The custom constraint is a constraint that allows defining custom client and server-side validation

To check the constraint on the client side a ClientFunctionName property should be set.

If it is not set, there will be no client-side validation

The implementation of function should look like this:
function clientFunctionName (value, client_id)

To check the function on the server side, the OnServerValidate event should be implemented. If it is not implemented, the validation will by default return true on server side.