Atomic groups can help you: '<div>((?><p>.*?</p>))</div>'.

But this feature is not supported in the re module yet (see issue433030). You can use the third-party regex module which is compatible with the re module and supports atomic grouping.

>>> import regex as re
>>> pattern = re.compile('<div>((?><p>.*?</p>))</div>', flags=re.DOTALL)
>>> print(re.sub(pattern, '\\1',
...              '<div><p>foo</p>123456789</div>\n'
...              '<div><p>bar</p></div>\n'))
<div><p>foo</p>123456789</div>
<p>bar</p>