VBA to set accepted inputs to specific cell, allow numbers and characters in excel

Can someone please help me, design below code to make it effective to Cell ("A1") only. thank you.

Private Sub Worksheet_Change(ByVal Target As Range)

Dim TestCell
Dim RE As Object

Dim REMatches As Object

Set RE = CreateObject("vbscript.regexp")
With RE
    .MultiLine = False
    .Global = False
    .IgnoreCase = True
    .Pattern = "[^0-9A-Z,]"
End With

For Each TestCell In Target.Cells
    Set REMatches = RE.Execute(TestCell.Value)
    If REMatches.Count > 0 Then
        MsgBox "Invalid:" & TestCell.Address & "-" & TestCell.Value
        TestCell.Value = ""
    End If
Next

End Sub