In the 5 part series linked below, Stephen Lewis from Experience provides a nice overview and deep dive of Validation in Craft and how it relates to the underlying validation in Yii.
Part 1: Foundations
If you’re working on a Craft plugin that accepts user input, chances are you’ll want to validate that data...
Part 2: AttributeTypes
This instalment is all about Craft’s AttributeTypes: what they are, what they do, and which ones are available.
Part 3: Attribute Rules
This instalment is all about attribute rules: what they are, how they affect validation, and which ones are available.
Part 4: Hidden Validator Settings
This instalment is all about “hidden” validator settings, which aren’t accessible via the usual Craft attribute rules.
Part 5: Custom Validators
This instalment is all about implementing your own custom validators.