Examples of tracking number formats are:
UK tracking
2 letters and 7 numbers (e.g. AA1234567)
4 letters and 10 numbers (e.g. AAAA1234567890).
International tracking of exports
2 letters and 9 numbers and GB (e.g. AA123456789GB)
12 numbers (e.g. 123456789012)
International tracking of imports
2 letters and 9 numbers and 2 letters (e.g. AA123456789AA)