Expressions below account for IIN numbers on CC’s of most major US issuers which includes validating starting character. These should be used in addition to Luhn Algorithm mod10 check.

These expressions can be used on clientside and codebehind to give your end users notification of invalid entry without wasting resources on a failed/invalid post. See snippet below. Expressions should technically be cross-platform.

Note: although very similar to those found on regular-expressions.info, there are some small differences which do not account for the old standards in my examples since these cards are no longer in circulation.

(C#)

string vs = @"((^4)[0-9]{15}$)|";
            string mc = @"((^5[1-5])[0-9]{14}$)|";
            string ax = @"((^3[4|7])[0-9]{14})$|";
            string ds = @"(^6(011|5[0-9]{2})[0-9]{12}$)";

string expirationmonth = @"((^[0-9]$)|(^[0-9][0-2]$))";

string ordertotalamount = @"((^[0-9]{1,5}$)|(^[0-9]{1,5}\.[0-9]{1,2}$))"; //does not account for currency symbols

System.Text.StringBuilder sbexp = new System.Text.StringBuilder();
            for (int i = 0; i <= 20; i++) //create list of years from now+20. same range amazon uses for card
            {
                sbexp.Append("(^" + (DateTime.Now.Year + i).ToString() + "$)|");
                if (i < 20) //don't append or on last
                {
                    sbexp.Append((DateTime.Now.Year + i).ToString());
                }
            }

            ((RegularExpressionValidator)validator).ValidationExpression = sbexp.ToString();

((RegularExpressionValidator)validator).ValidationExpression = vs+mc+ax+ds;

References
Wikipedia (IIN numbers), http://en.wikipedia.org/wiki/Bank_card_number
Wikipedia (Luhn Algorithm), http://en.wikipedia.org/wiki/Luhn_algorithm
RegularExpressions.info, http://www.regular-expressions.info/creditcard.html