Adding a new feature

In this example, we’ll look at an existing codebase and add a new feature. The codebase is like the one we used in the previous example, but it’s a bit more complex. This is because it has a few more functions involving different types of payment methods. Here’s the code:

```python
import re
def validate_card(card):
    # Visa cards start with 4 and have 13 or 16 digits.
    # MasterCard cards start with 51 through 55 and have 16 digits.
    visa_pattern = r'^4[0-9]{12}(?:[0-9]{3})?$'
    mastercard_pattern = r'^5[1-5][0-9]{14}$'
    if re.match(visa_pattern, card):
        return 'Visa'
    elif re.match(mastercard_pattern, card):
        return 'MasterCard'
    else:
        return None
def process_payment(card, cart):
    card_type = validate_card(card)
    if card_type is None:
        return 1  # Invalid card
    else:
        # Process the payment here
        # Return 0 if the payment is successful...