Regular Expression of a specific Date format

Question

I need help in forming a regular expression of a date to match YYYYMMDD format. Below are the details:

Input Validation: The value entered is length of 8 and all nu

Answer 1

This is a bad idea. The only way to validate that a date is correct is to attempt to convert it into a date. If the conversion fails then it's not a date; if it's successful then it might be. Dates are far too complex for a regular language to parse.

So, create a function that converts it into a date; you can make if far more generic than you have here, so it can then be reused for other purposes:

create or replace function validate_date (
     PDate in varchar2
   , PDateFormat in varchar2
     ) return date is
begin
   return to_date(PDate, PDateFormat);
exception when others then
   return null;
end;

This returns a date if it's able to validate that the date and date format match, otherwise returns NULL if there's any error. Your query then becomes:

select 'P' from dual where validate_date('20140506', 'yyyymmdd') is not null