Filter by selection using a control

[Sophia Kelly]

Hi there,
I have a field that contains a string of codes
corresponding to what groups the record belongs to. What
I'd like to do is have a button or list box that the user
could select one of these codes to filter the records to
show only those which contained one of these codes.
Wildcards don't seem to work with perameters in this way:
eg: *[Enter Code Here]* as a query/filter criteria. To
clarify, I'm able to filter correctly if the record only
has one code in this field, but not if it has several
codes (including the right one) in the field.
Any suggestions?

Thanks,
Sophia Kelly