The Definitive ANTLR 4 Reference, 2nd Edition

Direct Left-Recursive Alternative Patterns

ANTLR examines any left-recursive rule looking for one of four subexpression operator patterns.

binary

Any alternative of the form expr op expr or expr (op1|op2|...|opN) expr. op can be a single-token or multitoken operator. For example, a Java grammar might treat angle brackets individually instead of treating operators <=> and >= as single tokens. Here’s an alternative that handles comparison operators at the same precedence level:

expr: ...
    | expr ('<' '=' | '>' '=' | '>' | '<') expr
   ...
    ;

op can also be a rule reference. For example, we can factor out those tokens into another rule.

expr: ...
    | expr compareOps expr
   ...
    ;
compareOps