Python program to check the validity of a Password?

Password validation is crucial for application security. A strong password should meet specific criteria including minimum length, mixed case letters, numbers, and special characters. Python's re module provides regular expressions to check these requirements efficiently.

Password Validation Rules

A valid password must satisfy the following criteria −

• Minimum 8 characters long
• At least one lowercase letter (a-z)
• At least one uppercase letter (A-Z)
• At least one digit (0-9)
• At least one special character (_ @ $)
• No whitespace characters

Algorithm

Step 1: Import re module for regular expressions
Step 2: Get password input from user
Step 3: Check minimum length (8 characters)
Step 4: Verify lowercase letters using [a-z]
Step 5: Verify uppercase letters using [A-Z]
Step 6: Verify digits using [0-9]
Step 7: Verify special characters [_@$]
Step 8: Ensure no whitespace characters

Implementation

# Python program to check valid password
import re

password = input("Enter Password ::> ")
is_valid = True

while True:   
    if len(password) < 8: 
        is_valid = False
        break
    elif not re.search("[a-z]", password): 
        is_valid = False
        break
    elif not re.search("[A-Z]", password): 
        is_valid = False
        break
    elif not re.search("[0-9]", password): 
        is_valid = False
        break
    elif not re.search("[_@$]", password): 
        is_valid = False
        break
    elif re.search(r"\s", password): 
        is_valid = False
        break
    else: 
        print("This is a valid password!")
        break

if not is_valid: 
    print("Not a valid password")

Example Output

Enter Password ::> vbnA@hj9
This is a valid password!

Enter Password ::> weak
Not a valid password

Enhanced Function-Based Approach

import re

def validate_password(password):
    """Validate password against security criteria"""
    
    # Check length
    if len(password) < 8:
        return False, "Password must be at least 8 characters long"
    
    # Check for lowercase
    if not re.search("[a-z]", password):
        return False, "Password must contain lowercase letters"
    
    # Check for uppercase  
    if not re.search("[A-Z]", password):
        return False, "Password must contain uppercase letters"
    
    # Check for digits
    if not re.search("[0-9]", password):
        return False, "Password must contain digits"
    
    # Check for special characters
    if not re.search("[_@$]", password):
        return False, "Password must contain special characters (_ @ $)"
    
    # Check for whitespace
    if re.search(r"\s", password):
        return False, "Password cannot contain whitespace"
    
    return True, "Password is valid"

# Test the function
test_passwords = ["weak", "Strong123@", "NoSpecial123", "short@1A"]

for pwd in test_passwords:
    is_valid, message = validate_password(pwd)
    print(f"'{pwd}': {message}")

'weak': Password must be at least 8 characters long
'Strong123@': Password is valid
'NoSpecial123': Password must contain special characters (_ @ $)
'short@1A': Password is valid

Conclusion

Regular expressions provide an efficient way to validate password strength in Python. The re.search() method checks each security requirement systematically, ensuring robust password validation for applications.