I'm not very much experienced in regex but maybe I can throw some light. The parentheses are used to create a group, so that when replacing you can refer to it. That's the meaning of '\1': keep the original set of characters (the group) as it is.
You can use more than one group in the expression you want to match, for instance:
<p class="calibre2"><span class="none">
([0-9]+\.)</span>blablabla
([0-9])</p>
Then, when replacing, you can refer to each group using
\1 and
\2.
Of course you can use as many groups as required depending on the complexity of the search.