http://hi.baidu.com/w01fer/blog/item/93d185119a58ca0b213f2ea2.html

LUHN算法，主要用来计算信用卡等证件号码的合法性。

next section from:http://en.wikipedia.org/wiki/Luhn_algorithm

The Luhn algorithm or Luhn formula, also known as the " 10" or "mod 10" , is a simple formula used to validate a variety of identification numbers, such as , , in US and . It was created by scientist and described in , filed on January 6, 1954, and granted on August 23, 1960.

The formula verifies a number against its included check digit, which is usually appended to a partial account number to generate the full account number. This account number must pass the following test:

1. Counting from the check digit, which is the rightmost, and moving left, double the value of every second digit.
2. Sum the digits of the products together with the undoubled digits from the original number.
3. If the total ends in 0 (put another way, if the total modulo 10 is equal to 0), then the number is valid according to the Luhn formula; else it is not valid.

As an illustration, if the account number is 49927398716, it will be validated as follows:

1. Double every second digit, from the rightmost: (1×2) = 2, (8×2) = 16, (3×2) = 6, (2×2) = 4, (9×2) = 18
2. Sum all the individual digits (digits in parentheses are the products from Step 1): 6 + (2) + 7 + (1+6) + 9 + (6) + 7 + (4) + 9 + (1+8) + 4 = 70
3. Take the sum modulo 10: 70 mod 10 = 0; the account number is valid.

翻译过来就是:

1、从卡号最后一位数字开始，逆向将奇数位(1、3、5等等)相加。

2、将偶数位数字相加，但是这里有个麻烦。必须先将数字乘以2，如果结果是两位数，将两个位上数字相加。然后将这些结果加入总和中。

3、将奇数位总和加上偶数位总和，如果信用卡号码是合法的，结果应该可以被10整除。

现已Master Card为例:

//from:professional JavaScript for web developers

<script>

   function isValidMasterCard(sText){

    var reMasterCard=/^(5[1-5]\d{2})[\s\-]?(\d{4})[\s\-]?(\d{4})[\s\-](\d{4})$/;

    

    if(reMasterCard.test(sText)){

     var sCardNum=RegExp.$1+RegExp.$2+RegExp.$3+RegExp.$4;   

      alert(sCardNum);   

     //Luhn algorithm here

     return luhnCheckSum(sCardNum);

    }else{

     return

    }

   }

   

   function luhnCheckSum(sCardNum){

    var iOddSum=0;

    var iEvenSum=0;

    var bIsOdd=true;

    

    for(var i=sCardNum.length-1;i>=0;i--){

    //alert("length="+sCardNum.length);

    //alert("sCardNum.char("+i+")="+sCardNum.charAt(i));

     var iNum=parseInt(sCardNum.charAt(i));

     

     if(bIsOdd){

     //反向奇数求和

      iOddSum+=iNum;   

      

     }else{

      //偶数   

      if(iNum>9){

       iNum=eval(iNum.toString().split("").join("+"));

      }

      iEvenSum+=iNum;

     

     }

     bIsOdd=!bIsOdd;

    }

     return ((iEvenSum+iOddSum)%10==0);

   }

   

   alert(isValidMasterCard("5432 1234 5678 9012"));

   alert(isValidMasterCard("5432-1234-5678-9012"));

   

</script>