Permite en el campo

function permite( e , letras ) {
    var key = e.which,
        keye = e.keyCode,
        tecla = String.fromCharCode(key).toLowerCase();
    if (keye != 13) {
        if (letras.indexOf(tecla) == -1 && keye != 9 && (key == 37 || keye != 37) && (keye != 39 || key == 39) && keye != 8 && (keye != 46 || key == 46) || key == 161)
            e.preventDefault();
    }
}
permite( e , "asdghjklqweryuio")