In R12, the number of times the Oracle formatting failed for the VAT registration number even hen the number was correct was more than enough for some clients to simply turn the validation off completely. It also took them at least 6 months to change the validation for the Swiss VAT (MwSt) number format. So, it looks like they have learned from their mistakes and have instead provided the ability to override the validation but without turning it off completely.
How you ask?
First, make sure your tax has been setup with the default validation. This means that under your tax (Manage Taxes) you set the ‘Registration Number Controls and Defaults’ section to have a ‘Validation Type’ equal to the country your tax belongs to. Then instead of the ‘Validation Level’ being set to ‘error’ set it to warning instead.
So now when you save your newly entered VAT registration and the format is wrong (but its the only format you have) then you can simply override the warning!