[antlr-interest] Can antlr v3 lex star | tristar properly?
Harald Mueller
harald_m_mueller at gmx.de
Wed Nov 21 04:05:17 PST 2007
As Johannes pointed out, you can do manual left-factoring - but this somehow defies the reason to use ANTLR ...
A "somewhat less intrusive" way, which avoid the left-factoring, is the following:
stars : (STAR | TRISTAR)* EOF;
STAR : '*' ;
TRISTAR :
{input.LA(1)=='*' && input.LA(2)=='*' && input.LA(3)=='*'}? =>
STAR STAR STAR;
{...}? is a semantic predicate; and the => forces checking the predicate as with syntactic predicates.
This way, you could keep any actions inside your grammar - at times a nuisance with left-factoring and a primary reason to use some tool to disambiguate input ...
Regards
Harald
-------- Original-Nachricht --------
> Datum: Wed, 21 Nov 2007 12:54:31 +0200
> Von: Guntis Ozols <guntiso at latnet.lv>
> An: antlr-interest at antlr.org
> Betreff: [antlr-interest] Can antlr v3 lex star | tristar properly?
> Is there a way to lex this simple grammar (I am using ANTLRWorks 1.1.4):
>
> stars : (STAR | TRISTAR)* EOF;
> STAR : '*' ;
> TRISTAR : STAR STAR STAR;
>
> For '**' I keep getting: mismatched character '<EOF>' expecting '*'.
>
> I am out of options. Below is what I've tried:
> //tokens { STAR; TRISTAR;}
> //options { backtrack = true; }
> //options { k = 3; }
> //ST : ('***') => '***' {type = TRISTAR;}
> // | '*' {type = STAR;};
> //STAR options {greedy=false;}: '*' ;
> //TRISTAR : '***';
> //TRISTAR : ('***')=>'***';
> //TRISTAR options { greedy=true; } : '***';
> //TRISTAR options { greedy=true; } : ('***')=>'***';
> //TRISTAR options {greedy=false;} : '***';
>
> Guntis
>
--
GMX FreeMail: 1 GB Postfach, 5 E-Mail-Adressen, 10 Free SMS.
Alle Infos und kostenlose Anmeldung: http://www.gmx.net/de/go/freemail
More information about the antlr-interest
mailing list