@Kingston86
You've nearly completed a great job. However, in your split action, please make the following correction: Instead of referencing standard length, specify your delimiter. I assume all your filenames are suffixed with "_". Therefore, I've added that as the delimiter in the action below and add ExtractedsetFiles[0] into your List variable.
Method1:

Method2:
Instead of employing the split action, utilize the crop text action to trim your filename before the delimiter. Then, add your cropped filename text to your list variable.

Method3 - Regex: (it also works in similar way of method1):

Full Flow Details:

Code:
Variables.CreateNewList List=> FileList
Display.SelectFolder Description: $'''Please select your set file directory''' InitialDirectory: $'''C:\\''' IsTopMost: False SelectedFolder=> SelectedFolder ButtonPressed=> ButtonPressed
Folder.GetFiles Folder: SelectedFolder FileFilter: $'''*''' IncludeSubfolders: False FailOnAccessDenied: True SortBy1: Folder.SortBy.NoSort SortDescending1: False SortBy2: Folder.SortBy.NoSort SortDescending2: False SortBy3: Folder.SortBy.NoSort SortDescending3: False Files=> FileNameList
LOOP FOREACH CurrentFileItem IN FileNameList
# #Method1
Text.SplitText.SplitWithDelimiter Text: CurrentFileItem.Name CustomDelimiter: $'''_''' IsRegEx: False Result=> TextList
Variables.AddItemToList Item: TextList[0] List: FileList
# #Method2
Text.CropText.CropTextBeforeFlag Text: CurrentFileItem.Name ToFlag: $'''_''' IgnoreCase: False CroppedText=> CroppedFileName IsFlagFound=> IsFlagFound
IF IsFlagFound = $'''True''' THEN
Variables.AddItemToList Item: CroppedFileName List: FileList
END
END
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