SQL> select to_date('01@jan@12') from dual;

TO_DATE('
---------
01-JAN-12

SQL> create or replace
  2  function date_tester(p_str varchar2) return date is
  3    type format_masks is table of varchar2(20);
  4    l_mask format_masks
  5     := format_masks(
  6         'dd/mm/yyyy',
  7         'dd-mon-yy',
  8         'dd/mon/rr',
  9         'dd@mm@yy',
 10         'dd-mon@yy',
 11         'ddmmyy',
 12         'ddmonyy'
 13         );
 14    l_date date;
 15  begin
 16    for i in 1 .. l_mask.count
 17    loop
 18      begin
 19        l_date := to_date(p_str,l_mask(i));
 20        return l_date;
 21      exception
 22        when others then null;
 23      end;
 24    end loop;
 25    return null;
 26  end;
 27  /

Function created.

SQL>
SQL> select date_tester('01-jan-12') from dual;

DATE_TEST
---------
01-JAN-12

SQL> select date_tester('01-jan@12') from dual;

DATE_TEST
---------
01-JAN-12

SQL> select date_tester('01jan12') from dual;

DATE_TEST
---------
01-JAN-12

SQL> select date_tester('010012') from dual;

DATE_TEST
---------