The Definitive ANTLR 4 Reference, 2nd Edition

Parser Rules

Parsers consist of a set of parser rules in either a parser or a combined grammar. A Java application launches a parser by invoking the rule function, generated by ANTLR, associated with the desired start rule. The most basic rule is just a rule name followed by a single alternative terminated with a semicolon.

/** Javadoc comment can precede rule */
retstat : 'return' expr ';' ;

Rules can also have alternatives separated by the | operator.

stat:   retstat
    |   'break' ';'
    |   'continue' ';'
    ;

Alternatives are either a list of rule elements or empty. For example, here’s a rule with an empty alternative that makes the entire rule optional:

superClass
    :   'extends' ID
    |                    // empty means other ...