using System.Text.RegularExpressions;

public string CheckCC(string ccNumb){

//reg expression checks to see if the value in ccNum "LOOKS" like a valid cc number

//will return invalid if there are non num chars or the string is not the correct length

// checks for visa MC, amex, diners club, jcb and discover

string ccRegEx = @"^(?:4[0-9]{12}(?:[0-9]{3})?|5[1-5][0-9]{14}|6(?:011|5[0-9][0-9])[0-9]{12}|3[47][0-9]{13}|3(?:0[0-5]|[68][0-9])[0-9]{11}|(?:2131|1800|35\d{3})\d{11})$";

Regex r = new Regex(ccRegEx, RegexOptions .IgnoreCase);

// Match the regular expression pattern against the cc text string.

Match m = r.Match(ccNum);

if (!m.Success ) return "CC number is invalid" ;

}