The regular expression you're using requires that the
"[sftest]" be the first thing in the subject and must be
followed by a colon.
=v= This is the right way to go, as it parallels the prevailing
use of subject line keywords such as "[sftest]". Generally the
mailing list software will insert it at the front (or after an
initial "Re: "), but only after doing a very simple search to
see whether the string is any part of the subject line. That
lets it deal with stuff like:
Re: Fwd: Re: Fwd: Re: [sftest] ME TOO!
without throwing "[sftest]" in there four more times.