?<!<a[^>]+>[^<]*)" + oldstr+ "(?![^(</a>)]*?</a>)", newstr, RegexOptions.IgnoreCase);

        }  
    }

            string a = "xxxx <a>abcdxxxxyyyxxx</a> <a>xxxxyyy</a> xxxyyyxx <img alt=xxx/> yyy";
            Dictionary<string, string> dic = new Dictionary<string, string>();
            int index=0;
            foreach (Match match in Regex.Matches(a, "(<a>[^<]+</a>)|(<img[^>]+/>)"))
            {
                dic.Add("rep_" + index.ToString(), match.Value);
                a = a.Replace(match.Value, "rep_" + index.ToString());
                index++;
            }
            a = a.Replace("yyy", "DDD");
            foreach (string s in dic.Keys)
                a = a.Replace(s, dic[s]);
            Console.WriteLine(a);