You can search for
^(.*)(\r?\n\1)+$Don’t worry: all the duplicate lines will be selected, but not all deleted. Then give this as replace
\1Regex for first lowercase letter
If you have en ebook that split badly a paragraph, you can try (to merge the bad splitted paragraph), as find
</p>\n\n\s\s<p>([a-z])so that a paragraph beginning with a lowercase word be merged with the previous one.
But in Calibre editor you must check “case sensitive”, otherwise [a-z] will be read as [A-Z], and all first letters (lower and upper case) will be selected.
And as replace you could try
\1In this way you keep whatever letter is found with ([a-z]) in the result.
Regex for “space”
It can happen that you want replace a double space before <p>: use the \s. I.g. to replace
,</p>
<p>
You should use this find: ,</p>\n\n\s\s</p>
Regex replace tags around digits
If you would replace all <p>[some digits, such as "10", or "23", or "348"]</p>, you could use this regex code:
search: <p>(.*?\d{2})</p>
replace: <h6>\1</h6>
Indeed (.*?) is whatever, and (.*?\d{2}) is whatever number (with more than two digits).
regex replace all the text within an html tag
If you want, for example, delete all styles tag within several html in an epub file, keeping only the stylesheet linked in them, you can use this command:
<style type="text/css">(.|\n)*?<\/style>
Calibre editor in this case is better than Sigil, for epub files.