antlr left recursion for nesting boolean expressions

Question

I am writing an antlr grammar in which I'd like to be able to have nested expressions, which can be either "simple" expressions or boolean expressions (with optional parentheses). A simple expression is just one with an lhs and rhs, such as a = 5

I'd like to be able to support these types of expressions:

a = 5
a = 5 OR b = 10
a = 5 OR (b = 10 AND c = 12)
(a = 5 AND b = 10) OR (c = 12 AND D = 13)

My grammar looks like:

STRING: CHAR+;
fragment CHAR: ('a'..'z' | 'A'..'Z' | '0'..'9'); 

booleanOp: 'AND' | 'OR';
simpleExpr: STRING '=' STRING;
expr: simpleExpr | parenExpr | booleanExpr;
parenExpr: '(' expr ')';
booleanExpr: expr (booleanOp expr)+;

I'm getting an error that expr and booleanExpr are mutually left recursive. I understand why that is happening, but I'm not really sure how to work around this if I want to be able to nest boolean expressions within each other.

Answer 1

On the homepage of www.antlr.org you can see this sample grammar:

grammar Expr;
  prog: (expr NEWLINE)* ;
  expr: expr ('*'|'/') expr
  | expr ('+'|'-') expr
  | INT 
  | '(' expr ')' ;

A little editing and it will be what you need. This is for ANTLR 4. Which version are you using? I'm sure every version of ANTLR has an expression grammar sample.