??? \s should be valid, but change it to \s*

<p class="calibre3"><i class="calibre1">Red Sky At Morning
\s*</i>\s*\d+</p>
The second \s* is because it appears there is a possible leading space befor the single digit.
If that does not work, try the 8.4.101 preview at
https://download.calibre-ebook.com/preview/