Its not working. I don't know regEx, but I need use it.
if ($('input[name="due_date"]').val().match("^(0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])[- /.](19|20)\\d\\d$")) {
$('input[name="due_date"]').after("<span class='v_error'>Must fill</span>");
}