Oracle Database Discussions

Hi All,
We are trying to load certain data into oracle tables using sql loader. The filter condition is based on first character of 7th field in the record.
Ideally the control file should contain lines such as

------------start of control file ----------

LOAD DATA
truncate
DISCARDMAX 9999999
INTO table email_rep_correct
when substr(messageid,1,1)='<'
FIELDS TERMINATed by whitespace
trailing nullcols
(tdate date,
ttime char,
channel_name1 position(25:37) CHAR NULLIF channel_name1 =BLANKS,
channel_name2 position(38:49) CHAR NULLIF channel_name2 =BLANKS,
transtype char,
tsize integer external(8),
sender char,
orig_recipient char,
curr_recipient char,
messageid char
)

------------End of control file ----------

However the function substr in the when clause does not seem to work. is there any restriction on using function while loading using sql loader. Is there any workaround to the same.

Many thanks in advance
REgards
Yogesh