How do i add parens to this rule?

Question

I have a left-recursive rule like the following:

EXPRESSION   :  EXPRESSION BINARYOP EXPRESSION | UNARYOP EXPRESSION | NUMBER;

I need to add parens to it but i'm not sure how to make a left parens depend on a matching right parens yet still optional. Can someone show me how? (or am i trying to do entirely too much in lexing, and should i leave some or all of this to the parsing?)

Answer 1

You could add a recursive rule:

EXPRESSION      : EXPRESSION BINARYOP EXPRESSION
                | UNARYOP EXPRESSION
                | NUMBER
                | OPENPARENS EXPRESSION CLOSEPARENS
                ;

Answer 2

Yes, you're trying to do too much in the lexer. Here's how to get around the left-recursive rules:

http://www.antlr.org/wiki/display/ANTLR3/Expression+evaluator (see how the parser rule expr trickles down to the rule atom and then get called recursively from atom again)

HTH