Validation of field in User Registration

Hello anybody out there!
I have gotten hold of a php-script for validating Swedish social-security numbers and I want to connect it to a custom field that I call “Personnummer” in the User Registrations form in my Auction theme.

I have poor understanding of php-scripting as such, allthough I can navigate through on a basic level, so I do not know how I can apply the script to the particular custom field (Personnummer). Can anybody here help me?
It is a complete script in a separate php-file (see attached file).

//Magnus