How to build a negative lookahead parser in nom?

Question

How can I create a negative lookahead parser for nom?

For example, I'd like to parse "hello", except if it's followed by " world". The equivalent regex would be hello(?! world).

I tried to combine the cond, not and peek parsers

fn parser(input: &str) -> IResult<&str, &str> {
    cond(peek(not(tag(" world"))(input)), tag("hello"))(input)
}

but this doesn't work as cond expects the condition as bool instead of as IResult.

`not()` is generally something a parser combinator should avoid to use. It kind of the opposite of what a parser combinator is for — Stargateur, Feb 06 '23 at 19:34

score 2 · Accepted Answer · edited Feb 06 '23 at 19:19

2

terminated(tag("hello"), not(tag(" world")))

edited Feb 06 '23 at 19:19

mdcq

answered Feb 06 '23 at 18:47

MeetTitan

Oh, using `not` directly. Much more simple than I thought! Thanks. – mdcq Feb 06 '23 at 18:55

1 Answers1