You can add [\b] to match and allow backspace.

Code:

var regex = new RegExp("^[a-zA-Z0-9\b]+$");

Demo: http://jsfiddle.net/M3bvN/

UPDATE

Instead of extend your regex you can check if the pressed key is in a list of allowed keys (arrows, home, del, canc) and if so skip the validation.

This not prevent the user to copy/paste not allowed characters. so perform the validation control in the blur event too (and always on server side).

Code:

var keyCode = event.keyCode || event.which
// Don't validate the input if below arrow, delete and backspace keys were pressed 
if (keyCode == 8 || (keyCode >= 35 && keyCode <= 40)) { // Left / Up / Right / Down Arrow, Backspace, Delete keys
    return;
}

Demo: http://jsfiddle.net/M3bvN/3/

• onKeyValidate is an okay name, but a better name could be validateKeypress.

• It seems very silly to store a RegExp as a string, and then construct it every time. Why not just declare var alpha = /[ A-Za-z]/?

• keyChars appears to check against \x00, the null character, and \x08, the backspace character. Neither of these can ever be passed to onKeypress, so you can just take it out.

• The standard way to get the character code is event.which || event.keyCode.

• event is a global; I don't think you need to pass it in.

Here's a proposed rewrite:

var alpha = /[ A-Za-z]/;
var numeric = /[0-9]/; 
var alphanumeric = /[ A-Za-z0-9]/;

function validateKeypress(validChars) {
    var keyChar = String.fromCharCode(event.which || event.keyCode);
    return validChars.test(keyChar) ? keyChar : false;
}

The HTML will have to change to onkeypress="validateKeypress(alpha);".

.match() is returning either:

• An array of strings (just one in your existing code) in case of a match: in your case -- by luck -- it was coerced to a string, so it seemed to work as expected
• null in case of no match: that's why the entire input was lost

What you need is more like:

$('#custName').keyup(function() {
    $(this).val($(this).val().replace(/[^\w\s]+/g, ''));
});

As noticed by Hanlet, another potential issue with this approach is that some browsers will fire a keyup event for non printable characters. So, you may have to filter them with a test on event.keyCode.

What I did to make it work:

- Fixed the logical operators (& -> &&);
- Grouped the AND-groups with parentheses.

$('#sku').keypress(function(event)
{
    var kcode = event.keyCode;

    if (kcode == 8 ||
        kcode == 9 ||
        kcode == 46 ||
        kcode == 95 ||
        (kcode > 47 && kcode < 58) ||
        (kcode > 64 && kcode < 91) ||
        (kcode > 96 && kcode < 123))
    {
        return true;
    }
    else
    {
        return false;
    }
});

If you want to block the user input I would probably just handle this with an oninput regex. It's going to handle some additional edge cases, like pasting.

<InputText oninput="this.value = this.value.replace(/[^a-z0-9]/gi, '');" id="name" @bind-Value="@model.FirstName" />

For server side validation you can use the RegularExpressionAttribute on the model.FirstName property.