# DELETABLE PRODUCTIONS

use Parse::RecDescent;

$grammar =
q{
    commands :    command(s)

    command  :    directive
         |    evaluation

    directive:    'only prefix'
                { $thisparser->{deleted} = {infix=>1,postfix=>1} }
         |    'not prefix'
                { $thisparser->{deleted}{prefix} = 1 }

    evaluation:    <reject: $thisparser->{deleted}{prefix}> op var var
                { print "prefix $item[3] $item[2] $item[4]\n" }

              |    <reject: $thisparser->{deleted}{infix}> var op var
                { print "infix $item[2] $item[3] $item[4]\n" }

              |    <reject: $thisparser->{deleted}{postfix}> var var op
                { print "postfix $item[2] $item[4] $item[3]\n" }

    op        :    '+' | '-'

    var      :    /(?!\d)\w+/
};

$parse = new Parse::RecDescent ($grammar);

while (<DATA>)
{
    $parse->commands($_);
}

__DATA__
a + b
+ a b
a b +
not prefix
a + b
+ a b
a b +
only prefix
a + b
+ a b
a b +
not prefix
a + b
+ a b
a b +