I'm writing a Greasemonkey script, and I wanted to change the text of a forum post on a phpBB2 forum by using XPath to select the body of the post that occurs after a certain username, but the whole thing is a giant mess of tables.

<tr>
  <td>
    <span class="name">
      <a>
      <b>username</b>
    </span>
    <span></span>
  </td>
  <td>
    <table>
      <tbody>
        <tr></tr>
        <tr></tr>
        <tr>
          <td>
            <span class="postbody">text of post</span>
            <span></span>
          </td>
        </tr>
      </tbody>
    </table>
  </td>
</tr>

I need to get the postbody span that happens after the username in the b tag equals a certain name, and then mess with the text.