Use of group collections in regular expressions

简单实例：

    string s = "2005-2-21";
    Regex reg = new Regex(@"(?<y>d{4})-(?<m>d{1,2})-(?<d>d{1,2})",RegexOptions.Compiled);
         
    Match match = reg.Match(s);
    int year =  int.Parse(match.Groups["y"].Value);
    int month = int.Parse(match.Groups["m"].Value);
    int day = int .Parse(match.Groups["d"].Value);
    DateTime time = new DateTime(year,month,day);