# Checking Credit Cards (#122)

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1. Please do not post any solutions or spoiler discussion for this quiz
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1. Enjoy!

Suggestion: A [QUIZ] in the subject of emails about the problem helps
everyone
message,
if you can.

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

Before a credit card is submitted to a financial institution, it
generally makes
sense to run some simple reality checks on the number. The numbers are
a good
length and it’s common to make minor transcription errors when the card
is not
scanned directly.

The first check people often do is to validate that the card matches a
known
pattern from one of the accepted card providers. Some of these patterns
are:

+============+=============+===============+
| Card Type | Begins With | Number Length |
+============+=============+===============+
| AMEX | 34 or 37 | 15 |
±-----------±------------±--------------+
| Discover | 6011 | 16 |
±-----------±------------±--------------+
| MasterCard | 51-55 | 16 |
±-----------±------------±--------------+
| Visa | 4 | 13 or 16 |
±-----------±------------±--------------+

All of these card types also generate numbers such that they can be
validated by
the Luhn algorithm, so that’s the second check systems usually try. The
steps
are:

1. Starting with the next to last digit and continuing with every
other
digit going back to the beginning of the card, double the digit
2. Sum all doubled and untouched digits in the number
3. If that total is a multiple of 10, the number is valid

For example, given the card number 4408 0412 3456 7893:

Step 1: 8 4 0 8 0 4 2 2 6 4 10 6 14 8 18 3
Step 2: 8+4+0+8+0+4+2+2+6+4+1+0+6+1+4+8+1+8+3 = 70
Step 3: 70 % 10 == 0

Thus that card is valid.

Let’s try one more, 4417 1234 5678 9112:

Step 1: 8 4 2 7 2 2 6 4 10 6 14 8 18 1 2 2
Step 2: 8+4+2+7+2+2+6+4+1+0+6+1+4+8+1+8+1+2+2 = 69
Step 3: 69 % 10 != 0

That card is not valid.

This week’s Ruby Q. is to write a program that accepts a credit card
number as
a command-line argument. The program should print the card’s type (or
Unknown)
as well a Valid/Invalid indication of whether or not the card passes the
Luhn
algorithm.

On Apr 27, 2007, at 8:59 PM, Ruby Q. wrote:

1. Enjoy!
Before a credit card is submitted to a financial institution, it
patterns are:
| Visa | 4 | 13 or 16 |
digit going back to the beginning of the card, double the digit

a command-line argument. The program should print the card’s type
(or Unknown)
as well a Valid/Invalid indication of whether or not the card
passes the Luhn
algorithm.

For completeness and to make this Quiz exercise valid to more people,
can anyone include the information for other major credit cards from
major countries? Japan: JCB, et. al., U.K.: Barclay, et. al. , etc…
This would be good to expand the exercise and return more to our
friends in various areas!

For completeness and to make this Quiz exercise valid to more people,
can anyone include the information for other major credit cards from
major countries? Japan: JCB, et. al., U.K.: Barclay, et. al. , etc…

UK had Switch and Solo till recently which are now rebranded under the
Maestro umbrella. They’re not really credit cards and basically no
rules exist, you have to check tables to determine valid prefixes and
card number length. Additionally, Maestro card numbers may also be
Mastercard numbers, so they can’t be uniquely identified as being
Maestro.

For JCB and Diner’s:

JCB 3528-2589 Length: 16
Diners 3000-3029, 3040-3059, 36, 3815-3889, 389 Length: 14

Cheers,
-Tim

Ruby Q. [email protected] writes:

This week’s Ruby Q. is to write a program that accepts a credit
card number as a command-line argument. The program should print
the card’s type (or Unknown) as well a Valid/Invalid indication of
whether or not the card passes the Luhn algorithm.

May I make an additional suggestion? Treat spaces in the card number
as perfectly valid, and simply strip them before processing. One of
the most irritating features found in many online stores is the
requirement that the credit card number be typed in with no spaces.
(The other biggie is using a select box for the state abbreviation)

Now granted, for accepting the card number from the command line this
will mean that on the command line the card number was quoted, but
presumably most people’s programs will be a command-line wrapper
around a function/class that does the real work.

On Apr 27, 2007, at 9:22 PM, Tim B. wrote:

Maestro.

For JCB and Diner’s:

JCB 3528-2589 Length: 16
Diners 3000-3029, 3040-3059, 36, 3815-3889, 389 Length: 14

Cheers,
-Tim

Wow, those are some broad ranges.

On Apr 27, 2007, at 7:36 AM, Daniel M. wrote:

Now granted, for accepting the card number from the command line this
will mean that on the command line the card number was quoted…

It doesn’t have to mean that:

\$ ruby -e ‘p ARGV.join’ 1111 2222 3333 4444
“1111222233334444”

James Edward G. II

On Apr 27, 2007, at 10:37 PM, Tim B. wrote:

still uses them.

If you think those ranges are broad, the last UK-Switch/Solo table I
had has over 180 card prefixes with different card lengths…

My grandmother used to have a Diners Club card. Last time I saw that
was about 20 years ago too.
But of the cards accepted here in Tokyo, Diners Club is one that is
often listed! (in bars and restaurants of course)

Oh, I can only imagine. Processing cards with less deeply thought out
numbering schemes. The big boys planned that stuff back when

JCB 3528-2589 Length: 16

should be 3528-3589, obviously.

Diners 3000-3029, 3040-3059, 36, 3815-3889, 389 Length: 14

Wow, those are some broad ranges.

Well, those ranges DO include Carte Blanche I haven’t seen a
physical Diners Card in at least 20 years to be honest, not sure who
still uses them.

If you think those ranges are broad, the last UK-Switch/Solo table I
had has over 180 card prefixes with different card lengths…

On Apr 27, 2007, at 12:58 PM, Matthew M. wrote:

+------------+-------------+---------------+

So is a card number like “4012 3456 7890” a valid Unknown, or an
invalid Visa?

Visa Invalid

Always match the type first.

Similarly, for Unknown, should we accept any length, or go for the
common 16?

Unknown is for any type that doesn’t match, so it could be any length.

James Edward G. II

+------------+-------------+---------------+

So is a card number like “4012 3456 7890” a valid Unknown, or an invalid
Visa?
Similarly, for Unknown, should we accept any length, or go for the
common 16?

So I took this as an opportunity to try doing a little TDD, something
which is appealing but I’ve never really tried. It is fun to see
failure first and then fix. =) There are probably other tests I
could have included, but here is a start, in case people want to check
a few numbers or add more tests:

Obviously, you may need to change this to match your solution…

require “test/unit”
require “ccard”

class CreditCardCheckTest < Test::Unit::TestCase

def test_valid_numbers
assert CreditCard.new(“4408041234567893”).valid?
assert CreditCard.new(“6011111111111117”).valid?
end

def test_invalid_nubers
assert !CreditCard.new(“4417123456789112”).valid?
end

def test_valid_numbers_with_spaces
assert CreditCard.new(“4408 0412 3456 7893”).valid?
assert CreditCard.new(“6011 1111 1111 1117”).valid?
end

def test_invalid_nubers_with_spaces
assert !CreditCard.new(“4417 1234 5678 9112”).valid?
end

def test_valid_types
assert_equal CreditCard.new(“4408041234567893”).type, :visa
assert_equal CreditCard.new(“4408 0412 3456 7893”).type, :visa
assert_equal CreditCard.new(“6011 1111 1111 1117”).type, :discover
assert_equal CreditCard.new(“123456789”).type, :unknown
end

def test_invalid_types
assert_nil CreditCard.new(“4408 0412 3456 789”).type
end

end

On Apr 27, 2007, at 3:10 PM, anansi wrote:

would be obsolete don’t you think?
Both length and startingbytes have equal weight in the decision
wether a card belongs to a known Company or not.

You are right. I didn’t read well. It’s Unknown.

James Edward G. II

James Edward G. II wrote:

On Apr 27, 2007, at 12:58 PM, Matthew M. wrote:

So is a card number like “4012 3456 7890” a valid Unknown, or an
invalid Visa?

Visa Invalid

Always match the type first.

Nope thats no Visa number but a unknown one. Visa Cards have a certain
length (13 or 16) and if that length doesn’t fit (Matthew’s number here
had just 12 digits)it’s unknown otherwise the length would be
obsolete don’t you think?
Both length and startingbytes have equal weight in the decision wether a
card belongs to a known Company or not.

greets
(
)
(
/\ .-"""-. /
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|/| ,;;;;;, |/|
//\;-"""-;///\
// / . / \
(| ,-| \ | / |-, |)
//__\.-.-./__\
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(| |)
jgs ) (/

one must still have chaos in oneself to be able to give birth to a
dancing star

On Apr 27, 2007, at 8:39 PM, Raj S. wrote:

rules exist, you have to check tables to determine valid prefixes and
an American Express card that starts with 34 given those starting
numbers.

Tim corrected his typo:

JCB 3528-2589 Length: 16

should be 3528-3589, obviously.

So I don’t think a leading 34 is ambiguous. It would be American
Express, with an expected length of 15 digits.

-Mark

Mark Day wrote:

card number length. Additionally, Maestro card numbers may also be

Sorry, didn’t see that email for some reason, even though it’s right
here in my inbox. . .

Raj

Tim B. wrote:

For JCB and Diner’s:

JCB 3528-2589 Length: 16
Diners 3000-3029, 3040-3059, 36, 3815-3889, 389 Length: 14

There’s no way to differentiate between a JCB starting with 34 and an
American Express card that starts with 34 given those starting numbers.

Raj

On Apr 27, 2007, at 23:06 , James Edward G. II wrote:

certain length (13 or 16) and if that length doesn’t fit
(Matthew’s number here had just 12 digits)it’s unknown
otherwise the length would be obsolete don’t you think?
Both length and startingbytes have equal weight in the decision
wether a card belongs to a known Company or not.

You are right. I didn’t read well. It’s Unknown.

James Edward G. II

Do we display Valid/Invalid for unknown cards?

• And by the way: 4012 3456 7890 isn’t valid is it? So should that
not be “invalid Unknown” or “invalid Visa”?

/C

anansi wrote:

That’s a unknown and invalid one. To be a Visa one it would need to
be
13 or 15 bytes long.

13 or 16 of course…

greets
(
)
(
/\ .-"""-. /
//\/ , //\
|/| ,;;;;;, |/|
//\;-"""-;///\
// / . / \
(| ,-| \ | / |-, |)
//__\.-.-./__\
// /.-(() ())-.\ \
(\ |) ‘—’ (| /)
(| |)
jgs ) (/

one must still have chaos in oneself to be able to give birth to a
dancing star

On Apr 28, 2007, at 7:43 AM, Christoffer Lernö wrote:

Do we display Valid/Invalid for unknown cards?

• And by the way: 4012 3456 7890 isn’t valid is it? So should that
not be “invalid Unknown” or “invalid Visa”?

I would go ahead and display Valid/Invalid normally. That way people
can use the program on cards we didn’t cover, as long as they conform
to the standard.

James Edward G. II

Christoffer Lernö wrote:

James Edward G. II

Do we display Valid/Invalid for unknown cards?

We desplay unknown for unkown cards and valid/invalid for valid or
invalid cards, the first has nothing to do with the second.

Check first if it’s a known one which depends on both the length and the
starting bytes. Then output if it’s a known or unknown one.

After that, check the known or unknown number for validation and output
that result.

• And by the way: 4012 3456 7890 isn’t valid is it? So should that not
be “invalid Unknown” or “invalid Visa”?

/C

That’s a unknown and invalid one. To be a Visa one it would need to be
13 or 15 bytes long.

greets
(
)
(
/\ .-"""-. /
//\/ , //\
|/| ,;;;;;, |/|
//\;-"""-;///\
// / . / \
(| ,-| \ | / |-, |)
//__\.-.-./__\
// /.-(() ())-.\ \
(\ |) ‘—’ (| /)
(| |)
jgs ) (/

one must still have chaos in oneself to be able to give birth to a
dancing star

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