Look for the first "_" in a filename string - excel

Question

I want to loop through a directory and group files by the filenames up until the first _ in the filename.
Files are structured by
Manager Name_Employee Name_Assessment.xlsx
I want this to group together strings up until the first underscore. I can't designate a character length because managers have different names.
I'm thinking changing the If Right(filename,4) statement should do what I want, but I can't specify a character limit.
Public Sub Move_Files()
Dim sourceFolder As String, fileName As String
Dim destinationFolder As String, foundDestinationFolder As String
Dim missingFolders As String
sourceFolder = "C:\Assessment\"
If Right(sourceFolder, 1) <> "\" Then sourceFolder = sourceFolder & "\"
'Loop through *.xls files in source folder
missingFolders = ""
fileName = Dir(sourceFolder & "*.xls")
While fileName <> vbNullString
**If Right(fileName, 4) = ".xlsx" Then**
destinationFolder = Left(fileName, InStrRev(fileName, ".") - 1)
foundDestinationFolder = Find_Subfolder(sourceFolder, destinationFolder)
If foundDestinationFolder <> "" Then
Name sourceFolder & fileName As foundDestinationFolder & fileName
Else
missingFolders = missingFolders & vbCrLf & destinationFolder
End If
End If
fileName = Dir
Wend
If missingFolders = "" Then
MsgBox "All subfolders exist. All files moved to their respective destination folder"
Else
MsgBox "The following subfolders don't exist:" & vbCrLf & _
missingFolders
End If
End Sub
After it identifies all that fit within that manager's name, I want to group only those manager's employee files into a folder with their name up until the first _.

Answer 1

If the file extension is .xlsx, then change fileName = Dir(sourceFolder & "*.xls") to fileName = Dir(sourceFolder & "*.xlsx").
Consider the InStr function, which "returns a Variant (Long) specifying the position of the first occurrence of one string within another."
As is, destinationFolder = Left(fileName, InStrRev(fileName, ".") - 1) finds everything to the left of the last period.
So if you want to the destination folder to be Manager Name, then perhaps destinationFolder = Left$(fileName, InStr(fileName, "_") - 1)
Note that this can be made more robust to handle the possibility of no _ in the file name, or the file name beginning with _, with an initial If InStr(fileName, "_") > 1).