Preventing Google from indexing the contents of a div by reversing the string

I want to prevent that Google indexes the contents of one on my page

Then I think you shouldn't put that content on the page, period.

You could try using the googleon/googleoff tags, per this article:

Tell Google to Not Index Certain Parts of Your Page

<!--googleoff: index-->
don't index this content
<!--googleon: index-->

Then again, I find this article which states that it isn't possible:

http://productforums.google.com/forum/#!topic/webmasters/qrBI_v-N4N0

How to tell Google not to? =============

You don't!

If it is content, If it is part of that page, then it Will be Crawled, and may be Indexed and Ranked

You cannot use a Meta-Tag, or a HTML tag to tell Google to ignore, discount, not use, refer or touch part of your content.

I asked on Google forums and the answer is: It doesn't

Updating this thread. While google will still crawl and index, you can prevent it from showing up in search results with data-nosnippet attribute in the HTML. Can be used in <div>, <span>, and <section> elements.

Example: <p><span data-nosnippet>Harry Houdini</span> is undoubtedly the most famous magician ever to live.</p>

See here

I used server side codes to hide div from googlebot

<?php if(self::isNotGoogleBot()):?>
<div id="noindex"></div>
<?php endif?>
public static function isNotGoogleBot()
    {
        $ua = strtolower($_SERVER['HTTP_USER_AGENT']);
        if(strpos($ua,'googlebot') === false && strpos($ua,'mediapartners-google') === false)return true;
        return false;
    }