Backrefs is a wrapper around Python's built-in Re and the 3rd party Regex library. Backrefs adds various additional back references (and a couple other features) that are known to some regular expression engines, but not to Python's Re and/or Regex. The supported back references actually vary depending on the regular expression engine being used as the engine may already have support for some.
from backrefs import bre
>>> pattern = bre.compile(r'(\p{Letter}+)')
>>> pattern.sub(r'\C\1\E', 'sometext')
'SOMETEXT'https://facelessuser.github.io/backrefs/
MIT