Find max in a specific data range

Hi all,
I'm looking to set up a database for part numbers. The part numbers are created by a combination of category codes and a sequential number.
It will end up looking something like xx-yy-xyz-001 xx-yy-xyz-002 aa-bb-abc-001 aa-bb-abc-002 and so on
I've set up a user interface so you can select the first categories from a combobox, but since the number is sequential i'd like to have it set up so you can either enter an unused number OR automatically generate the first unused number in the string.

So...The code will need to find all other numbers previously entered that have the same starting digits up to the number, and then find the maximum number and add one to that. (i can worry about no getting duplicate numbers later)

any help will be greatly appreciated and thanks in advance.

After reading through i realize i should've been more specific. All i need to learn right at the moment is how to search through all previously entered numbers and find ones that match the current number being created up to the last three digits. Then with that set i need to find the max so that i can add one to that and get my new number. Also with that set i can be sure that the number i'm entering hasn't been entered yet.

sorry and thanks aga