David Bremner venit, vidit, dixit 2021-04-24 12:57:17:
> Erwan Hingant <erwan.hingant@mailo.com> writes:
> 
> > Hello,
> >
> >   I have some troubles with regex negative lookahead. When searching 
> > in 
> > all directories but one (say inbox), I do the following query:
> >
> >  > notmuch search folder:"/^(?!inbox)/"
> >
> 
> As far as I know, lookaheads are not supported by POSIX regex, which is
> what notmuch uses. regex(7) is a bit terse, but the relevant section
> seems to be
> 
>        An atom is a regular expression enclosed in "()" (matching a match  for
>        the  regular  expression),  an  empty  set  of  "()" (matching the null
>        string)(!), a bracket expression (see below), '.' (matching any  single
>        character),  '^' (matching the null string at the beginning of a line),
>        '$' (matching the null string at the end of a line), a '\' followed  by
>        one  of the characters "^.[$()|*+?{\" (matching that character taken as
>        an ordinary character),  a  '\'  followed  by  any  other  character(!)
>        (matching  that character taken as an ordinary character, as if the '\'
>        had not been present(!)), or a single character with no other  signifi‐
>        cance  (matching  that character).  A '{' followed by a character other
>        than a digit is an ordinary character, not the beginning of a bound(!).
>        It is illegal to end an RE with '\'.
> 
> I'm not sure your whole problem, but maybe regex is not the right answer
> here. In general they should be a last resort in notmuch, for efficiency
> reasons. Does "not folder:inbox"  do what you want?

Yes, that solves the problem as stated (all folders except inbox).

As for the error message: both "^" and "(?!inbox)" are assertions in
regex lingo, not atoms. A negative assertion like "(?!inbox)" asserts
whether the previous atom is considered a match or not (under the
assumption that the atom matches something).

The error message seems to say that you cannot use an assertion
without a preceding atom that it applies to.

But, assertions are really for the case where you want to base a match
result (true/false) on parts of the expression that you do not want to
be part of the match itself. As far as I understand, matching in xapian
is solely about the true/false result, so I'm wondering when you would
really need lookahead and lookbehind (instead of groups).

Michael
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