function inputLimiter(e, allow, value) {
        var AllowableCharacters = '';
        if (allow == 'Letters') { AllowableCharacters = ' ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'; }
        if (allow == 'Numbers') { AllowableCharacters = '1234567890'; }
        if (allow == 'NameCharacters') { AllowableCharacters = ' ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-.\''; }
        if (allow == 'NameCharactersAndNumbers') { AllowableCharacters = '1234567890 ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-\''; }
        var k;
        k = document.all ? parseInt(e.keyCode) : parseInt(e.which);
        if (k != 13 && k != 8 && k != 0) {
            if ((e.ctrlKey == false) && (e.altKey == false)) {
                return ((AllowableCharacters.indexOf(String.fromCharCode(k)) != -1) && (value.length < 5));
            } else {
                return true;
            }
        } else {
            return true;
        }
    }

<input type="text" onkeypress="return inputLimiter(event,'Numbers', this.value)" />

Restricts the characters that are entered into a textbox. Also limits the length of the value to 5.

Javascript Character input restriction script, with length limiter

javascript

JavaScript