[antlr-interest] Lookahead predicates in the Lexer?

[Terence Parr]

That predicate should work.  If that predicate fails, then that rule will fail and the input will not be consumed for B.
Ter
On Nov 12, 2012, at 3:29 PM, Gerald Rosenberg wrote:

> In Antlr4, is there a way to do a fixed lookahead in the lexer predicate 
> without capturing the lookahead token(s)?  In v3, predicates could be 
> used for this purpose.
> 
> csvRule : A ( Comma B )* ;
> 
> A : P Q R -> pushMode(Alphabet)
> 
> mode Alphabet;
> B : X Y Z { ~Comma }? -> popMode
>    : X Y Z ;
> 
> In v4 , the "~Comma" is presumed to be native code.
> 
> Basically, looking for a clean, workable way to not require the use of a 
> semicolon to explicitly terminate input that matches the csvRule, yet 
> have a representation in the lexer that can be used as the popMode trigger.
> 
> I do realize that I can write a predicate method to do a stream scan, 
> but would prefer a non-native code solution if possible.  Also realize 
> that, in the simplest case, csvRule could be pushed down into the 
> Lexer.  Where A and B  are not just single terminals in the parser, 
> other rules would have to be pushed down also, making for a bit of a mess.
> 
> Thanks,
> Gerald
> 
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