Validating credit card numbers in SAS

Major credit card issuing networks (including Visa, MasterCard, Discover, and American Express) allow simple credit card number validation using the Luhn Algorithm (also called the “modulus 10″ or “mod 10″ algorithm). The following code demonstrates an implementation in SAS. The code also validates the credit card number by length and by checking against a short of known test account numbers.

data eft_valid eft_invalid;
	input acct_nbr $ 1-30;
	length AddIt $2;

	/* check length for Visa/Discover/MC/Amex */
	if length(acct_nbr) not in (15,16)
	then do;
		put 'Not 15/16 digits' acct_nbr=;
		output eft_invalid;
		/* to stop processing, uncomment out the delete statement */
/*		delete;*/
	end;

	/* check for known test credit card number accounts */
	/* reference: https://www.paypal.com/en_US/vhelp/paypalmanager_help/credit_card_numbers.htm  */
	if acct_nbr in ('378282246310005', '371449635398431', '378734493671000', '6011111111111110', 
	'6011000990139420', '5555555555554440', '5105105105105100', '4111111111111110', '4012888888881880', '4222222222222')
	then do;
		put 'fake credit card number' acct_nbr=;
		output eft_invalid;
		/* to stop processing, uncomment out the delete statement */		
/*		delete;*/
	end;

	/* Luhn algorithm (also called modulus 10 or mod 10) */
	/* mostly copied from Patrick http://support.sas.com/forums/thread.jspa?threadID=4274 */
	ChkSum = 0; /* reset */
	ChkStr=left(reverse(acct_nbr));
	do pos=1 to length(ChkStr);
		if mod(pos,2) then /* odd positions */
			do;
			AddIt=substr(ChkStr,pos,1);
			end;
		else /* even positions: digit*2 */
			do;
			AddIt=put(2*input(substr(ChkStr,pos,1),2.),2.);
			end;

		/* add digits */
		do i=1 to length(AddIt);
			ChkSum+input(substr(AddIt,i,1),2.);
			end;
	end;

	/* Check if ID is valid or not (if ChkSum ends with Zero) */
	if mod(ChkSum,10)=0 then
		do;
		put 'This is a valid ID: ' acct_nbr= ChkSum=;
		output eft_valid;
		end;
	else
		if mod(ChkSum,10) ne 0 then
		do;
		put 'This is a invalid ID: ' acct_nbr= ChkSum=;
		output eft_invalid;
	end;
	drop ChkSum i pos ChkStr AddIt;
datalines;
378282246310005
371449635398431
378734493671000
6011111111111110
6011000990139420
5555555555554440
5105105105105100
4111111111111110
4012888888881880
4222222222222
123456789
3847592
48573726264859560
2843759
00028434305834
442308239586
;
run; 

Patrick’s SAS code was very helpful, but without resetting ChkSum, SAS would fail all credit card numbers following an invalid card number.

This code was executed on 177,172 credit card records and found only 40 invalid numbers. Most of them were obvious like 4000000000000000. YMMV.

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