[antlr-interest] Simple expression grammar

[Johannes Luber]

Maciej Bakalarz schrieb:
> Hi !
> 
> I am newbie to ANTLR. I am trying to make very simple grammar which will 
> be used to build AST for Guard Condition expressions. Guard Conditions 
> expression are used as a labels in automata transitions.
> 
> This is my grammar:
> 
> grammar GuardCondition2;
> 
> options {
>     output=AST;
>     ASTLabelType=CommonTree;
> }
> 
> prog:
>     logical_or_expression+
>     ;
> 
> logical_or_expression:
>     logical_and_expression (OR_OP^ logical_and_expression)*
>     ;
> 
> logical_and_expression
>     : rel_expression (AND_OP^ rel_expression)*
>     ;
> 
> rel_expression
>     : expression (REL_OP^ expression)*
>     ;
> expression
>     : ALFANUM
>     ;
> 
> LPAREN     :    '(' ;
> RPAREN    :    ')' ;
> REL_OP       :      '==' | '<' | '>' | '<=' | '>='|'!=' ;
> OR_OP     :     '||' ;
> AND_OP     :     '&&' ;
> ALFANUM     :     (ID|INT)+;
> ID      :       ('a'..'z'|'A'..'Z')+ ;
> INT     :       '0'..'9'+ ;
> WS      :       (' '|'\t')+ {skip();} ;
> 
> 
> This grammar is working fine. Using this grammar I can parse expressions 
> like "a>=3 || b<=4 && c>=4".
> 
> I need to parse expressions which are using nested parenthesis, like:
> "( a>=3 || b<=4 ) && c>=4" or "((a>=3 || b<=4) && c>=4) || a>=3 )"
> 
> I wast trying to modify grammar to achieve this goal but I it always 
> came up that I am using Left Recursion.
> 
> Does anybody knows how to modify grammar so nested parenthesis would be 
> possible ?

Left-recusrion isn't that much of a showstopper as you believe. If your 
grammars orientates itself on the Java or C# grammar, then you can use 
<http://www.antlr.org/wiki/display/ANTLR3/Left-Recursion+Removal> to get 
rid of the left-recursion.

Johannes