bitwarden-android/src/App/Behaviors/EmailValidationBehavior.cs

using System;
using System.Text.RegularExpressions;
using Xamarin.Forms;

namespace Bit.App.Behaviors
{
    public class EmailValidationBehavior : Behavior<Entry>
    {
        private const string EmailRegex = @"^(?("")("".+?(?<!\\)""@)|(([0-9a-z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-z])@))" +
            @"(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-z][-\w]*[0-9a-z]*\.)+[a-z0-9][\-a-z0-9]{0,22}[a-z0-9]))$";

        private static readonly BindablePropertyKey IsValidPropertyKey = BindableProperty.CreateReadOnly("IsValid", typeof(bool), typeof(EmailValidationBehavior), false);
        public static readonly BindableProperty IsValidProperty = IsValidPropertyKey.BindableProperty;

        public bool IsValid
        {
            get { return (bool)GetValue(IsValidProperty); }
            private set { SetValue(IsValidPropertyKey, value); }
        }

        protected override void OnAttachedTo(Entry entry)
        {
            entry.TextChanged += HandleTextChanged;
            base.OnAttachedTo(entry);
        }

        void HandleTextChanged(object sender, TextChangedEventArgs e)
        {
            IsValid = Regex.IsMatch(e.NewTextValue, EmailRegex, RegexOptions.IgnoreCase, TimeSpan.FromMilliseconds(250));
            ((Entry)sender).BackgroundColor = IsValid ? Color.Default : Color.Red;
        }

        protected override void OnDetachingFrom(Entry entry)
        {
            entry.TextChanged -= HandleTextChanged;
            base.OnDetachingFrom(entry);
        }
    }
}