my $mw = new MainWindow;
my $mw2 = $mw->Text()->pack();
my $mw3 = $mw->Button(-text=>"Check brackets",
                      -command => [ \&parser, $mw2 ]
                      )->pack();
MainLoop;

sub parser {
    my $txt = shift;
    my $grammar = q {
startrule: print open text end

print: "print"
open: "'"
text: /([^']*)/
end: "'"
};
    my $parser = Parse::RecDescent->new($grammar);
    if ( defined( $parser->startrule( $txt->get( "1.0", "end" )))) {
        print "NO ERROR\n";
    } else {
        print "TEXT COULD NOT BE PARSED\n";
    }
}