長すぎワロタ
なお動作確認はしていない

Sub GetProcessTargetFilesImpl(ByVal folder As Object, ByVal tester As Object, ByVal result As Collection)
For Each file In folder.Files
If tester.Test(file.Name) Then
result.Add file
End If
Next
For Each subFolder In folder.SubFolders
GetProcessTargetFilesImpl subFolder, result
Next
End Sub

Function GetProcessTargetFiles(ByVal rootFolderPath As String) As Collection
Dim fso As Object
Set fso = CreateObject("Scripting.FileSystemObject")
Dim tester As Object
Set tester = CreateObject("VBScript.RegExp")
tester.Pattern = "^2018-\d\d-\d\d\.xlsx$"
tester.IgnoreCase = True
Set GetProcessTargetFiles = New Collection
GetProcessTargetFilesImpl fso.GetFolder(rootFolderPath), tester, GetProcessTargetFiles
End Function