При попытке стереть маску после символа " - " символы перестают удаляться

window.addEventListener("DOMContentLoaded", function() {
  [].forEach.call(document.querySelectorAll('.CalendarBase > input'), function(input) {
    var keyCode;

    function mask(event) {
      event.keyCode && (keyCode = event.keyCode);
      var pos = this.selectionStart;
      if (pos < 3) event.preventDefault();
      var matrix = "____-____",
        i = 0,
        def = matrix.replace(/\D/g, ""),
        val = this.value.replace(/\D/g, ""),
        new_value = matrix.replace(/[_\d]/g, function(a) {
          return i < val.length ? val.charAt(i++) : a
        });
      i = new_value.indexOf("_");
      if (i != -1) {
        i < 5 && (i = 3);
        new_value = new_value.slice(0, i)
      }
      var reg = matrix.substr(0, this.value.length).replace(/_+/g,
        function(a) {
          return "\\d{1," + a.length + "}"
        }).replace(/[+()]/g, "\\$&");
      reg = new RegExp("^" + reg + "$");
      if (!reg.test(this.value) || this.value.length < 5 || keyCode > 47 && keyCode < 58) {
        this.value = new_value;
      }
      if (event.type == "blur" && this.value.length < 5) {
        this.value = "";
      }
    }

    input.addEventListener("input", mask, false);
    input.addEventListener("focus", mask, false);
    input.addEventListener("blur", mask, false);
    input.addEventListener("keydown", mask, false);

  });

});

<input name="brithday" type="text" class="form-control form-control-filter" placeholder="Дата рождения">