The definitive source for a correct PID regexp is probably the BBC
themselves, and it just so happens that they provide one:
https://github.com/bbc/programmes-pages-service/blob/master/src/Domain/ValueObject/Pid.php#l14
^[0-9b-df-hj-np-tv-z]{8,}$
HTH.
--
James Scholes
http://twitter.com/JamesScholes