Ok I’m going the other route of nodes instead of python, regular expressions, or using C# as you’re admittedly a newer user. Yes regular expressions and a more robust python code will help, but don’t go too far into the deep end as a newer user. Stay with Dynamo for now, and learn to think the problem though.
Think the problem though - if all files which are backups and end with “.****.rvt” then every file that is a backup will have two decimals in the file name. This will hold true for only backups as long as you aren’t using decimals in your paths or in file names (which you shouldn’t be as that’ll screw up a lot of Revit stuff), you then can count the number of times a decimal occurs in your path. String.AllIndiciesOf wired into a List.Count (lacing should be longest) to produce a series of 1 and 2 values. If it’s a 1, it’s a main file. if it’s a 2, it’s a backup. Use an == node to allow you to quickly check for the one or two value, and filter your list of files accordingly. The end result looks like this:
Simple and you can do it easily. If you do use decimals in your file names, first please stop doing that after today (this includes in the names of Revit families), then post back so we can help you code a better solution.
Note on the other solutions: Searching for “00” works in some cases, but that reference model you’ve added to a few hundred times over the years can end with “.0101.rvt”, or even “.9999.rvt”. Believe it or not you can even have a “.99999.rvt” (try renaming your last backup and saving your Revit file - you might be surprised what you get). Because of this the number sequence and spacing of the decimals is unreliable as a source - it’s unlikely to change but it could. Sorting by file name, dropping the extension, and then iterating each item against the next to remove the leading string from the trailing, adding a 0 to the front of any remaining strings and attempting to convert the result to a number is likely the "right’ way to filter this - if it worked then the file was most certainly a backup (after all it had the same name followed by .****.rvt. Sequencing of the iteration is the hard part there.