Javascript, how to prevent invalid characters from showing in a text field

Let's say I only want the user to input alphabetic and numeric (no punctuations of any kind) within a text field. I can detect this through the various key events, but what I really want to accomplish is to prevent the invalid character from even showing in the text field.

For example, if I keyed in A, it would show in the text field since it's valid. But when I type in a '>', it is invalid and i don't want it to show in the text field. I've tried using event.preventDefault(), but it's not working.

Suggestions, anyone? Here is my code snippet:

<!DOCTYPE html>
<script src="//"></script>
    $(function() {
        $("#txtText").keyup(function(event) {
            var sInput = String.fromCharCode(event.keyCode);
            if ("ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".indexOf(sInput) !== -1) {
            } else {
                console.log("Invalid character");
                event.preventDefault(); // NOT WORKING, CHARACTER STILL SHOWS IN THE TEXT FIELD.

    <input id="txtText" />

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Russ SuterSenior Software DeveloperCommented:
You could try something like this:

$(document).ready(function() {
    $('#contactNumber').keyup(function() {
        var numbers = $(this).val();
        $(this).val(numbers.replace(/\D/, ''));

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Or, instead of event.PreventDefault() you could just return false. That should do the trick.
RobOwner (Aidellio)Commented:
Problem is the event you're using.  Keyup has already inserted the character.  Use Keydown instead.


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elepilAuthor Commented:
You nailed it, Rob. Here's a part 2 to my question at

Thanks to Russ, too, for responding.
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