The Definitive ANTLR 4 Reference, 2nd Edition

Left-Recursive Rule Transformations

If you turn on the -Xlog ANTLR command-line option, you can find the transformed left-recursive rules in the log file. Here’s what happens to rules stat and expr from Expr.g4 shown earlier:

// use "antlr4 -Xlog Expr.g4" to see transformed rules
stat:   expr[0] ';' ;  // match an expr whose operators have any precedence

expr[int _p]           // _p is expected minimum precedence level
    :   ( INT          // match primaries (non-operators)
        | ID
        )
        // match operators as long as their precedence is at or higher than
        // expected minimum
        ( {4 >= $_p}? '*' expr[5]  // * has precedence 4
        | {3 >= $_p}? '+' expr[4]  // + has precedence 3
        )*
    ;

Whoa! ...