PO module error trapping - need code help

[Richter12x2]

I've been working with the javascript_validation function, trying to get it to display a given message if the length of the PO number is either 0 or more than 12 places, to prevent embarassing program crashes when someone writes a novel in the PO number field, but I'm not terribly good at coding

 

I can get it to do either, but not both at the same time. Whenever I try, it ignores the entire function - what's wrong with this statement?

function javascript_validation()

{

$validation_string = 'if (payment_value == "' . $this->code . '") {' . "\n" .

' var purchase_order_number = document.checkout_payment.purchase_order_number.value;' . "\n" .

' if (purchase_order_number.length == 0) || (purchase_order_number.length >= 13) {' . "\n" .

' error_message = error_message + "' . MODULE_PAYMENT_PO_TEXT_JS_PO_NUMBER . '";' . "\n" .

' error = 1;' . "\n" .

' }' . "\n" .

'}

' . "\n";

return $validation_string;

}