在php中替换嵌套标签中的Text

I need to do replace some text. For that I need a regexp pattern. Here the text is:

<div CLASS="CHAP_BM_CON">Outer <div CLASS="CHAP_BM_FIRST">DIV inner</div> 
Outer ends here</div><div CLASS="CHAP_BM_FIRST">Separate DIV Even inner</div>

In the above paragraph we can see one nested DIV. I need reg exp pattern to satisfy the below output like this:

<div CLASS="CHAP_BM_CON">Outer <div CLASS="CHAP_BM_FIRST">DIV inner <!--CHAP_BM_FIRST--></div> 
Outer ends here<!--CHAP_BM_CON--></div><div CLASS="CHAP_BM_FIRST">Separate DIV Even     inner<!--CHAP_BM_FIRST--></div>

How can we write the regexp for this case?

I tried the below code ..

<?php
function parseTagsRecursive($input)
{

 $regex = '/<div tagname="(.*?)" (.*?)>(.*?)<\/div>/si';    
if (is_array($input)) {

    $input = '<div tagname='.$input[1].' '.$input[2].'>'.$input[3].'<!--'.$input[1].'--></div>';

}   

return preg_replace_callback($regex, 'parseTagsRecursive', $input);

}

$content = '<div CLASS="CHAP_BM_CON">Outer <div CLASS="CHAP_BM_FIRST">DIV inner</div> 
Outer ends here</div><div CLASS="CHAP_BM_FIRST">Separate DIV Even inner</div';


echo $ver = parseTagsRecursive($content);