@Ahmad - thx for the link. So is it the case now that every time you want to do a multiline regex you have to put (?s) at the front of it? If so, that's... ummm, nasty.
The other find/replace operations I did of things like this:
<p class="xxx"> </p>\s+<p class"xxx"> </p>
worked without any (?s) so clearly I am missing something. Most of the regex operations I do in Sigil are multiline - in particular repairing broken paragraphs but also stripping repeated blank lines, replacing <br><br> separated text with proper paragraphs etc.
However I also tend to rip out those horrific giant <style> sections with hundreds of lines of utterly pointless font declarations. Or <script> sections containing javascript from html conversions that early Sigil versions couldn't cope with.
I'm not clear on when you need (?s) versus when you don't?