import re
re.sub(rb'([a-z])()([A-Z])', rb'\g<1>. \g<3>', b'djkfN ewEr')

  
>>> import re
>>> 
>>> s = 'abcaNDEFbNDEFaNGHIdef'
>>> 
>>> out = re.sub(r'([a-z])([A-Z])', r'\1. \2', s)
>>> out
'abca. NDEFb. NDEFa. NGHIdef'
>>>