Either: 'regex' or 'callback' ('function' is also kept for backward compatibility). If registering a subclass of HTML_QuickForm_Rule you can pass anything here, preferrably NULL or empty string.
string
$data1
Name of function, regular expression, classname of HTML_QuickForm_Rule subclass or an instance of such class.
string
$data2
Object parent of above function, name of the file containing the subclass of HTML_QuickForm_Rule.
