Regex in header_check is not working

[Dr Michael Daly]

Hi
I cannot get a regex to work in postfix 2.11.1. The regex is designed to
reject Subject lines that have ALL CAPITALS;

/(^Subject: [^a-z]+$)/ REJECT <--not working

Testing the above for 'AAAA' via command line with:
postmap -q 'Subject: AAAA' regexp:/etc/postfix/header_checks

....shows its not 'rejected'

Online PCRE regex checkers seem to indicate the expression is fine.

Main.cf has:
header_checks = pcre:/etc/postfix/header_checks
(have also tried: header_checks = pcre:/etc/postfix/header_checks)

Have many other headercheck regex's working.

Any clues please?
Michael