Sub test()

Dim str As String
str = "str 9999 9999 str"

Dim RegExp As Object
Set RegExp = CreateObject("vbscript.regexp")

With RegExp
    .IgnoreCase = True
    .MultiLine = False
    .Pattern = "[!0-9]"
    .Global = True
End With

output = RegExp.Replace(str, "")
MsgBox output

Set RegExp = Nothing

End Sub