using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var ser = new string[]{

            "210700X109M",

            "210700x109M",

            "181284+234K",

            "210899-198K",

            "321287-187K",

            "121200-188K",

            "121200A188K",

            "121200B188K",

            "121200C188K",

            "121200D188K",

            "121200E188K",

            "121200F188K",

            "121200G188K",

            "121200H188K",

            "121200I188K",

            "121200P188K",

            "121200K188K",

            "121200L188K",

            "121200X188K",

            "121000-1883",

            "141200-1971"

        };

        foreach(var str in ser){

            string output = Test(str);

            Console.WriteLine(output);

        }

    }

    private static readonly Regex Pattern = new Regex("^(0[1-9]|[12]\\d|3[01])(0[1-9]|1[0-2])([5-9]\\d\\+|\\d\\d-|[01]\\d[ABCDEFYXWVU])\\d{3}[\\dA-Z]$", RegexOptions.Compiled);

​

    public static string Test(string input)

    {

        return Pattern.Replace(input, match =>

        {

            string newMatch = match.Groups[0].Value;

            return newMatch + " <<<";

        });

    }

}