Hm, I also can't seem to get the result from the regexp you suggested.
Code:
preprocess_regexps = [(re.compile(r'(?s)-->'), lambda m: '')]
preprocess_regexps = [(re.compile(r'(?s)-->'), lambda m: '')]
Still returns this in the final HTML:
Code:
<p class="calibre_2">-->--> </p>