Credit card prediction in Javascript

I'm writing a tool that onkeydown will run the current value being entered in an input box to check to see if it matches a regex for one of the 4 major types of credit cards.

I feel like it kind of works, but it's flaky so I wanted to figure out what was causing it to give faulty response (eg sometimes it'll output 2 values instead of one). Is it because I need to set a flag variable before looping? Upon match of the correct card, I'm just returning from the loop through the object, so I thought that'd be sufficient enough...

The criteria for the regexes were pulled from this site:

$(function() {

var $cardNumber = $('#js-cardnumber');

var ccMap = {};

ccMap.cards = {
    'amex': '^3[47][0-9]{13}$',
    'discover': '^6(?:011|5[0-9]{2})[0-9]{12}$',
    'mastercard': '^5[1-5][0-9]{14}$',
    'visa': '^4[0-9]{12}(?:[0-9]{3})?$'
};


$cardNumber.keydown(function() {
for (var cardType in ccMap.cards) {
    if (ccMap.cards.hasOwnProperty(cardType)) {
        var regex = ccMap.cards[cardType];
        if (regex.match($(this).val())) {
            console.log(cardType);
            return;
        }
    }
}
});
});​

Here's a fiddle

It seems like you're using the regular expressions in a wrong way.

If you want to check a string against a regular expression, you can use the match() method of the string:

string.match(regexp) // returns boolean

You're doing it the wrong way:

if ( regex.match($(this).val()) ) {

tries to interpret the current value as a regular expression. Must be this way:

if ( $(this).val().match(regex) ) {

You can also cache the regular expressions to make your script more efficient:

ccMap.cards = {
    'amex': /^3[47][0-9]{13}$/,  // store an actual regexp object, not a string
    // ...

// The way you test changes, now you're able to use the "test"
// method of the regexp object:
if ( regex.test($(this).val()) ) {

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