using System;

using System.Linq;

using System.Collections.ObjectModel;

using System.Collections.Generic;

using System.Text.RegularExpressions;

using System.ComponentModel.DataAnnotations;

​

#nullable enable

​

public class Program

{

    public static void Main()

    {

        var attr = new RegularExpressionAttribute("^\\+\\d{1,3} +[0-9]+$");

        attr.IsValid("123456").Dump();

        attr.IsValid("").Dump();

        attr.IsValid(null).Dump();

    }

}