import re
stroka = """1. Last login: Sun Mar 11 23:53:01 2018 from 192.168.88.26
2. Lasted: 30 minutes
3. etc"""
slovo_dla_poiska = "Last"
regularka = re.compile(r'\b{}\b'.format(slovo_dla_poiska))
rezultat = "\n".join(["X" if regularka.search(line) else line for line in stroka.split('\n')])
print(rezultat)
aW1wb3J0IHJlCnN0cm9rYSA9ICIiIjEuIExhc3QgbG9naW46IFN1biBNYXIgMTEgMjM6NTM6MDEgMjAxOCBmcm9tIDE5Mi4xNjguODguMjYKMi4gTGFzdGVkOiAzMCBtaW51dGVzCjMuIGV0YyIiIgpzbG92b19kbGFfcG9pc2thID0gIkxhc3QiCnJlZ3VsYXJrYSA9IHJlLmNvbXBpbGUocidcYnt9XGInLmZvcm1hdChzbG92b19kbGFfcG9pc2thKSkKcmV6dWx0YXQgPSAiXG4iLmpvaW4oWyJYIiBpZiByZWd1bGFya2Euc2VhcmNoKGxpbmUpIGVsc2UgbGluZSBmb3IgbGluZSBpbiBzdHJva2Euc3BsaXQoJ1xuJyldKQpwcmludChyZXp1bHRhdCk=