[antlr-interest] Confused about backtracking

[Terence Parr]

> expr	:	ID suffix;

will try to match the sep '.'.  backtrack takes first alt that wins and the decision for suffix needs to backtrack due to your program rule. therefore it attempts '.' expr and fails.

allowing ;;;;…;;;;… as a program is not a good idea ;)

T
On Nov 26, 2011, at 4:21 AM, franck102 wrote:

> I think I am missing something fundamental about backtracking. The grammar
> below won't parse input such as
> 
> a=b.
> c=d;
> 
> ... even though I would expect it to backtrack after realizing that a=b.c
> leads to a dead-end. What am I missing?
> 
> Thanks!
> 
> PS: I am not looking for refactoring options, I have the issue in a complex
> grammar that can't easily be refactored.
> 
> =====================================
> grammar Test;
> 
> options {
> 	output=AST;
> 	backtrack=true;
> }
> 
> program
> 	: statement* EOF
> 	;
> 	
> statement
> 	:	ID '=' expr
> 	|	sep
> 	;
> 
> expr	:	ID suffix;
> 
> suffix	:	( '.' expr )*
> 	;
> 
> sep	:	';' | '.';
> 
> // LEXER
> // ==========================================
> 
> ID
> 	:	('a'..'z'|'A'..'Z'|'_') ('a'..'z'|'A'..'Z'|'0'..'9'|'_')*
> 	;
> 
> 
> WS  :   ( ' '
>        | '\t'
>        | '\r'
>        | '\n'
>        ) {$channel=HIDDEN;}
>    ;
> 
> 
> --
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