[antlr-interest] How to use greedy in antlr3

[Kay Roepke]

Hi Martina!

On 28. Nov 2006, at 16:00 , Pantzagias, Martina wrote:

> This works fine, but if I add the second alternative an warning  
> occurs (--> multiple alternatives):
>
> Tag
>     :    '{' (options{greedy=false;}: .)* '}'
>     |    '{!{' (options{greedy=false;}: .)* '}!}'
>
>
> How can I solve this problem?

There are two ways:
One with a warning, but correct behavior, the other without a warning  
but a (slight) performance hit.

The important thing to note with ANTLR is that it will always do the  
matching in the order of the alts in a rule.
In your rule ANTLR tells you that it disabled alt2 for input like  
'{', because the first alt already matches.
Simply reversing the alts will match correctly.
The second option you can take is to use a syntactic predicate to  
disambiguate (and to shut up the warning).
The grammar below will make it clear (hopefully :)...

grammar Greedy;

tags:	TAG+;
/* This rule doesn't warn */
/*
TAG	:	('{!{') => '{!{' (options{greedy=false;}: .)*  
{ System.err.println("alt1"); }'}!}'
	|	'{'	(options{greedy=false;}: .)*{ System.err.println("alt2"); } '}'
	;
*/

/* This warns, but will match correctly */
TAG	:	'{!{' (options{greedy=false;}: .)* { System.err.println("{! 
{"); } '}!}'
	|	'{' (options{greedy=false;}: .)* { System.err.println("plain  
{"); }'}'
	;

WS	:	(' ' | '\t' | '\n' | '\r') { skip(); }
	;

In AW, you need to use the debugger and watch the "Output" to see the  
System.err... calls.

HTH,
-k

-- 
Kay Röpke
http://classdump.org/