Not true. Postfix regexp (and pcre) matches are case insensitive by default, adding the /i flag makes them case sensitive. This should be quite clear in the postfix docs quoted above. This documented default behavior may be different from other software you're familiar with.
You're welcome to discuss postfix header checks further on the postfix-users list.
-- Noel Jones
I am not here to argue on details. It's just getting side tracked. I meant in general. A couple of 100 tests that I ran trying to solve this issue included both case sensitive and case insensitive and all the other possible and impossible options I could think of. Perhaps I missed that little detail, but postfix-user list members and Postfix creator Mr. Wetsie Venema in person said that the regexp were correct. I have discussed it there before as well.