Show error message to user if incorrect amount of numbers entered

[Baz]

Hi there
On the data base you have to enter a seven digit part no
code, it then searches for the record. Is there anyway I
can show a message if not all seven digits are entered.
Some part nos are only 5 digits but the the lookup table
has them in this format e.g. 0012345, if the user only
enters 5 digits it does not find it. Or alternativly is
there away to automatically insert zeros if the user
inputs less than seven digits. Hope this all makes sense.

Thanks