MainPage.xaml

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:Validators="clr-namespace:XamarinFormValidation.Validators.Implementations"
             xmlns:Behaviors="clr-namespace:XamarinFormValidation.Behaviors"
             xmlns:local="clr-namespace:XamarinFormValidation" x:Class="XamarinFormValidation.MainPage">
    <StackLayout Padding="15">
        <Label>Enter the mobile number</Label>
        <Entry>
            <Entry.Behaviors>
                <Behaviors:ValidationBehavior PropertyName="Text">
                    <Behaviors:ValidationBehavior.Validators>
                        <Validators:RequiredValidator />
                        <Validators:FormatValidator
                            Message="Mobile number must be 10 digit" 
                            Format="^[0-9]{10}$" />
                    </Behaviors:ValidationBehavior.Validators>
                </Behaviors:ValidationBehavior>
            </Entry.Behaviors>
        </Entry>
    </StackLayout>
</ContentPage>