Custom validation with Serial Number

Hey, I’m working on a website that is supposed to differentiate user accounts at registration. Firstly, users who do not yet have our product can register, and we will manually approve such registrations in order to avoid possible registrations of our competition. On the other hand, we would like to offer users who already own our product the possibility to be auto approved and thus have direct access to the website. For this purpose I have already used the “user role radio”, but now I would like to authenticate these “Existing customers” by entering a correct serial number. Is there a possibility to store the basic pattern of the serial number? For example: Our serial numbers always begin with SN10 followed by 6 individual numbers.

Thank you very much in advance!