I used calibre.
Don't know what nested tags are, but I'll give an example:
<p class="calibre3"> text here <i class="calibre2"> text here </i> text here </p>
Toxaris' regex did something like this to various paragraphs:
<p></em>some text that didn't need em <em>text that needed the em</em>regular text <em> </p>
Didn't make sense to me. Maybe my first <i class="calibre2">(\w+)</i> had something to do with it.