[a-zA-Z]{2}\d{6}
[a-zA-Z]{2}
means two letters
\d{6}
means 6 digits
If you want only uppercase letters, then:
[A-Z]{2}\d{6}
[a-zA-Z]{2}\d{6}
[a-zA-Z]{2}
means two letters
\d{6}
means 6 digits
If you want only uppercase letters, then:
[A-Z]{2}\d{6}