select t3.cert_no from table_name t3 where regexp_like(t3.cert_no, '^(\d{15}|\d{18})$')

SELECT REGEXP_SUBSTR('first field, second field , third field', ', [^,]*,') FROM dual

SELECT REGEXP_INSTR ('hello itmyhome', 'e') FROM dual; 

select regexp_replace('','','abc') from dual;

 select to_date(substr('',7,8),'yyyyMMdd') from dual;

select trunc((to_char(sysdate, 'yyyyMMdd') -

             to_char(to_date(substr('', 7, 8),

                              'yyyy-MM-dd'),

                      'yyyyMMdd')) / 10000)

  from dual;

select to_date(substr(t.cert_no, 7, 8), 'yyyyMMdd')

  from table_name t

 where to_date((case when length(t.cert_no) = 18 then

                substr(t.cert_no, 7, 8) when length(t.cert_no) = 15 then

                '' || substr(t.cert_no, 7, 6) end),

               'yyyyMMdd') < to_date('', 'yyyyMMdd')