How to prevent invalid characters from being typed into input fields

Question

Onkeydown, I run the following JavaScript:

function ThisOnKeyDown(el) {
   if (el.title == 'textonly') {
       !(/^[A-Za-zÑñ-\s]*$/i).test(el.value) ? el.value = el.value.replace(/[^A-Za-zÑñ-\s]/ig, '') : null;
   }
   if (el.title == 'numbersonly') {
       !(/^[0-9]*$/i).test(el.value) ? el.value = el.value.replace(/[^0-9]/ig, '') : null;
   }
   if (el.title == 'textandnumbers') {
       !(/^[A-Za-zÑñ0-9-\s]*$/i).test(el.value) ? el.value = el.value.replace(/[^A-Za-zÑñ0-9-\s]/ig, '') : null;
   }
}

One of these three title attributes is given to various input fields on the page. The code works so far as invalid characters are correctly erased, but not until the next character is entered. I want to find a way to simply deny the invalid input in the first place. I appreciate your help!

Edit: I create the events globally. Here's how I do that:

      function Globalization() {
      var inputs = document.getElementsByTagName('input');
      for (i = 0; i < inputs.length; i++) {
          inputs[i].onfocus = createEventHandler(
              ThisOnFocus, inputs[i]);
          inputs[i].onblur = createEventHandler(
              ThisOnBlur, inputs[i]);
          inputs[i].onkeydown = createEventHandler(
              ThisOnKeyDown, inputs[i]);
          inputs[i].onkeyup = createEventHandler(
              ThisOnKeyUp, inputs[i]);
      }
  }

Globalization() is run body.onload

Therefore, a typical input field has HTML without function calls like this:

          <input id="AppFirstName" style="width: 150px;" type="text" maxlength="30" title="textonly"/>

Answer 1

To prevent it from being set in the first place, you can return false on the keydown event handler, thus preventing the event from propagating any further.

I wrote the example below using jQuery, but you can use the same function when binding traditionally.

Though it's important to validate on the server-side as well, client-side validation is important for the sake of user friendliness.

$("input.number-only").bind({
    keydown: function(e) {
        if (e.shiftKey === true ) {
            if (e.which == 9) {
                return true;
            }
            return false;
        }
        if (e.which > 57) {
            return false;
        }
        if (e.which==32) {
            return false;
        }
        return true;
    }
});