If else validation in code behind

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ASP.NET

Hi,

I am validating page in code behind using if-else statement. i didn't need asp.net validator for this.
If else statement working fine. but the problem is that page is still submitting when it execute else condition.
I want when i click save button and when execution reach in else condition , the page set to invalid. or it i need to prevent page when code is in else statement

EntityMpdb1 obentitympdb1 = new EntityMpdb1();
            BalLoginTable obupdatepensioner = new BalLoginTable();
            DataSet dspensionrecord = new DataSet();
            BalLoginTable obbalpensionrecord = new BalLoginTable();
            try
            {
                obentitympdb1.Authority_No = Txtauthorityno.Text;
                Match match = Regex.Match(Txtauthoritydate.Text, @"^([1-9]|0[1-9]|1[0-2])[- / .]([1-9]|0[1-9]|1[0-9]|2[0-9]|3[0-1])[- / .](1[9][0-9][0-9]|2[0][0-9][0-9]|3[0][0-9][0-9])$");
                if (match.Success)
                {
                    string val = match.Groups[1].Value;
                     
                }
                else
                {

                    spstartdate.InnerText = "Please enter correct date";                 
 
                }
  obupdatepensioner.updatepensionderdetail(obentitympdb1);
}
catch
{
}