using System;

using System.Text.RegularExpressions;

public class Program

{

    private const string IMAGE_FILENAME_PATTERN = @"^(?<OrderCode>(cm[\d]{11}_[\d]{4,}|[a-z0-9]{5,})).*\.(jpg|png|jpeg)$";

    private static bool IsImageFile(string path, out string orderCode)

        {

            orderCode = null;

            var match = Regex.Match(path, IMAGE_FILENAME_PATTERN, RegexOptions.IgnoreCase);

            if (match?.Success == true)

            {

                orderCode = match.Groups["OrderCode"].Value;

                return true;

            }

            return false;

        }

    public static void Main()

    {

        bool ok = IsImageFile("CM20000289800_10018_01.jpg", out string code);

        Console.WriteLine(string.Format("OK: {0}", ok));        

        Console.WriteLine(string.Format("order: {0}", code));

    }

}