With jQuery or PHP, I must customize the dom structure from the the_content function in WordPress. In certain posts I personally use the h3 element, and I must give a wrapper that consists of this content before the next h3.

So I must convert this:

<h3>Title</h3>
<p>This is just regular text</p>
<h3>Next title</h3>

Into this:

<div class='wrapper'>
  <h3>Title</h3>
  <p>This is just regular text</p>
</div>
<div class='wrapper'>
  <h3>Next title</h3>
</div>

Thanks!

presuming the content only includes <h3>s and <p>s,and they're welled formated like:

<div id="content">
   <h3>title</h3>
    <p>..........</p>
    <p>..........</p>
   <h3>another title</h3>
    <p>.........</p>
   <h3>yet another title</h3>
    <p>..........</p>
 </div>

you might do this in jQuery.

  //get the main post content
  $content=$("#content");
  $h3s=$content.find('h3');
  $h3s.each(function(index){
       if(index==0)$(this).before('<div class="wrapper">');           
       else $(this).before('</div><div class="wrapper">');
  });
  //remeber to close the last one
  $content.append('</div>');