How to search the symbol in the file name?

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C#

Dear Experts,
Problem: after the batch uploading files I need to recognizing and sorting files by names (by mask or flag). For example, file name "Tripolskoe_23_2341_MD_123-XXX.PDF". My utility should recognize the file name and determine this file per flag "MD". What condition for search should be put? What code I can use in my case?
Regards,
Darya

What I have tried:

private void DropSpot_DragEnter(object sender, System.Windows.Forms.DragEventArgs e)
		{
			// We only want to accept files, so we only set our DragDropEffects 
			// if that's what's being dragged in
			if (e.Data.GetDataPresent(DataFormats.FileDrop, false)==true)
			{
				e.Effect = DragDropEffects.All;				
			}
		}
		ArrayList Files = new ArrayList();
		private void DropSpot_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
		{
			// Get a list of all objects in the Drop Data, that are files
            
			string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
			// Iterate through the dropped files
			for (int i=0;i<files.Length;i++)
			{
				// Add the to our ArrayList
				Files.Add(files[i]);
				// Create our new List View item
				ListViewItem item = new ListViewItem();
				// Get a file info object
				// we use this for getting file size, etc.
				System.IO.FileInfo fInfo = new System.IO.FileInfo(files[i]);								
				item.Text = System.IO.Path.GetFileName(fInfo.Name);
				item.SubItems.Add(fInfo.Length.ToString());
				item.SubItems.Add("Pending");								
				FileListView.Items.Add(item);
				FileListView.Tag = Files[Files.Count-1];
			}
			// Refresh the file list - for good measure
			this.Refresh();
			// If we added files, clear the instruction label
			
		}