binop.phpeg

-init {
    $operators = array(
        '+' => 10,
        '-' => 10,
        '*' => 20,
        '/' => 20,
        '^' => 30
    );

    $right_associative = array(
        '^' => TRUE,
    );

    $precedence = 0;
    $precedence_stack = array();
}

binary
  = l:primary
    ( op:operator
      ( ?-> $precedence > $operators[$op]
      / ?-> $precedence < $operators[$op]
        ( ?-> !isset($right_associative[$op])
          r:primary
          &( nextop:operator ?-> $operators[$op] >= $operators[$nextop]
           / !operator
           )
          { $l = array($op, $l, $r); }

        / ?-> isset($right_associative[$op])
          r:binary
          { $l = array($op, $l, $r); }

        / ?{ array_push($precedence_stack, $precedence);
             $precedence = $operators[$op];
             return TRUE; }
          ( r:binary
            { $precedence = array_pop($precedence_stack);
              $l = array($op, $l, $r); }
          / ?{ $precedence = array_pop($precedence_stack);
               return FALSE; }
          )
        )
      )
    )*
    -> $l

operator
  = [-+*/^]

primary
  = "(" e:binary ")" -> $e
  / i:[0-9]+ -> intval($i)