lexer grammar Test2;

@members {
  public static void main(String[] args) {
    Test2 lexer = new Test2();
    lexer.setCharStream(new ANTLRStringStream("cb"));
    System.out.println("Token="+lexer.nextToken());
  }
}

CAB:
  ( ( 'c' 'a' )
  | ( { false }? 'c' 'b' )
  )
  ;