using System;

using System.Text.RegularExpressions;

public class Program

{

    private const string TOPIC_PREFIX = "com.apple.mgmt.External.";

    public static void Main()

    {

        var i1 = "7fb7e49b-5d33-4860-9ccc-eada0a4f6b07";

        var i2 = "com.apple.mGmt.External.7fb7e49b-5d33-4860-9ccc-eada0a4f6b07";

        var i3 = "com.apple.mgmt.External.com.apple.mgmt.External.7fb7e49b-5d33-4860-9ccc-eada0a4f6b07";

        var o1 = AddTopicPrefix(i1);

        var o2 = AddTopicPrefix(i2);

        var o3 = AddTopicPrefix(i3);

        Console.WriteLine("---");

        Console.WriteLine(o1);

        Console.WriteLine(o2);

        Console.WriteLine(o3);

        Console.WriteLine("---");

    }

    private static string AddTopicPrefix(string topic)

    {

        try

        {

            // This is done to make sure that the UUID in the topic is lowercased, which is what the topics typically are.

            // I don't know if Apple is case-sensitive with these, so we will just make sure it is a known working format.

            string input = topic.ToLower();

​

            // External is typically capitalized, so we will make sure it is here after lowercasing the whole string above.

            // It was done this way so that we didn't have to conditionally lowercase everything in the call above.

            input = input.Replace("external", "External");

​

            string result = string.Empty;

​

            string UUIDstr = "([a-z0-9]{8}-([a-z0-9]{4}-){3}[a-z0-9]{12})$";

            Regex fullRegex = new Regex("^" + TOPIC_PREFIX + UUIDstr);

            Regex partialRegex = new Regex("^" + UUIDstr);

​

            if (fullRegex.IsMatch(input))

            {

                result = input;

            }

            else if (partialRegex.IsMatch(input))

            {

                result = TOPIC_PREFIX + input;

            }

​

            return result;

        }

        catch (Exception)

        {

            return string.Empty;

        }

    }

}