ValidationExample.xaml

...
<Entry>
    <Entry.Behaviors>
        <Behaviors:ValidationBehavior Group="{x:Reference form}" PropertyName="Text">
            <Behaviors:ValidationBehavior.Validators>
                <v:FormatValidator Format="^[0-9]{6}$" Message="Village Code must be 6 digit" />
                <v:RequiredValidator />
            </Behaviors:ValidationBehavior.Validators>
        </Behaviors:ValidationBehavior>
    </Entry.Behaviors>
</Entry>
...