Referential constraints on 'unlike' fields

[rob@xxxxxxxxx]

I am trying to do the following:
Create table GDWEB/FTPUSER
       ( 
...
       ID400 character (10),
...
        constraint FTPUSER_InvalidID400        check( 
         (ID400 is null) or 
         ( 
 
          (substr(ID400,4,1) between '0' and '9') and 
          (substr(ID400,5,1) between '0' and '9') and 
          (substr(ID400,6,1) between '0' and '9') and 
          (substr(ID400,7,1) between '0' and '9') and 
          (substr(ID400,8,1) between '0' and '9') and 
          (substr(ID400,9,1) between '0' and '9') and 
          (substr(ID400,10,1) between '0' and '9')) ), 
 
        constraint FTPUSER_DupID400 unique (ID400), 
        constraint FTPUSER_REFID400 foreign key (substr(ID400,1,3))
        references gdweb/ftpcompany (userprefix) 
) 

Now, in file FTPCOMPANY the field USERPREFIX is character(3).  In file 
FTPUSER the field ID400 is character(10).  I just want to make sure that 
the first three characters of ID400 were in the field USERPREFIX in the 
file FTPCOMPANY.

I was hoping to avoid a trigger.

I tried the above and get:  SQL0104-Token ( was not valid. Valid tokens: ) 
,.

I tried 
              constraint FTPUSER_InvalidID400        check( 
               (ID400 is null) or 
               ((substr(ID400,1,3) in (select USERPREFIX from
      GDWEB/FTPCOMPANY)) and 
                (substr(ID400,4,1) between '0' and '9') and 
                (substr(ID400,5,1) between '0' and '9') and 
                (substr(ID400,6,1) between '0' and '9') and 
                (substr(ID400,7,1) between '0' and '9') and 
                (substr(ID400,8,1) between '0' and '9') and 
                (substr(ID400,9,1) between '0' and '9') and 
                (substr(ID400,10,1) between '0' and '9')) ),
and got SQL0546-
Message . . . . :   CHECK condition of constraint *N not valid.  
Cause . . . . . :   The CHECK condition of constraint *N is not valid for 
one 
  of the following reasons:  
    -- a column level CHECK condition refers to some other column in the 
table
    -- the CHECK condition refers to a column that is not in this table  
    -- the CHECK condition refers to a special register  
    -- the CHECK condition uses a column function (such as AVG or COUNT) 
or a 
  user-defined function  
    -- the CHECK condition contains a subselect  
    -- the CHECK condition uses the NODENAME scalar function  
    -- the CHECK condition uses an expression involving LOBs.  

Rob Berendt
-- 
"They that can give up essential liberty to obtain a little temporary 
safety deserve neither liberty nor safety." 
Benjamin Franklin