Public Function Test(Txt As String, RExpr As String) As Boolean
Dim regEx As New RegExp
    If RExpr <> "" Then
        With regEx
            .Global = True
            .MultiLine = True
            .IgnoreCase = True
            .Pattern = RExpr
        End With
        Test = regEx.Test(Txt)
    End If
End Function

Sub RegEx_Tester()
        Set objRegExp_1 = CreateObject("vbscript.regexp")
        objRegExp_1.Global = True
        objRegExp_1.IgnoreCase = False
        objRegExp_1.Pattern = ^(97(8|9))?d{9}(d|X)$                ' nell'esempio incontrato era:     objRegExp_1.Pattern=[a-z,A-Z]*@[a-z,A-Z]*.com. A me qualsiasi espressione da errore al primo carattere dicendo: "Errore di compilazione. Si aspettava espressione."
        ....l'esempio continua con
        strToSearch="ABC@xyz.com"
        Set regExp_Matches = objRegExp_1.Execute(strToSearch) 'che nel mio caso sarebbe TextBox1.Text    
        If regExp_Matches.Count = 1
        MsgBox("This string is a valid email address.")
        End if
End Sub

Public Function Test2(Txt As String) As Boolean
  Const sPattern = "ISBN(-1(?:(0)|3))?:?x20(s)*[0-9]+[- ][0-9]+[- ][0-9]+[- ][0-9]*[- ]*[xX0-9]"
  Dim regEx  As Object
  Set regEx = CreateObject("VBScript.RegExp")
  With regEx
      .Global = True
      .MultiLine = False
      .IgnoreCase = True
      .Pattern = sPattern
  End With
  Test2 = regEx.Test(Txt)
End Function

Public Function Test(Txt As String) As Boolean
Dim regEx As New RegExp
With regEx
            .Global = True
            .MultiLine = True
            .IgnoreCase = True
            .Pattern = "^(97(8|9))?d{9}(d|X)$" 
End With
Test = regEx.Test(Txt)
End Function

Private Sub TextBox1_Exit(ByVal Cancel As MSForms.ReturnBoolean)
    If Not Test(Me.TextBox1.Value) Then
        MsgBox "Valore errato"
        Cancel = True
    End If
End Sub

Option Base 1
Sub a()
Dim arr() As Integer
isbn = Range("A1").Text
L = Len(isbn)
ReDim arr(L)
molt = Array(1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3)
For j = 1 To L
  arr(j) = Val(Mid(isbn, j, 1))
Next
For j = 1 To L
  s1 = s1 + arr(j) * molt(j)
Next
Debug.Print s1
End Sub

Public Function CheckIsbn13(codice As String) As String
    Dim i As Integer
    Dim r As Integer
    Dim somma As Long
    codice = Replace(codice, "-", "")
    If (Len(codice) < 12) Or (Len(codice) > 13) Then
        MsgBox "Codice Errato"
        Exit Function
    End If
    For i = 1 To 12
        r = CInt(Mid(codice, i, 1))
        If i Mod 2 = 1 Then
            somma = somma + r
        Else
            somma = somma + r * 3
        End If
    Next
    r = somma Mod 10
    If r <> 0 Then r = 10 - r
    CheckIsbn13 = CStr(r)
End Function

Public Function Sbn13_Ok(codice As String) As Boolean
    Dim i As Integer
    Dim somma As Long
    codice = Replace(codice, "-", "")
    If Len(codice) <> 13 Then
        Sbn13_Ok = False
    Else
        For i = 1 To 12
            somma = somma + CInt(Mid(codice, i, 1)) * (3 - 2 * (i Mod 2))
        Next
        r = somma Mod 10
        Sbn13_Ok = CStr(IIf(somma Mod 10 = 0, 0, 10 - somma Mod 10)) = Mid(codice, 13, 1)
    End If
End Function

Sub test()
MsgBox Sbn13_Ok(Range("A1"))
End Sub

'---------------------------------------------------------------------------------------
' Procedure : CheckIsbn
' Author    : scossa
' Date      : 04/11/2014
' Purpose   :
'---------------------------------------------------------------------------------------
'
Public Function CheckIsbn(ByVal sIsbn As String) As Variant

  Dim vRet_ As Variant
  Dim nLen As Long
  nLen = Len(sIsbn)
  If nLen = 13 Then
    vRet_ = Evaluate("IF(MOD(SUM({1,3,1,3,1,3,1,3,1,3,1,3,1}*INDEX(MID(" & sIsbn & ",{1,2,3,4,5,6,7,8,9,10,11,12,13},1),)),10)=0,""OK"",""ERRATO"")")
  ElseIf nLen = 10 Then
    vRet_ = Replace(Evaluate("11-MOD(SUM({10,9,8,7,6,5,4,3,2}*INDEX(MID(" & Replace(sIsbn, "X", "") & ",{1,2,3,4,5,6,7,8,9},1),)),11)"), 10, "X") = Right(sIsbn, 1)
  Else
    vRet_ = ""
  End If

  CheckIsbn = vRet_

End Function

Private Sub GoBack()
    Me.TextBox1.BackColor = &H80000005
    blExit = True
    CommandButton2_Click
    Unload Me
    Sheets("Menu").Select
End Sub

Private Sub EmptyISBN()
    With Me.TextBox1
        If MsgBox(Prompt:="Campo Obligatorio!" & vbNewLine & vbNewLine _
            & "REINTENTAR para volver a ingresar o CANCELAR para Salir", _
            Buttons:=vbRetryCancel, Title:="Puesto de Libros") = vbCancel _
            Then
            .BackColor = &H80000005
            GoBack
        Else
            .BorderStyle = fmBorderStyleSingle
            .BorderColor = RGB(255, 80, 20)
            .BackColor = RGB(255, 255, 10)
            TextBox11.Text = ""
        End If
    End With
End Sub

Private Sub ValidISBN()
    With TextBox1
        TextBox11.Text = "ISBN VÁLIDO"
        .BorderStyle = fmBorderStyleSingle
        .BorderColor = RGB(0, 0, 70)
        .BackColor = RGB(150, 200, 255)
    End With
End Sub

Private Sub InvalidISBN()
    TextBox11.Text = "ISBN NO VÁLIDO"
    With TextBox1
        If MsgBox(Prompt:="Ingresar un ISBN válido" _
            & vbNewLine _
            & vbNewLine _
            & "REINTENTAR para volver a ingresar o CANCELAR para Salir", _
            Buttons:=vbRetryCancel, _
            Title:="Puesto de Libros") <> vbCancel _
        Then
            .BackColor = &H80000005
            .BorderStyle = fmBorderStyleSingle
            .BorderColor = RGB(255, 80, 20)
            .BackColor = RGB(255, 255, 10)
            .SetFocus
        Else
            GoBack
        End If
        .Text = ""
    End With
End Sub

Private Sub CommandButton5_Click()
Dim cdgo, chrcontr As String
Dim Res As VbMsgBoxResult
    If Not blExit Then
        With TextBox1
            cdgo = TextBox1.Text
            If cdgo = "" Then
                EmptyISBN
            ElseIf cdgo = "111" Then
                TextBox11.Text = "SÍN ISBN"
                .BorderStyle = fmBorderStyleSingle
                .BorderColor = RGB(0, 0, 70)
                .BackColor = RGB(150, 200, 255)
            ElseIf Len(cdgo) = 13 Then
                TextBox10.Text = CheckIsbn13(TextBox1.Text)
                If CInt(Mid(cdgo, 13, 1)) = TextBox10.Text Then
                    ValidISBN
                Else
                    InvalidISBN
                End If
            ElseIf Len(cdgo) = 10 Then
                If IsbnOk(cdgo) Then
                    ValidISBN
                Else
                    InvalidISBN
                End If
            Else
                InvalidISBN
            End If
        End With
    End If
End Sub

Public Function Sbn13_Ok(codice As String) As Boolean
    Dim i As Integer
    Dim somma As Long
    codice = Replace(codice, "-", "")
    If Len(codice) <> 13 Then
        Sbn13_Ok = False
    Else
        For i = 1 To 12
            somma = somma + CInt(Mid(codice, i, 1)) * (3 - 2 * (i Mod 2))
        Next
        Sbn13_Ok = CStr(IIf(somma Mod 10 = 0, 0, 10 - somma Mod 10)) = Mid(codice, 13, 1)
    End If
End Function

codice = Trim(Replace(codice, "-", ""))