Validate Visa number : Credit Card « Regular Expressions « JavaScript Tutorial

<HTML>
    <head>
        <title>Visa Example</title>
        <script type="text/javascript">
                function luhnCheckSum(sCardNum) {

                    var iOddSum = 0;
                    var iEvenSum = 0;
                    var bIsOdd = true;

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

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

                        if (bIsOdd) {
                            iOddSum += iNum;
                        } else {
                            iNum = iNum * 2;
                            if (iNum > 9) {
                                iNum = eval(iNum.toString().split("").join("+"));
                            }
                            iEvenSum += iNum;
                        }

                        bIsOdd = !bIsOdd;
                    }

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

                function isValidVisa(sText) {
                    var reVisa = /^(4\d{12}(?:\d{3})?)$/;

                    if (reVisa.test(sText)) {
                        return luhnCheckSum(RegExp.$1);
                    } else {
                        return false;
                    }
                }


                function validate() {
                    var oInput1 = document.getElementById("txt1");
                    if (isValidVisa(oInput1.value)) {
                        alert("Valid");
                    } else {
                        alert("Invalid!");
                    }

                }
        </script>
    </head>
    <body>

        <P>Visa Number: <input type="text" id="txt1" /><br />
        <input type="button" value="Validate" onclick="validate()" /></p>

    </body>
</html>
Quote from:
Professional JavaScript for Web Developers
by Nicholas C. Zakas (Author)

# Paperback: 672 pages
# Publisher: Wrox (April 22, 2005)
# Language: English
# ISBN-10: 0764579088
# ISBN-13: 978-0764579080

26.2.Credit Card
	26.2.1.	Matching a 16 digit credit card number separated by a single space
	26.2.2.	Matching a 16 digit credit card number with variable number of space characters
	26.2.3.	Validate Visa number
	26.2.4.	Validate MasterCard