# PARSE LOGICAL EXPRESSIONS

use Parse::RecDescent;

$grammar =
q{
    expr    :    '[' <leftop: atom /,?/ atom>(?) ']'     { $item[2] }
        |       '[[' atomlist ']]'     { $item[2] }
        |    disj  no_garbage     { $item[1] }
        |    <rightop: atom '=' num>
        |    <leftop: atom '<<' num>

    atomlist: <leftop: atom /,?/ atom>

    no_garbage: /^\s*$/
          | <error: Trailing garbage: $text>

    disj    :    <leftop: conj /(or)/ conj >

    conj    :    <leftop: unary 'and' unary >

    and    :     'and'

    unary    :    'not' atom
        |    '(' expr ')'    { $item[2] }
        |    atom

    atom    :    /(?!and|or)[a-z]/

    num    :     /\d+/

};

$parse = new Parse::RecDescent ($grammar) or die "bad grammar";

$input = '';

use Data::Dumper;

print "> ";
while (<>)
{

    if (/^\.$/) { print STDERR Data::Dumper->Dump([$parse->expr($input)]);
              $input = '' }
    else        { $input .= $_ }
    print "> ";
}