regexp syntax, nnmail split: "s1 not-followed-by s2"

[Karl Kleinpaste <karl@jprc.com>]

I want to experiment with an acquaintance's 2-rule anti-spam trap.  If:
1. Neither To: nor Cc: contains one's own address, and
2. The origin is not a known mailing list,
then it's probably spam.  I've got a bunch of other spam-avoidant
rules, but I'd like to try these out.  Unfortunately, it's not clear
to me how I go about expressing a not-followed-by condition in a
regexp.  Any suggestions?

Obviously, this simple rule is not perfect, as it will mis-classify as
spam any mail in which one was properly addressed using Bcc:.

Also, in looking at the documentation for nnmail-split-fancy, because
I thought it might be necessary for my anti-spam effort (but now that
I think I understand -fancy, it won't help -- I have nnmail-crosspost
set to nil), the 1st line of the example contains a token `mail' (not
a string) which should be enclosed in quotes as a string.