That still didn't work, it wouldn't accept any password I input, however complex. However I found something that did work, so I'm posting it here in case it helps anyone else:
WAValidateRX(((isset($_POST["Registration_group_2_Password"]))?$_POST["Registration_group_2_Password"]:"") . "","#.*^(?=.{8,20})(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9])(?=.*\W).*$#",true,10);