message-subject-trailing-was-regexp

message-subject-trailing-was-regexp is a variable defined in `message.el'.
Its value is

"[ 	]*\\((*[Ww][Aa][Ss]:[ 	]*.*)\\)"

Documentation:
*Regexp matching "(was: )" in the subject line.

If `message-subject-trailing-was-query' is set to t, the subject is
matched against `message-subject-trailing-was-regexp' in
`message-strip-subject-trailing-was'. You should use a regexp creating very
few false positives here.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.