That checks if the last char is uppercase letter - as you would also have a seperate rule to check length = 7 then this wouldn't need to be handle in the regex. The rule would be set to must match.
[A-Z]$
To include check for 6 digits at start you could use: ^[0-9]{6}[A-Z]$
Also if you want to check the 'randomness' and see if there are sequences in the numeric part somthing such as this could be added as another rule - whic must not match: