import re
# Define the input string
str = "###Word1 Word2\nWord3\nWord4 Word5 Word6\n##Word7\n#Word8 Word9\n"
# Use re.sub to replace the pattern with the desired format
str = re.sub(r'#+(.*)\n', r'_\1_\n', str)
print(str)
'''
run:
_Word1 Word2_
Word3
Word4 Word5 Word6
_Word7_
_Word8 Word9_
'''