Python program to check credit card number is valid or not

Credit card numbers must follow specific formatting rules to be considered valid. We need to check if a credit card number meets all the required criteria including format, length, and digit patterns.

Credit Card Validation Rules

A valid credit card number must satisfy these conditions ?

• Start with 4, 5, or 6

• Be exactly 16 digits long

• Contain only numeric digits

• May have digits grouped in four sets separated by hyphens

• Must not use spaces, underscores, or other separators

• Must not have 4 or more consecutive identical digits

Implementation

Here's a complete solution to validate credit card numbers ?

import re

def is_valid_credit_card(card_number):
    # Check if card has hyphens
    if card_number.count("-") > 0:
        # Split by hyphens
        parts = card_number.split("-")
        
        # Must have exactly 4 groups
        if len(parts) != 4:
            return False
        
        # Each group must have exactly 4 digits
        for part in parts:
            if len(part) != 4 or not part.isdigit():
                return False
    else:
        # No hyphens - must be 16 digits
        if len(card_number) != 16 or not card_number.isdigit():
            return False
    
    # Remove hyphens for further validation
    clean_number = card_number.replace("-", "")
    
    # Check if starts with 4, 5, or 6 and is 16 digits
    if not re.match(r"^[456][0-9]{15}$", clean_number):
        return False
    
    # Check for 4+ consecutive same digits
    if re.search(r"([0-9])\1{3,}", clean_number):
        return False
    
    return True

# Test cases
test_cards = [
    "5423-2578-8632-6589",
    "4253625879615786", 
    "5555-5555-5555-5555",  # Invalid: 4+ consecutive same digits
    "51-67-89-45",          # Invalid: not 4 digits per group
    "4424444424442444"      # Invalid: 4+ consecutive same digits
]

for card in test_cards:
    result = is_valid_credit_card(card)
    print(f"Card: {card} -> Valid: {result}")

Card: 5423-2578-8632-6589 -> Valid: True
Card: 4253625879615786 -> Valid: True
Card: 5555-5555-5555-5555 -> Valid: False
Card: 51-67-89-45 -> Valid: False
Card: 4424444424442444 -> Valid: False

How the Validation Works

The validation process follows these steps ?

1. Format Check: If hyphens are present, ensure exactly 4 groups of 4 digits each

2. Length Check: Without hyphens, the number must be exactly 16 digits

3. Starting Digit: Must begin with 4, 5, or 6

4. Consecutive Digits: No 4 or more identical consecutive digits allowed

Key Regular Expressions

• ^[456][0-9]{15}$ - Matches numbers starting with 4, 5, or 6 followed by 15 digits

• ([0-9])\1{3,} - Detects 4 or more consecutive identical digits

Conclusion

This credit card validator checks format, length, starting digit, and consecutive digit patterns. Use regular expressions for pattern matching and string methods for format validation.