Regex Python - Modify random generated string

I need to read from file, two strings, one 'static' string from file, and one random dynamically generated one, which is also written on a file.

Then, replace one character in the random generated string with another random one.

And repeat the process. Until I get the "static" string which is on a file, in this case "THIS IS A STRING".

I'm pretty lost trying to achieve this, this is what I have so far:

import string
import random
import os
import re

file = open('file.dat', 'r')
file=file.read().split(',')
print file

def id_generator(size=28, chars=string.ascii_uppercase + string.digits):
    return ''.join(random.choice(chars) for _ in range(size))

if os.path.exists('newfile.txt'): os.remove('newfile.txt') else: file = open("newfile.txt", "w") file.write(id_generator()) file.close() if re.search(r"THIS IS A STRING", file): print("success!")

I'm trying to achieve this with re module, since it should read character by character, finding it's position in the random generated string.

Not just comparing but also finding the position of the matching characters (if any)

The file.dat file contains the THIS IS A STRING string, which I call the 'static' one, it doesn't changes, it should be matched by the random generated ones process .

The newfile.txt prints the random generated string.

So, in a nutshell, how can I read the string on file.dat character by character, and same goes for the random generated string on newfile.txt?

I hope I've explained myself.