请教大师们一个正则表达式的问题

Question

#region 获取网页源码
        public string getWebSouce(string url)
        {
            WebClient client = new WebClient();
            try
            {
                _websource = client.DownloadString(new Uri(url));
            }
            catch
            {
                _websource = "";
            }
            string pattern = @"(?i)\bcharset=(?<charset>[-a-zA-Z_0-9]+)";
            string charset = System.Text.RegularExpressions.Regex.Match(_websource, pattern).Groups["charset"].Value;
            try
            {
                string tmp = _websource;
                Encoding encode = Encoding.GetEncoding(charset);
                _websource = encode.GetString(client.Encoding.GetBytes(tmp.ToCharArray()));
                //转为utf-8编码
                // _websource = Encoding.UTF8.GetString(encode.GetBytes(tmp.ToCharArray()));            }
            catch
            {            }
            //client.DownloadStringAsync(new Uri(url));
            // client.DownloadStringCompleted += new System.Net.DownloadStringCompletedEventHandler(client_DownloadStringCompleted);
            return _websource;
        }大师们,能不能帮解释一下这段代码呢
尤其是正则部分 

Answer 1

(?<name>exp) 匹配exp,并捕获文本到名称为name的组里，也可以写成(?'name'exp) [-a-zA-Z_0-9]+ 匹配- 字母 _数字