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Python Exercise for Regular Expressions: Password Validator Program

Objective:
Write a Python program that asks the user to enter a password and checks whether it is valid based on the following rules:

The password must be at least 8 characters long.
It must contain at least one uppercase letter (A–Z).
It must contain at least one lowercase letter (a–z).
It must contain at least one number (0–9).
It must contain at least one special character (e.g., !@#$%^&*()_+-=[]{}|;:'",.<>?/).

Your program should:

Tell the user if the password is valid or not.
If invalid, print the specific rule(s) that were not met.

Code Challenge: Allow the user to keep trying until they enter a valid password.

Solution

import re

def is_valid_password(password):
    if len(password) < 8:
        return False
    if not re.search(r"[A-Z]", password):  # At least one uppercase letter
        return False
    if not re.search(r"[a-z]", password):  # At least one lowercase letter
        return False
    if not re.search(r"\d", password):     # At least one digit
        return False
    if not re.search(r"[!@#$%^&*(),.?\":{}|<>]", password):  # Special character
        return False
    return True

# Example usage
password = input("Enter your password: ")
if is_valid_password(password):
    print("Valid password")
else:
    print("Invalid password")

Here’s an enhanced version that tells the user exactly which rules their password failed:

import re

def validate_password(password):
    errors = []

    if len(password) < 8:
        errors.append("Password must be at least 8 characters long.")
    if not re.search(r"[A-Z]", password):
        errors.append("Password must contain at least one uppercase letter.")
    if not re.search(r"[a-z]", password):
        errors.append("Password must contain at least one lowercase letter.")
    if not re.search(r"\d", password):
        errors.append("Password must contain at least one number.")
    if not re.search(r"[!@#$%^&*(),.?\":{}|<>]", password):
        errors.append("Password must contain at least one special character.")

    return errors

# Example usage
password = input("Enter your password: ")
validation_errors = validate_password(password)

if not validation_errors:
    print("Valid password.")
else:
    print("Invalid password:")
    for error in validation_errors:
        print("-", error)

For more python exercises, see Python Exercises