高分求正则啦！解决办法

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);