Input Mask, Validation Rule, Search Form

I have a form that acts as a filter/search engine -- it inputs criteria into a query, then opens a report based on the criteria/query. One of the criteria options is "Supplier Number", which is to be entered into an unbound text box.

All of our suppliers have 10 digits, usually in this format: A123400000 (i.e. First character is either a Letter or digit, followed by four or five digits, then followed by four or five zeros)

Our system works with ten digit numbers, while the people in the dept are used to just typing in "A1234" assuming the zeros are irrelevant.

How do I create a validation rule or input mask where a user can simply type "A1234", then it will return zeros for the remaining of the ten required characters?

I realize the usual response to these type of simple question is "Search the Threads", but I have done so already and still don't have an answer!