Input mask use on form

[jk]

Hi,

I have two fields (invoice number) and (serial number) that the users
continue to make mistakes on input since they are both number fields but the
amount of digits are differerent by just 3 digits.How can i use a code on the
form to limit one field to nine digits and to place focus with a limit of 12
digits for the other field?Can this be done and can a message box be utilized
as well?

thanks for the support