using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string input = "sample DD81UN DN55 1PT WRONG text 123456";

        Regex r = new Regex(@"\b([A-PR-UWYZ](?:(?:\d{1,2}|\d[A-HJ-KSTUW])|(?:[A-HK-Y]\d(?:\d|[A-Z])?)))\s?(\d[ABD-HJLNP-UW-Z]{2})\b");

        string output = r.Replace(input, "$1 $2");

        Console.WriteLine(output);

        var results = r.Matches(output);

        foreach(var result in results) {

            Console.WriteLine(result);

        }

    }

}