Not regex, but a partwise approach to renaming problematic picture filepaths

picRenamer.cs

internal class Program
    {
        private static readonly string dirPath = @"C:\Users\delta\Desktop\560_25p";

        static void Main(string[] args)
        {
            string[] paths = GetSortedPicturesFrom(dirPath).ToArray();

            foreach (string path in paths)
            {
                try
                {
                    string[] parts = path.Split(',');
                    string badPart = parts.Where(x => x.Contains("RC ") && x.Contains("R ")).First();
                    int badIdx = parts.ToList().IndexOf(badPart);
                    List<string> fixingParts = badPart.Split(' ').ToList();
                    for (int i = 0; i < fixingParts.Count; i++)
                    {
                        if (int.TryParse(fixingParts[i], out int validNum))
                        {
                            fixingParts[i] += ",";
                            break;
                        }
                    }
                    parts[badIdx] = string.Join(" ", fixingParts);
                    string fixedStr = string.Join(",", parts);
                    File.Move(path, fixedStr);
                }
                catch (Exception)
                {
                    Console.WriteLine($"Fail on {path}");
                }
            }

            Console.WriteLine($"Done");
            Console.ReadKey();
        }

        private static IEnumerable<string> GetSortedPicturesFrom(string searchFolder)
        {
            string[] filters = new string[] { "jpg", "jpeg", "png", "gif", "tiff", "bmp", "svg" };
            List<string> filesFound = new List<string>();
            foreach (var filter in filters)
                filesFound.AddRange(Directory.GetFiles(searchFolder, string.Format("*.{0}", filter), SearchOption.AllDirectories));
            return filesFound.Where(x => !x.Contains("pattern"));
        }
    }