[antlr-interest] Antlr won't backtrack?

[Vitaliy]

Hi Ter,

Thanks for your quick response!

I'm still confused:

1.       What can I do to suggest Antlr to consider the other alternative, and backtrack instead of failing?

2.       Shouldn't the backtrack option do just that - make Antlr backtrack if it has failed upon a given input?

3.       Why would the variable alternative effect its behavior?

Thanks allot for your help,
Vitaliy

From: Terence Parr [mailto:parrt at cs.usfca.edu]
Sent: Sunday, February 08, 2009 19:46
To: Vitaliy
Cc: antlr-interest at antlr.org
Subject: Re: [antlr-interest] Antlr won't backtrack?


On Feb 8, 2009, at 2:48 AM, Vitaliy wrote:
root                        : statements EOF;
statements            : (statement)+;
statement             : label
                                  | keyword
                                  | variable
                                  ;

identifier               : Identifier;
label                       : identifier COLON;
keyword                : KEYWORD (identifier)?;
variable                 : identifier;


"keyword label:" will always match rule keyword and then fail.  ANTLR will match greedily because you told it to shut up and backtrack.  It's hiding the warning that identifier can match now or later (during statements loop).

Ter


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