validate_credit_card.js

// takes the form field value and returns true on valid number
function valid_credit_card(value) {
  // accept only digits, dashes or spaces
	if (/[^0-9-\s]+/.test(value)) return false;

	// The Luhn Algorithm. It's so pretty.
	var nCheck = 0, nDigit = 0, bEven = false;
	value = value.replace(/\D/g, "");

	for (var n = value.length - 1; n >= 0; n--) {
		var cDigit = value.charAt(n),
			  nDigit = parseInt(cDigit, 10);

		if (bEven) {
			if ((nDigit *= 2) > 9) nDigit -= 9;
		}

		nCheck += nDigit;
		bEven = !bEven;
	}

	return (nCheck % 10) == 0;
}