Create a Validation rule for IBAN Account nos in a form field

[Guest]

Hi,

I have a form linked to a table.
One of the fields in the table has this input mask (for IBAN account nos.):

LL00\-0000\-0000\-0000\-0000\-9999\-9999;;

This forces the first two characters to be upper case alphabetic, then up to
18 numbers (mandatory) followed by up to 8 optional numbers.

But I would like to be able to validate this on the form side with a
validation rule and validation text for the error message - as currently it
just shows the message 'not valid for this input mask' ... which is not very
user friendly.

Any idea how I can either create an Validation Rule like that for the form,
or get Access 2003 to show a nice error message when the data entry doesn't
fit the input mask?

thanks for any help or ideas

Philip