string S = "<input type=\"hidden\" name=\"38482131042916\" value=\"73711242558722\" id=\"h_38482131042916\" />";
string patten = "value=\"\\d+\"";
MatchCollection mc = Regex.Matches(S, patten);
if (mc.Count > 0)
{
S = S.Substring(mc[0].Index, mc[0].Length);
}
S = S.Substring(7, S.Length - 8);
string result=Regex.Match(str,@"(?i)(?<=<input[^>]*?value=(""))[^""]*?(?=\1)").Value;