Suppose that a bank only permits passwords that are strings from the alphabet = {a, b, c, d, 1, 2, 3, 4} that follow the rules:

 the length is at least five characters

 it begins with a letter {a, b, c, d}

 it ends with two digits {1, 2, 3, 4}

the set of legal passwords forms a regular language l. construct a regular expression r such that l(r) = l.

#Python

import re

password = input("Enter password: ")

p = re.compile('^[ABCDabcd]+.*[^e-zE-Z5-9_]{2,}[1-4][1-4]+$')

match = p.search(password)

if match == None:

    print('L(r) != L -> Incorrect password')

else:

    print('L(r) = L -> Correct password')

Explanation:

The regular expression needed is:

^[ABCDabcd]+.*[^e-zE-Z5-9_]{2,}[1-4][1-4]+$

To understand why see the step by step of the regex:

Note: if you don't have python you can use an online regex checker like myregextester, note that the string must be at least five characters long because you need one letter at the beginning, at least two characters in the middle and two numbers at the end.

1

explanation:

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hope this is what you're talking about