I've got an event registration form that I've customized putting Jquery into a custom .tpl override file. It's all working fine, except I'm not sure how to go about adding a custom validation rule. I don't want to override any of the existing validation, I just want to add an extra rule. However, I'm not sure exactly how I should implement this.
I've found this post (https://civicrm.org/comment/6311) but that seems to apply to adding validation rules for ALL forms, and involves editing core files, which I don't want to do.
And yes I know Webforms would be able to handle this, but this system is on Joomla so Webforms is not an option :-)