[antlr-interest] How to do "not" in a syntactic predicate?

[Naveen Chawla]

Yes, if you put "sentence: q b;" above q then it accepts "asdqwe", even
though the *a* in q is followed by *b*. And in fact, on my side, accepts *a*
regardless of what it's followed by. So the predicate seems to have no
effect, on my side.

I haven't yet been able to reproduce the problem itself in simpler form,
and I agree about trying to find other ways, although unable so far. A
working workaround of "not followed by" would fix the whole problem though.

Jim posted in 2007 a PEG-style "followed by" solution using "foo: a
{input.mark();} b {input.rewind();}".. which "takes *a* and *b* and then
rewinds as if it it has just taken *a* "  so I was wondering if there was a
solution for "not followed by" using mark(), rewind() and maybe reset()
which "takes *a* and *b* and then (something or another?)..." ?
2009/10/14 Indhu Bharathi <indhu.b at s7software.com>

>  Here is the complete grammar:
>
>
>
> grammar Test;
>
>
>
> q             :               a ((b)=>NOWAY | /*nothing*/);
>
>
>
> a              :               'asd';
>
> b             :               'qwe';
>
>
>
> fragment NOWAY:          ;
>
>
>
>
>
> ‘q’ will accept “asd” but not “asdqwe”. Am I missing something?
>
>
>
>
>
> BTW, if you can post a simplified example of your exact problem, you might
> get more elegant solutions. This is only a hack for the specific problem you
> posted. Maybe your original problem can be solved in a more elegant way.
> Cases that require hacks of this kind are very rare and arises only while
> parsing some very obscure languages.
>
>
>
> Cheers, Indhu
>
>
>
>
>
> *From:* Naveen Chawla [mailto:naveen.chwl at googlemail.com]
> *Sent:* Wednesday, October 14, 2009 11:38 PM
> *To:* Indhu Bharathi
> *Cc:* Jim Idle; antlr-interest at antlr.org
>
> *Subject:* Re: [antlr-interest] How to do "not" in a syntactic predicate?
>
>
>
> Yikes, that doesn't seem to work, it seems to give a normal positive
> predicate for me. (Or something). But not "if a not followed by b" => a.
>
>
>
> ?
>
> 2009/10/14 Indhu Bharathi <indhu.b at s7software.com>
>
> The other say I replied from my ipod and was not able to test it with
> ANTLR. I checked it now and the code doesn’t work for me too. However you
> can try a variant:
>
>
>
> q             :               a ((b)=>NOWAY | /*nothing*/)
>
>                 ;
>
>
>
> fragment NOWAY
>
>                 :               ;
>
>
>
> This is tested J But note that this is only a hack and use it only when
> there is no other alternative.
>
>
>
> Going by your “one of my "smaller" rules is "consuming" something that
> should belong to a "larger" rule” description, it looks like you can solve
> that problem by turning off greedy. Grep for ‘greedy’ in the book or wiki.
>
>
>
> Cheers, Indhu
>
>
>
>
>
> *From:* antlr-interest-bounces at antlr.org [mailto:
> antlr-interest-bounces at antlr.org] *On Behalf Of *Naveen Chawla
> *Sent:* Wednesday, October 14, 2009 3:50 PM
> *To:* Jim Idle
> *Cc:* antlr-interest at antlr.org
> *Subject:* Re: [antlr-interest] How to do "not" in a syntactic predicate?
>
>
>
> Jim, being new to predicates I realised soon after this that you were
> talking about token lookaheads. And yes I do need a syntactic lookahead (but
> thas is negative). In beginner-speak, one of my "smaller" rules is
> "consuming" something that should belong to a "larger" rule. This would be
> correct if that particular something (e.g. "a") was *not* followed by a
> syntactic construct conforming to "b". Hence (a !b)=>a (for the smaller
> rule) seems the simplest solution to this to me. Is Indhu's version correct
> for doing this trick? It doesn't seem to work for me (but I might be doing
> something else wrong). If not correct, what is the correct way? My target is
> Java.
>
> 2009/10/5 Jim Idle <jimi at temporal-wave.com>
>
> Use a semantic predicate rather than syntactic. You possibly need a bated
> predicate here too:
>
>
>
> { input.LA(1) == A && input.LA(2) != B}?=>
>
>
>
> However, if you need that kind of syntactic predicate, then I suggest you
> may be approaching your problem incorrectly.
>
>
>
> Jim
>
>
>
> *From:* antlr-interest-bounces at antlr.org [mailto:
> antlr-interest-bounces at antlr.org] *On Behalf Of *Naveen Chawla
> *Sent:* Monday, October 05, 2009 7:02 AM
>
>
> *To:* antlr-interest at antlr.org
> *Subject:* [antlr-interest] How to do "not" in a syntactic predicate?
>
>
>
> If I do
>
>
>
> (a ~b)=> a
>
>
>
> meaning "take this alternative if you encounter an a when not followed by
> b"
>
>
>
> I get a syntax error: unexpected token b
>
> Is it the right syntax to use '~'?
>
>
>
> N
>
>
>
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