$(document).ready(function () {
    $('.input_parcelnr').keypress(function (event) {
        if ($.inArray(event.which, [0, 8, 13, 48, 49, 50, 51, 52, 53, 54, 56, 55, 56, 57]) == -1) {
             event.preventDefault();
            return false;
        }
    });
});

//onkeypress = "if (event.keyCode != 13) { if (event.keyCode < '0'.charCodeAt(0) || event.keyCode > '9'.charCodeAt(0)) { event.keyCode = 0; return false; };}"