Limit the prefix vba function to dynamic range

[Vincent88]

I want to limit below vba code to the column A in dynamic range ONLY. Right now if I enter something outside the range, it shows error and disrupts other functions in Workbook_SheetChange. I attach my file for easy ref. Please HELP ! HELP
File attached Excel file

'Formate Column A
If Target.Column = 1 Then
    Dim s As String
    Dim arr As Variant

    s = Target.Value
    If s = "" Then
      Target.NumberFormat = "General"
    Else
      With CreateObject("vbscript.regexp")
        .Pattern = "[^0-9]"
        .Global = True
        .IgnoreCase = True
        arr = Split(Application.Trim(.Replace(s, " ")), " ")
      End With
      Target.Value = arr
      Target.Value = Target.Value * 1
      Target.NumberFormat = """REQ0000000""General"
    End If

 

[GWteB]

You are welcome and thanks for letting me know.