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4.20. Validate Credit Card Numbers

Problem

You’re given the job of implementing an order form for a company that accepts payment by credit card. Since the credit card processor charges for each transaction attempt, including failed attempts, you want to use a regular expression to weed out obviously invalid credit card numbers.

Doing this will also improve the customer’s experience. A regular expression can instantly detect obvious typos as soon as the customer finishes filling in the field on the web form. A round trip to the credit card processor, by contrast, easily takes 10 to 30 seconds.

Solution

To keep the implementation simple, this solution is split into two parts. First we strip out spaces and hyphens. Then we validate what remains.

Strip spaces and hyphens

Retrieve the credit card number entered by the customer and store it into a variable. Before performing the check for a valid number, perform a search-and-replace to strip out spaces and hyphens. Replace all matches of this regular expression with blank replacement text:

[-]
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Recipe 3.14 shows you how to perform this initial replacement.

Validate the number

With spaces and hyphens stripped from the input, the next regular expression checks if the credit card number uses the format of any of the six major credit card companies. It uses named capture to detect which brand of credit card the customer has:

^(?:
(?<visa>4[0-9]{12}(?:[0-9]{3})?) |
(?<mastercard>5[1-5][0-9]{14}) |
(?<discover>6(?:011|5[0-9]{2})[0-9]{12}) |
(?<amex>3[47][0-9]{13}) |
(?<diners>3(?:0[0-5]|[68][0-9])[0-9]{11}) |
(?<jcb>(?:2131|1800|35[0-9]{3})[0-9]{11})
)$
Regex options: Free-spacing
Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10, Ruby 1.9
^(?:
(?P<visa>4[0-9]{12}(?:[0-9]{3})?) |
(?P<mastercard>5[1-5][0-9]{14}) |
(?P<discover>6(?:011|5[0-9]{2})[0-9]{12}) |
(?P<amex>3[47][0-9]{13}) |
(?P<diners>3(?:0[0-5]|[68][0-9])[0-9]{11}) |
(?P<jcb>(?:2131|1800|35[0-9]{3})[0-9]{11})
)$
Regex options: Free-spacing
Regex flavors: PCRE, Python

Java 4 to 6, Perl 5.8 and earlier, and Ruby 1.8 do not support named capture. You can use numbered capture instead. Group 1 will capture Visa cards, group 2 MasterCard, and so on up to group 6 for JCB:

^(?:
(4[0-9]{12}(?:[0-9]{3})?) |          # Visa
(5[1-5][0-9]{14}) |                  # MasterCard
(6(?:011|5[0-9]{2})[0-9]{12}) |      # Discover
(3[47][0-9]{13}) |                   # AMEX
(3(?:0[0-5]|[68][0-9])[0-9]{11}) |   # Diners Club
((?:2131|1800|35[0-9]{3})[0-9]{11})  # JCB
)$
Regex options: Free-spacing
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby

Standard JavaScript does not support named capture or free-spacing. Removing whitespace and comments, we get:

^(?:(4[0-9]{12}(?:[0-9]{3})?)|(5[1-5][0-9]{14})|↵
(6(?:011|5[0-9]{2})[0-9]{12})|(3[47][0-9]{13})|↵
(3(?:0[0-5]|[68][0-9])[0-9]{11})|((?:2131|1800|35[0-9]{3})[0-9]{11}))$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

If you don’t need to determine which type the card is, you can remove the six capturing groups that surround the pattern for each card type, as they don’t serve any other purpose.

Follow Recipe 3.6 to add this regular expression to your order form to validate the card number. If you use different processors for different cards, or if you just want to keep some statistics, you can use Recipe 3.9 to check which named or numbered capturing group holds the match. That will tell you which brand of credit card the customer has.

Example web page with JavaScript

<html>
<head>
<title>Credit Card Test</title>
</head>

<body>
<h1>Credit Card Test</h1>

<form>
<p>Please enter your credit card number:</p>

<p><input type="text" size="20" name="cardnumber"
  onkeyup="validatecardnumber(this.value)"></p>

<p id="notice">(no card number entered)</p>
</form>

<script>
function validatecardnumber(cardnumber) {
  // Strip spaces and dashes
  cardnumber = cardnumber.replace(/[ -]/g, '');
  // See if the card is valid
  // The regex will capture the number in one of the capturing groups
  var match = /^(?:(4[0-9]{12}(?:[0-9]{3})?)|(5[1-5][0-9]{14})|↵
(6(?:011|5[0-9]{2})[0-9]{12})|(3[47][0-9]{13})|(3(?:0[0-5]|[68][0-9])↵
[0-9]{11})|((?:2131|1800|35[0-9]{3})[0-9]{11}))$/.exec(cardnumber);
  if (match) {
    // List of card types, in the same order as the regex capturing groups
    var types = ['Visa', 'MasterCard', 'Discover', 'American Express',
                 'Diners Club', 'JCB'];
    // Find the capturing group that matched
    // Skip the zeroth element of the match array (the overall match)
    for (var i = 1; i < match.length; i++) {
      if (match[i]) {
        // Display the card type for that group
        document.getElementById('notice').innerHTML = types[i - 1];
        break;
      }
    }
  } else {
    document.getElementById('notice').innerHTML = '(invalid card number)';
  }
}
</script>
</body>
</html>

Discussion

Strip spaces and hyphens

On an actual credit card, the digits of the embossed card number are usually placed into groups of four. That makes the card number easier for humans to read. Naturally, many people will try to enter the card number in the same way, including the spaces, on order forms.

Writing a regular expression that validates a card number, allowing for spaces, hyphens, and whatnot, is much more difficult that writing a regular expression that only allows digits. Thus, unless you want to annoy the customer with retyping the card number without spaces or hyphens, do a quick search-and-replace to strip them out before validating the card number and sending it to the card processor.

The regular expression [-] matches a character that is a space or a hyphen. Replacing all matches of this regular expression with nothing effectively deletes all spaces and hyphens.

Tip

Credit card numbers can consist only of digits. Instead of using [-] to remove only spaces and hyphens, you could use the shorthand character class \D to strip out all nondigits.

Validate the number

Each of the credit card companies uses a different number format. We’ll exploit that difference to allow users to enter a number without specifying a company; the company can be determined from the number. The format for each company is:

Visa

13 or 16 digits, starting with 4.

MasterCard

16 digits, starting with 51 through 55.

Discover

16 digits, starting with 6011 or 65.

American Express

15 digits, starting with 34 or 37.

Diners Club

14 digits, starting with 300 through 305, 36, or 38.

JCB

15 digits, starting with 2131 or 1800, or 16 digits starting with 35.

If you accept only certain brands of credit cards, you can delete the cards that you don’t accept from the regular expression. When deleting JCB, make sure to delete the last remaining | in the regular expression as well. If you end up with || or |) in your regular expression, it will accept the empty string as a valid card number.

For example, to accept only Visa, MasterCard, and AMEX, you can use:

^(?:
4[0-9]{12}(?:[0-9]{3})? |         # Visa
5[1-5][0-9]{14} |                 # MasterCard
3[47][0-9]{13}                    # AMEX
)$
Regex options: Free-spacing
Regex flavors: .NET, Java, XRegExp, PCRE, Perl, Python, Ruby

Alternatively:

^(?:4[0-9]{12}(?:[0-9]{3})?|5[1-5][0-9]{14}|3[47][0-9]{13})$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

If you’re searching for credit card numbers in a larger body of text, replace the anchors with \b word boundaries.

Incorporating the solution into a web page

The section Example web page with JavaScript shows how you could add these two regular expressions to your order form. The input box for the credit card number has an onkeyup event handler that calls the validatecardnumber() function. This function retrieves the card number from the input box, strips the spaces and hyphens, and then validates it using the regular expression with numbered capturing groups. The result of the validation is displayed by replacing the text in the last paragraph on the page.

If the regular expression fails to match, regexp.exec() returns null, and (invalid card number) is displayed. If the regex does match, regexp.exec() returns an array of strings. The zeroth element holds the overall match. Elements 1 through 6 hold the text matched by the six capturing groups.

Our regular expression has six capturing groups, divided by alternation. This means that exactly one capturing group will participate in the match and hold the card number. The other groups will be empty (either undefined or the empty string, depending on your browser). The function checks the six capturing groups, one by one. When it finds one that is not empty, the card number is recognized and displayed.

Extra Validation with the Luhn Algorithm

There is an extra validation check that you can do on the credit card number before processing the order. The last digit in the credit card number is a checksum calculated according to the Luhn algorithm. Since this algorithm requires basic arithmetic, you cannot implement it with a regular expression.

You can add the Luhn check to the web page example for this recipe by inserting the call luhn(cardnumber); before the “else” line in the validatecardnumber() function. This way, the Luhn check will be done only if the regular expression finds a valid match, and after determining the card brand. However, determining the brand of the credit card is not necessary for the Luhn check. All credit cards use the same method.

In JavaScript, you can code the Luhn function as follows:

function luhn(cardnumber) {
  // Build an array with the digits in the card number
  var digits = cardnumber.split('');
  for (var i = 0; i < digits.length; i++) {
    digits[i] = parseInt(digits[i], 10);
  }  
  // Run the Luhn algorithm on the array
  var sum = 0;
  var alt = false;
  for (i = digits.length - 1; i >= 0; i--) {
    if (alt) {
      digits[i] *= 2;
      if (digits[i] > 9) {
        digits[i] -= 9;
      }
    }
    sum += digits[i];
    alt = !alt;
  }
  // Check the result
  if (sum % 10 == 0) {
    document.getElementById('notice').innerHTML += '; Luhn check passed';
  } else {
    document.getElementById('notice').innerHTML += '; Luhn check failed';
  }
}

This function takes a string with the credit card number as a parameter. The card number should consist only of digits. In our example, validatecardnumber() has already stripped spaces and hyphens and determined the card number to have the right number of digits.

First, we split the string into an array of individual characters. Then we iterate over the array to convert the characters into integers. If we don’t convert them, the sum variable will end up as a string concatenation of the digits, rather than the integer addition of the numbers.

The actual algorithm runs on the array, calculating a checksum. If the sum modulus 10 is zero, then the card number is valid. If not, the number is invalid.

See Also

Techniques used in the regular expressions in this recipe are discussed in Chapter 2. Recipe 2.3 explains character classes. Recipe 2.5 explains anchors. Recipe 2.8 explains alternation. Recipe 2.9 explains grouping. Recipe 2.11 explains named capturing groups. Recipe 2.12 explains repetition.

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