Password Strength Checker using Regular Expressions

1) Project Overview

The Password Strength Checker helps users create strong and secure passwords by analyzing their password's structure and providing instant feedback.

This program uses regular expressions (regex) to evaluate if a password:

Contains uppercase and lowercase letters
Has at least one digit
Includes a special character
Meets a minimum length requirement

💡 Why it's useful: Weak passwords are a common cause of data breaches. This tool teaches users to understand password security and helps developers implement real-time validation in registration systems.

2) Learning Objectives

By completing this project, students will:

🧠 Understand Regular Expressions (re module) and pattern matching
🧩 Learn string validation using multiple conditions
💬 Practice user input handling and formatted output
🔄 Use functions for modular and reusable code
📈 Learn how to rate password strength dynamically (weak, medium, strong)

3) Step-by-Step Explanation

Follow these steps to build the password checker:

Import the Required Library – Python's built-in re (Regular Expressions) library helps us define complex validation patterns easily
Create the Password Validation Function – We'll define check_password_strength(password) to test the password against several regex patterns, calculate how many conditions it passes, and return a message indicating whether it's weak, medium, or strong
Define Validation Criteria – We'll check for:
- Minimum length (8 characters)
- At least one uppercase letter
- At least one lowercase letter
- At least one digit
- At least one special symbol (@, #, $, %, &, *, etc.)
Use Regex Patterns – Examples: [A-Z] → Uppercase letters, [a-z] → Lowercase letters, [0-9] → Digits, [@#$%^&*()_+=!~] → Special characters
Display the Result – Use conditional logic to classify: Weak → Less than 3 conditions met, Medium → 3–4 conditions met, Strong → All conditions met

4) Complete, Well-Commented, and Verified Python Code

You can copy this into a file named password_checker.py and run it.

Try it Yourself »

# ---------------------------------------------------
# 🔐 Password Strength Checker using Regular Expressions
# ---------------------------------------------------
# Author: Your Name
# Level: Intermediate
# Verified with Python 3.8+
# ---------------------------------------------------

import re

def check_password_strength(password):
    """Evaluates and returns the strength of a password."""
    
    # Initialize score
    score = 0

    # Criteria 1: Length at least 8
    if len(password) >= 8:
        score += 1

    # Criteria 2: Contains uppercase letter
    if re.search(r"[A-Z]", password):
        score += 1

    # Criteria 3: Contains lowercase letter
    if re.search(r"[a-z]", password):
        score += 1

    # Criteria 4: Contains a digit
    if re.search(r"[0-9]", password):
        score += 1

    # Criteria 5: Contains special character
    if re.search(r"[@#$%^&*()_+=!~]", password):
        score += 1

    # Determine strength
    if score <= 2:
        strength = "Weak 🔴"
    elif score == 3 or score == 4:
        strength = "Medium 🟡"
    else:
        strength = "Strong 🟢"

    return strength


# --------------------------
# Main Program Execution
# --------------------------
if __name__ == "__main__":
    print("🔐 Password Strength Checker")
    print("-" * 35)

    # Get password input from user
    user_password = input("Enter your password: ")

    # Check strength
    result = check_password_strength(user_password)

    # Display result
    print(f"\nPassword Strength: {result}")

    # Suggest improvement if weak
    if "Weak" in result:
        print("❗ Tip: Use uppercase, lowercase, digits, and symbols to make it stronger.")
    elif "Medium" in result:
        print("⚠️  Almost there! Add more variety (special chars or numbers).")
    else:
        print("✅ Excellent! Your password is strong and secure.")

✅ Verified: Tested in Python 3.8 and 3.11 — runs perfectly with no errors.
✅ Libraries Used: Only re (built-in).

5) Sample Output

Example 1: Weak Password

🔐 Password Strength Checker
-----------------------------------
Enter your password: hello

Password Strength: Weak 🔴
❗ Tip: Use uppercase, lowercase, digits, and symbols to make it stronger.

Example 2: Medium Password

Enter your password: Hello123

Password Strength: Medium 🟡
⚠️ Almost there! Add more variety (special chars or numbers).

Example 3: Strong Password

Enter your password: Hello@123

Password Strength: Strong 🟢
✅ Excellent! Your password is strong and secure.

6) Extension Challenge

🎯 Make it more advanced

Goal: Enhance your password checker with these features:

Add a Tkinter GUI: Create a user-friendly interface with a password input box and color-coded strength indicator
Visual Strength Bar: Use ASCII or progress bar (tqdm or tkinter.ttk.Progressbar) to visualize strength
Password Generator Integration: Combine this with a password generator to suggest strong random passwords automatically

7) Summary

In this project, you built a real-time password strength analyzer using Python's re module.

You practiced:

Regex pattern matching
Input validation
Conditional logic
Modular function design

💡 "Cybersecurity starts with strong passwords — and you've just automated that with Python!"

You've now taken a real-world problem and solved it using intermediate-level programming logic that's practical, reusable, and secure.

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