With just a regex, there's nothing simple about a search for a tag with a specific "class=" and then removing that tag (and it's closing tag). For example, build a general-purpose regex that successfully deletes the tag with class "deleteme", along with the matching closing tag:
Code:
<span class="keep01">Here <span class="deleteme">is <span class="alsokeep">the</span> text <span class="dontdelete">in the</span> book</span> that should all remain intact.</span>
This operation is trivial with a real HTML editor (I use Expression Web), but Sigil (and Calibre) don't have anything that can do this, and that's a simple operation compared to what you can do with a real editor.