Field_A  Field_B
A123     X
123A     Y
1234     Z

SQL> create table t (fielda varchar2(10), fieldb varchar2(10));

Table created.

SQL> host type t.ctl
load data
infile *
into table t
fields terminated by ","  TRAILING NULLCOLS
(
fielda char,
fieldb char "case when substr(:fielda,1,1) between 'A' and 'Z' then 'X'
                  when substr(:fielda,4,1) between 'A' and 'Z' then 'Y'
                  else 'Z'
             end"
)
begindata
A123
123A
1234

SQL> host sqlldr michel/michel control=t.ctl log=t.log

SQL*Loader: Release 10.2.0.4.0 - Production on Jeu. Sept. 10 21:01:36 2009

Copyright (c) 1982, 2007, Oracle.  All rights reserved.

Commit point reached - logical record count 3

SQL> select * from t;
FIELDA     FIELDB
---------- ----------
A123       X
123A       Y
1234       Z

3 rows selected.

case when fielda is null then null 
         when regexp_like(substr(fielda, 1, 3), '[[:digit:]]') and
              (substr(fielda, 4, 2) is null 
              then 'W'
         when regexp_like(substr(fielda, 1, 4), '[[:digit:]]') and
              (substr(fielda, 5, 1)) is null 
              then 'W'
         when regexp_like(substr(fielda, 1, 5), '[[:digit:]]') and
              then 'X'
	 when regexp_like((substr(fielda, 1, 4), '[[:digit:]]') and
	      regexp_like(substr(fielda, 5, 1), '[[:digit:]]') 
              then 'X'
         when regexp_like(substr(fielda, 1, 1), '[[:digit:]]') and
              regexp_like(substr(fielda, 2, 4), '[[:digit:]]') and
              then 'Y'
              else 'Z'