问从字符串转换为日期时间时，未将字符串识别为有效的DateTime

EN

        var dateString = "09/25/2012 11:00:36:156";
        var modifiedString = dateString.Substring(0, dateString.Length - 4) + "." + dateString.Substring(20);
        //var modifiedString = dateString.Substring(0, dateString.Length - 4);
        var dateValue = Convert.ToDateTime(modifiedString);

void Main()
{
    string datestring = "09/25/2012 11:00:36:156";

    string regexpr = @"(?x)(?i)
                    (\d{1,4}) [./-]
                    (\d{1,2}) [./-]
                    (\d{1,4}) [\sT]  (\d+):(\d+):(\d+) \s? (A\.?M\.?|P\.?M\.?)?";

    DateTime capture = new DateTime(); // set to default value in case datestring isn't valid

    if (Regex.IsMatch(datestring, regexpr)
        && DateTime.TryParse(Regex.Match(datestring, regexpr).Groups[0].Value, out capture))
    {    
        // convert is succeded
    }
    else
    {
        // Handle invalid dateString
    }

    Console.WriteLine(capture);
}