Get Maximum Number

Question

I have a problem
I have INV_REG table where fixed assets are stored
and there inventory number (INV_NO) genereates like this:
ABC-025145-2009
DONOR-SRNO-YEAR
now i want to get maximum serial number which will based on donor. i draw following query but i cannot get maximum number kindly guide me...
select inv_no, substr(inv_no,instr(inv_no,'-')+1,instr(inv_no,'-')-1) sr_no
from inv_reg
where don_code = :don_code;
result:
INV_NO     SR_NO
AC-HDIT-0272-2003     HD
ABC-9428-2006     9428
ABC-9426-2006     9426

Answer 1

i am using oracle database 9i...Should be always the first line in a post ;)
Again assuming no digits for DONOR:
SQL> with t as (
  select 'AC-HDIT-0272-2003' inv_no from dual union all
  select 'ABC-9428-2006' from dual union all
  select 'ABC-9426-2006' from dual
select inv_no, trim ('-' from rtrim (replace (inv_no, rtrim (inv_no, '0123456789-')), '0123456789')) ser_no
  from t
INV_NO                    SER_NO                  
AC-HDIT-0272-2003         0272                    
ABC-9428-2006             9428                    
ABC-9426-2006             9426                    
3 rows selected.