Variable: message-subject-re-regexp
message-subject-re-regexp is a customizable variable defined in
message.el.gz.
Value
"^[ ]*\\([Rr][Ee]\\(\\[[0-9]*\\]\\)* ?:[ ]*\\)*[ ]*"
Documentation
Regexp matching "Re: " in the subject line.
Source Code
;; Defined in /usr/src/emacs/lisp/gnus/message.el.gz
(defcustom message-subject-re-regexp
"^[ \t]*\\([Rr][Ee]\\(\\[[0-9]*\\]\\)* ?:[ \t]*\\)*[ \t]*"
"Regexp matching \"Re: \" in the subject line."
:group 'message-various
:link '(custom-manual "(message)Message Headers")
:type 'regexp)