正则获取 某段 DIV 中 的内容

string html = "<div class='aa'><div class='left'>324324<div>dsfsdf</div><h1>aa</h1></div></div>";
            // 获取第一个 相呼应的标记
            //Regex reg = new Regex(@"<div class='left'>([\s\S]+?)<\/div>");
            // 获取前后对应的标记
            Regex reg = new Regex(@"(?is)<div class='left'[^>]*>(?><div[^>]*>(?<o>)|</div>(?<-o>)|(?:(?!</?div\b).)*)*(?(o)(?!))</div>");
            // 获取第一条
            Match first = reg.Match(html);
            // 获取匹配到的所有集合
            MatchCollection list = reg.Matches(html);
            foreach (Match item in list)
            {
                string value = item.Value;
            }

        public static string GetElementByClassName(string htmlConetnt, string label, string className)
        {
            Regex reg = new Regex(string.Format(@"(?is)<{0} class=""{1}""[^>]*>(?><{0}[^>]*>(?<o>)|</{0}>(?<-o>)|(?:(?!</?{0}\b).)*)*(?(o)(?!))</{0}>", label, className));
            Match first = reg.Match(htmlConetnt);
            return first.Value;
        }

        public static string GetElementById(string htmlConetnt, string label, string id)
        {
            Regex reg = new Regex(string.Format(@"(?is)<{0} id=""{1}""[^>]*>(?><{0}[^>]*>(?<o>)|</{0}>(?<-o>)|(?:(?!</?{0}\b).)*)*(?(o)(?!))</{0}>", label, id));
            Match first = reg.Match(htmlConetnt);
            return first.Value;
        }

        public static string GetLabel(string htmlConetnt, string label)
        {
            Regex reg = new Regex(string.Format(@"(?is)<{0}[^>]*>(?><{0}[^>]*>(?<o>)|</{0}>(?<-o>)|(?:(?!</?{0}\b).)*)*(?(o)(?!))</{0}>", label));
            Match first = reg.Match(htmlConetnt);
            return first.Value;
        }