Vim Regex : How to search for A AND B NOT C

To represent a NOT, use the negative assertion \@!.

For example, "NOT Bush" would be:

^\(.*Bush\)\@!

or using \v:

\v^(.*Bush)@!

Important: note the leading ^. While it's optional if you only use positive assertions (one match is as good as any other), it is required to anchor negative assertions (otherwise they can still match at the end of a line).

Translating "Bush AND Clinton AND NOT (Carter OR Obama)":

\v^(.*Bush)&(.*Clinton)&(.*Carter|.*Obama)@!

Addendum

To explain the relationship between \& and \@=:

One&Two&Three

is interchangeable with:

(One)@=(Two)@=Three

The only difference is that \& directly mirrors \| (which should be more obvious and natural), while \@= mirrors Perl's (?=pattern).

If you want to use Perl-style regular expressions after vim, forget about \&: it is a vim-specific feature which is useless since vim also has lookaheads, so any r1\&r2 can be rewritten as as \%(r1\)\@=r2. But lookaheads are better as there is a negative version of it and they are also available in most of Perl-style regular expression engines. Your (Bush AND Clinton AND NOT (Carter OR Obama)) can be expressed in the following way:

g/^\%(.*\%(Carter\|Obama\)\)\@!\%(.*Bush\)\@=.*Clinton/

Or, with very magic:

g/^\v%(.*%(Carter|Obama))@!%(.*Bush)@=.*Clinton/

See :h /\@=

About inner logic: look-ahead is like branches: for regex (reg1)@=reg2 assuming that reg2 matches at position N (match starts at position N), regex engine checks whether reg1 also matches at this position. If it does not, then the position is discarded and regex engine tries next possible match for reg2. Same for the negative look-ahead, but with the difference that regex engine discards the position if reg1 does match.

Example:

Regex: (.b)@!a.

String: aba.

1. Found match: a matches at position 0 (aba). Trying to match look-ahead: . matches a (aba) and b matches b (aba), look-ahead matches, discarding position.
2. Position 1 (aba) does not match a.
3. Found match: a matches at position 2 (aba). Trying to match look-ahead: . matches a (aba), but b does not match: no symbols left, look-ahead fails. Result: regex matches at position 2.