Zahl aus String filtern mit fuehrender Null

Sub a()
Dim strWeek As String, strWeeknew As String, strFormel As String
Dim pos1 As Long, pos2 As Long, pos3 As Long
Sheet3.Range("Y51").Activate
strFormel = ActiveCell.Formula
'MsgBox strFormel
pos1 = InStr(strFormel, "_") 'Position erster Unterstrich
pos2 = InStr(pos1 + 1, strFormel, "_") 'Position zweiter Unterstrich und Wort Week
pos3 = InStr(pos2 + 1, strFormel, "_") 'Position dritter Unterstrich
strWeek = Mid(strFormel, pos2 + 1, pos3 - pos2 - 1)
strWeeknew = ExtractNumber(strWeek)
End Sub
Private Function ExtractNumber(str As String) As Long
Dim i As Byte, ii As Byte
For i = 1 To Len(str)
If IsNumeric(Mid(str, i, 1)) Then Exit For
Next i
For ii = i To Len(str)
If Not IsNumeric(Mid(str, ii, 1)) Then Exit For
Next ii
ExtractNumber = Mid(str, i, Len(str) - (ii - i))
End Function