string URL = "<img[^>]*src=(\"|')(?<src>[^(\"|')]*)(\"|')[^>]*>";               
                           MatchCollection MC = new Regex(URL, RegexOptions.IgnoreCase).Matches(strHTML);
                List<string> arrOutPut = new List<string>();

                foreach (Match m in MC)
                {
                    arrOutPut.Add(m.Groups["src"].Value);
                }

------解决方案--------------------

 string str = "<img [这里面的内容不确定] src=\"a.gif\"> ";
           str = Regex.Replace(str, @"(?is)(<img((?!src=)[^>])*src=['""])([^'""]+)(['""][^>]*>)", "$1img/$3$4");

------解决方案--------------------

C# code

            string content = @"<img [这里面的内容不确定] src=""a.gif""> ";
            Regex reg = new Regex(@"(?is)(<img[^>]+src="")([^""]+)("")");
            Match m = reg.Match(content);
            content = reg.Replace(content, "$1image/$2$3");
            MessageBox.Show(content);