My postfix didn't have pcre installed:
fatal: dict_open: unsupported dictionary type: pcre: Is the postfix-pcre package installed?
But the basic regex ones worked, as outlined here:
header_checks = regexp:/etc/postfix/header_checks
The syntax in the header_checks file is:
Be aware too that the users name comes before the email address in the header line starting with From, eg:
From: Mr Smith <firstname.lastname@example.org>
So your regex will need to take that into account.
But I found a simpler solution that doesn't require regexes as long as you don't mind not being able to control the rule based on the recipient.
In modern versions of postfix you can add a redirect command to sender_access
Assuming you have a line in your main.cf like so:
smtpd_sender_restrictions = check_sender_access hash:/etc/postfix/sender_access
You can add a line to sender_access to redirect the email to another address like so:
email@example.com REDIRECT firstname.lastname@example.org
Just remember to do postmap hash:sender_access after saving sender_access