docx to epub, questions about regex search & replace

I use calibre to convert my Word docx files to epub, and think it's great.

Now I'd like to try to automate some of the manual editing I do in Word before each conversion, by using the regex Search & replace facility, if possible.

Questions:

What exactly is the regex function being applied to? Is it working on some kind of html, or an internal docx format of the text?

Can the regex function recognize a multi-line target? And if so, replace it with fewer lines? And how?

Thanks.