I don't have a solution for a sigil regex, but if you want to try the calibre editor, you can use the function mode.
The function mode gives you the possibility of more advanced text processing.
It should be perfect for your problem:
Open your book with the calibre editor.
Choose one of the html-files on the left panel and click Ctrl+F.
Insert "<table[^>]*>.*?</table>" in the field "Find:"
Choose Mode "Regex-function" and click create/edit.
In the new window you will have the basic structure of a function.
In the upper field you can insert a name (maybe "clean_tables") and the the given basic function you should replace with:
Code:
import regex
def replace(match, number, file_name, metadata, dictionaries, data, functions, *args, **kwargs):
return regex.sub(r'<p[^>]*>(.*?)</p>',r'\1',match.group())
Confirm with OK and you can use the new function.
Klecks