DHCP filters

Some odd MAC's have started appearing in my DHCP server and I wondered if there was a way to filter the MAC in a particular format.

I.E.: if the mac does not meet the 12 character hex pattern normally used for MAC's then reject the request.

As none of these odd MAC addresses can be added to the deny filters because they don't meet standard, I am hoping that there is a way to filter them further up the line.