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import re
with open ('file1.txt') as fil1:
     for line in fil1:
         re.sub('^*{3}[*]*','**',line)

import fileinput

with fileinput.FileInput('file1.txt', inplace=True) as fil1:
    for line in fil1:
        print(re.sub('^*{3}[*]*','**', line.rstrip())) # inside the `with` clause, print writes to the file automatically