Custom validation rules on frontend form What would be the best way to add some custom validation rules on forms submitted on the front-end without using specific field types for it?

I'm using the Guest Entries plugin that allows anonymous posts to the targeted section.  I'm new to Yii and Craft CMS and I cannot seem to find a way to extend the validation rules for the EntryModel (at least I suspect it needs to get done there).
If your validation rules are truly custom and aren't covered by the default Craft validation rules, the only way I can think of getting them into the Guest Entries EntryModel is to fork the plugin and add your own MyPluginEntryModel that extends EntryModel adding a rules method with your custom validation logic.

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