[antlr-interest] Expression Parser - identifying invalid expressions

[kumarsriram]

Hi,

I'm writing a glorified formula parser and borrowed some of the 
concepts from java.g bundled with the examples in antlr. I'm pretty 
new to the world of parsers, so pardon my ignorance.

I have the following sub-rules for conditional, relational, 
multiplicative, additive expressions that handle simple formulae.

The trouble is, my parser doesn't seem to catch invalid cases. For 
example, if there's a conditional expression of the 
form "expr1>expr2 & expr3<expr4" it doesn't throw an exception to 
indicate that "&" is invalid (it must be "&&"). It just takes the 
first predicate "expr1>expr2" and matches it against a "relExpr" 
rule and proceeds further.

So I wrote an explicit look ahead as follows:

// logical or (||)  
condExpr
	:(logicalAndExpression VAR )=>logicalAndExpression (LOR^ 
expr2:logicalAndExpression)+ 
	| logicalAndExpression
	;
    }
// logical and (&&)  
logicalAndExpression
	:(equalityExpr VAR)=>(expr1:equalityExpr) (LAND^ 
equalityExpr)+
	| equalityExpr
	;
    }

Is there any other simpler way of specifying the same? Otherwise, I 
need to perform an explicit lookahead in every rule.

Regards,
Sriram


The complete grammar is:

class MyParser extends Parser;
options {
	buildAST = true;
	k=1;
}

// logical or (||)  
condExpr
	:(logicalAndExpression VAR )=>logicalAndExpression (LOR^ 
expr2:logicalAndExpression)+ 
	| logicalAndExpression
	;
	exception
	catch [RecognitionException ex] {
       throw ex;
    }
// logical and (&&)  
logicalAndExpression
	:(equalityExpr VAR)=>(expr1:equalityExpr) (LAND^ 
equalityExpr)+
	| equalityExpr
	;
    }
// logical and (&&) 
equalityExpr
	:(relExpr (NOT_EQUAL^ | JAVA_EQUAL^))=> relExpr ((NOT_EQUAL^ 
| JAVA_EQUAL^) relExpr)* 
	| relExpr
	;
    }
// boolean relational expressions 
relExpr
	:	addExpr	 (GT^|GT_EQ^|LT^|LT_EQ^addExpr))*
	;
		
// binary addition/subtraction 
addExpr
	:	multExpr ((PLUS^ | MINUS^) multExpr)*
	;

// multiplication/division/modulo 
multExpr
	:	unaryExpr ((STAR^ | DIV^ | MOD^ ) unaryExpr)*
	;

unaryExpr
	:	MINUS^ unaryExpr
	|	PLUS^ unaryExpr
	|	unaryExprNotPlusMinus
	;

unaryExprNotPlusMinus
	:	LNOT^ unaryExpr
	|	postfixExpr
	;

postfixExpr:
	(id:VAR LPAREN)=>
		// Matches function call syntax like "pow(a,b)" 
		id2:VAR^
		(
 	      	parenArgs
		)?
	|atom;

parenArgs:	
	LPAREN^ (addExpr(COMMA addExpr)*)? RPAREN^;

atom:
	VAR 
	| NUM
	| CHAR_LITERAL
	| STRING_LITERAL
	| INPUT_VAR
	| LPAREN! condExpr RPAREN!;








 
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