Textfields with permanent prefix

Always display prefix in inputfield no matter what the user does in the input

HTML

<form>
    <div class="ui-field-contain">
        <label for="pan">Member #</label>
        <input id="pan" name="pan" type="text" value="  /" />
    </div>
</form>

JavaScript

var prefix = '  /';
var backSpaceCode = 8;
var resetCursor = function (e) {
    var len = this.value.length;
    this.setSelectionRange(len, len);
};
var checkInput = function(e) {
    var currentValue = this.value;
    if (currentValue.length != prefix.length && currentValue.substr(0, prefix.length) !== prefix) {
         this.value = prefix;   
    }
    checkDelete(e);
    resetCursor(e);
};

var checkDelete = function (e) {
    var currentValue = this.value;
    var keyCode = e.which || e.keyCode;
    if (currentValue.length === prefix.length && keyCode === backSpaceCode) {
        e.preventDefault();
    }
};

var input = $('#pan').keydown(checkInput).keyup(checkInput).focus(resetCursor).mouseup(resetCursor);