Parser built with Marpa::XS

parser.pl

use 5.10.1;
use strict;
use Marpa::XS;

my $grammar = Marpa::XS::Grammar->new({
    actions => 'Action',
    start   => 'Parser',
    rules => [
        { lhs => 'Parser', rhs => [ qw/Expression/ ], action => 'Parser' },
        { lhs => 'Expression', rhs => [qw/Term Plus Term/], action => 'Expression' },
        { lhs => 'Expression', rhs => [qw/Term/], action => 'Expression' },
        { lhs => 'Term', rhs => [qw/Factor Star Factor/], action => 'Term' },
        { lhs => 'Term', rhs => [qw/Factor/], action => 'Term' },
        { lhs => 'Factor', rhs => [qw/Number/], action => 'Number' },
    ],
    terminals => [qw/Number Plus Star/],
});

sub Action::Parser {
    my (undef, $expr) = @_;
    return $expr;
}
sub Action::Expression {
    my (undef, $left, undef, $right) = @_;
    if ($right) {
        return $left + $right;
    }
    return $left;
}

sub Action::Term {
    my (undef, $left, undef, $right) = @_;
    if ($right) {
        return $left * $right;
    }
    return $left;
}
sub Action::Number {
    my (undef, $number) = @_;
    return $number;
}

$grammar->precompute;

my %tokens = (
    Number => qr/^\d+/,
    Plus   => qr/^\+/,
    Star   => qr/^\*/,
);

my $line = <>;

my $r = Marpa::XS::Recognizer->new({
    grammar => $grammar,
});

while (length($line)) {
    $line =~ s/^\s+//;

    for my $token_name (@{$r->terminals_expected}) {
        my $re = $tokens{$token_name};
        my (@res) = ($line=~m/^($re)/);

        if (@res) {
            defined $r->alternative($token_name, $res[0], 1)
                or next;

            $line = substr $line, length($res[0]);

            my $ok = eval {
                $r->earleme_complete;
                1;
            };
            if (!$ok) {
                die "Error";
            }
        }
    }
}

$r->end_input;
say ${$r->value};