using System;

using System.ComponentModel.DataAnnotations;

public class Program

{

    public const string Regex1 = "abc";

    public const string Regex2 = "xyz";

    public const string CompositeRegex = Regex1 + "|" + Regex2;

    public static void Main()

    {

        Console.WriteLine(Regex1);

        Console.WriteLine(Regex2);

        Console.WriteLine(CompositeRegex);      

    }

}

​

public class TestClass

{

    [RegularExpression(Program.Regex1  + "|" + Program.Regex2)]

    public string TestProperty {get; set;}

}