Algorithm for Credit Card Type Detection Based on Card Numbers

Credit Card Number Structure and Detection Principles

The credit card number, technically known as PAN (Primary Account Number), consists of the first six digits forming the IIN (Issuer Identification Number) following the ISO/IEC 7812 international standard. Although the complete IIN database is not publicly available, accurate card type identification can be achieved through regular expressions based on known number patterns.

Identification Patterns for Major Credit Card Types

Visa: Numbers start with digit 4, with regular expression ^4[0-9]{6,}$. Visa has historically issued 13-digit and 16-digit PANs, with current specifications allowing lengths from 12 to 19 digits.

MasterCard: Traditional identification range is 51-55, expanded to 222100-272099 after 2016. The complete regular expression is: ^5[1-5][0-9]{5,}|222[1-9][0-9]{3,}|22[3-9][0-9]{4,}|2[3-6][0-9]{5,}|27[01][0-9]{4,}|2720[0-9]{3,}$. Note that MasterCard system includes cards outside this IIN range, such as some Maestro cards.

American Express: Starts with 34 or 37, pattern ^3[47][0-9]{5,}$, typically 15 digits in length.

Diners Club: Starting digits include 300-305, 36, or 38, expression ^3(?:0[0-5]|[68][0-9])[0-9]{4,}$. Some co-branded cards with MasterCard start with 5 and have 16 digits, should be processed as MasterCard.

Discover: Starts with 6011 or 65, pattern ^6(?:011|5[0-9]{2})[0-9]{3,}$.

JCB: Begins with 2131, 1800, or 35, expression ^(?:2131|1800|35[0-9]{3})[0-9]{3,}$.

Implementation Considerations and Best Practices

Input Preprocessing: In practical applications, users may input numbers with spaces or punctuation (e.g., "4444 4444 4444 4444"). All non-digit characters should be removed before processing to avoid identification failures due to formatting issues.

Length Validation Strategy: PAN lengths vary among different issuers, with Visa potentially 12-19 digits. Strict length restrictions should be avoided. Minimum validation length is 7 digits (complete IIN plus 1 check digit) to ensure Luhn algorithm verification.

Luhn Algorithm Verification: After type identification, the Luhn algorithm should be used to validate number authenticity, preventing invalid numbers from entering processing pipelines.

Interface Design Considerations: Input fields should provide sufficient space, recommended to support 32-character display and 64-character input capacity, accommodating space-separated input habits.

Special Case Handling

For non-standard IIN cards within the MasterCard system, particularly Maestro cards, an "exclusion method" strategy is recommended: when a number doesn't match other known type patterns, default to MasterCard processing. This approach offers good practicality in most commercial scenarios.

Python Implementation Example

The following code demonstrates complete credit card type detection implementation:

import re

def detect_card_type(card_number):
    # Remove spaces and punctuation
    cleaned_number = re.sub(r"[^0-9]", "", str(card_number))
    
    # Define card type patterns
    patterns = {
        "Visa": r"^4[0-9]{6,}$",
        "MasterCard": r"^5[1-5][0-9]{5,}|222[1-9][0-9]{3,}|22[3-9][0-9]{4,}|2[3-6][0-9]{5,}|27[01][0-9]{4,}|2720[0-9]{3,}$",
        "American Express": r"^3[47][0-9]{5,}$",
        "Discover": r"^6(?:011|5[0-9]{2})[0-9]{3,}$",
        "Diners Club": r"^3(?:0[0-5]|[68][0-9])[0-9]{4,}$",
        "JCB": r"^(?:2131|1800|35[0-9]{3})[0-9]{3,}$"
    }
    
    for card_type, pattern in patterns.items():
        if re.match(pattern, cleaned_number):
            return card_type
    
    # Default processing
    return "MasterCard"

# Test examples
print(detect_card_type("4111 1111 1111 1111"))  # Visa
print(detect_card_type("5500 0000 0000 0004"))  # MasterCard
print(detect_card_type("3400 0000 0000 009"))   # American Express

This implementation includes input cleaning, pattern matching, and special case handling, suitable for most e-commerce scenarios. For actual deployment, appropriate adjustments based on specific business requirements and supported card types are recommended.