[Doitsu]

I haven't looked at your code in detail, but it looks like you're using both bs4 and regular expressions, which, IMHO, isn't a good idea.

The invalid code that Sigil is reporting is caused by the following lines in LSH.xhtml

before:

<div class="illus">
<img alt="" class="img065" src="../Images/lsh-23-065.png"/></div>

after:

<div class="illus">

<img alt="" class="mobionly" height="441px" src="../Images/lsh-23-065.png" width="630px"/>

<img alt="" class="kf8only" src="../Images/lsh-23-065.png" style="width: 100%;height: auto;"/>

For some reason your code removed the closing </div> tag.

I don't know what the exact cause is, but since you seem to be using regular expressions, it might help to normalize the HTML input using the Sigil bs4 prettyprint_xhtml function. When I ran the following plugin prior to running yours, I didn't get any error messages:

Spoiler:

#!/usr/bin/env python
# -*- coding: utf-8 -*-
from sigil_bs4 import BeautifulSoup

def run(bk):
    html_id = 'LSH.xhtml'
    html = bk.readfile(html_id)
    soup = BeautifulSoup(html, 'html.parser')
    normalized_html = str(soup.prettyprint_xhtml(indent_level=0, eventual_encoding="utf-8", formatter="minimal", indent_chars="  "))
    bk.writefile(html_id, normalized_html)

    return 0

def main():
    print('I reached main when I should not have\n')
    return -1

if __name__ == "__main__":
    sys.exit(main())