Save outlook attachment as file using email subject as file name w/o invalid characters

I have the very basic code that will grab (using rules) specific inbound emails and save down the attachment to a local drive.
I would like to be able to identify the attachments by a number that is unfortunately included in the email subject instead of the attachment display name.
The subject is also riddled with invalid characters (example: OTTC WO Advice Report - Customer: Murex LLC Reference: 1895478 Transport Unit: TTE 2546 / TTE 1975 / D. OWENS Order #: 183006-1.1 and in this example, all I need is the 1895478 to ID it correctly)

Any advice on how to pull out the invalid characters would be awesome!!

Public Sub saveAttachtoDisk(itm As Outlook.MailItem)
Dim objAtt As Outlook.Attachment
Dim saveFolder As String
saveFolder = "O:\1 - Terminals And Transloads\1 - Oiltanking Texas City\EDI Truck BoLs\"
For Each objAtt In itm.Attachments
objAtt.SaveAsFile saveFolder & "\" & itm.Subject & " " & objAtt.DisplayName
Set objAtt = Nothing
Next
End Sub