简单实例：

string s = "2005-2-21";
Regex reg = new Regex(@"(?\d{4})-(?\d{1,2})-(?\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);