We added this new feature to extend the way you can validate your data. You could already validate your data on presence, and uniqueness. From now on, you can also validate by an expression definition.
The validates by expression feature is built at properties and is placed under the property settings. Go to the data model in the builder bar left. Either choose an existing model or create a new one. Pick a property and scroll down. You can define your validation expression in this input field.