Does anyone know what a ValidationValueDelegate is and does?

I am looking to validation some text on an entry control.

e.g. I have an email address and would like to run it through a validation regex and display the text as red if it is not valid.

I noticed that a BindableProperty has a Constructor that takes a variable ValidationValueDelegate. Brilliant I thought that sounds JUST what I need.

I go to the BindableProperty API documentation

and look for ValidateValueDelegate

to find out what it is and does... but yep... 404

So does anyone here know what it does or is? Can I use it to implement my control and turn the text red on incorrect validation