Looking for a certain regexp

From: Robert TV (ducott_at_hotmail.com)
Date: 06/30/04 

Date: Tue, 29 Jun 2004 23:16:33 GMT

Hi, has onyone ever created a regexp to match common credit card formulas? I
have been asked to "validate" some CC#'s we have on file. I do not know the
formulas by which credit cards are generated. All I know is Visa always
starts with 4, Mastercard with 5, and Amex with 3 I think. It would be nice
to have:

$cardtype = param('cardtype');
$cardnumber = param('cardnumber');

if ($cardtype eq "Visa") {
  unless ($cardnumber =~ /some valid visa formula/) {
    print "It's Good";
  } else {
    print "It's Bad";
  }
}

And so forth. You can find e-commerce websites all over the place that do a
basic check but I don't know the formulas used. At the very least, I would
like to validate the first number as being 4,5,3 etc. and that the scalar is
at least 13 characters long. I do not know how to match just the very first
char of a scalar ... can some one help me with this? EG:

$cardtype = param('cardtype');
$cardnumber = param('cardnumber');

if ($cardtype eq "Visa") {
  unless ($cardnumber =~ /first character is 4/ && $cardnumber =~ /at least
13 characters long/) {
    print "It's Good";
  } else {
    print "It's Bad";
  }
}

I am very new to writing regexp's and my present knowledge is limited to
validating characters used, not their positions. This was my best guess to
match 4 as first (untested):

unless ($cardnumber =~ /4.*/) { #does not match character length as 13
however
  print "It's Good";

} else {
  print "It's Bad";
}

Thank you all.
Robert

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