Decimal data validation using macros - excel

Question

I have a macro enabled work sheet in which i have data validations for columns where i want to regulate the data input. I cannot use regular data validation feature in excel as it fails to function as we copy data from other sources which is the case in my requirement.So i am implementing data validations through macros . I have a scenario where one column can input only decimal data. The conditions are as follows the input data is of length 9 which constitutes only 2 decimal positions. I have partly written a macro for this validation which does not work(When i make a invalid input macro is not triggered therefore no msgbox pop up) and i am stuck at this point.Please help me out here to find a different if condition for the validation. The macro i have written is as follows:
Set AffectedCells = Intersect(Target, Target.Parent.Range("F:F"))
If Not AffectedCells Is Nothing Then
For Each Cell In AffectedCells
If Not (Cell.Value * (10 ^ 2) Mod 10) <> 0 Then
MsgBox "The value you entered is not valid."
Application.Undo 'undo insert
Exit Sub 'stop checking after one invalid data was found.
End If

Answer 1

This needs to be pasted on the sheet code space for the sheet you want the macro to run on.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim xCell As Range, AffectedCells As Range
Set AffectedCells = Intersect(Target, Target.Parent.Range("F:F"))
If Not AffectedCells Is Nothing Then
For Each xCell In AffectedCells
If Not (xCell.Value * (10 ^ 2) Mod 10) <> 0 Then
MsgBox "The value you entered is not valid."
Application.EnableEvents = False
Application.Undo
Application.EnableEvents = True
Exit Sub
End If
Next xCell
End If
End Sub

Answer 2

Is this what you are trying? I have commented the code so you should not have a problem undrstanding it. But if you do then simply ask.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim xCell As Range, AffectedCells As Range
On Error GoTo Whoa
Application.EnableEvents = False
Set AffectedCells = Intersect(Target, Columns(6))
If Not AffectedCells Is Nothing Then
For Each xCell In AffectedCells
'~~> Avoid cases like IP address 10.1.2.234
'~~> Check if the number contains decimal
If IsNumeric(xCell.Value) And _
Int(xCell.Value) <> xCell.Value Then
'~~> Check the position of the decimal
'~~> Check the length of the string
If Mid(xCell.Value, Len(xCell.Value) - 2, 1) <> "." Or _
Len(xCell.Value) > 9 Then
'
'~~> INVALID INPUT: Do what you want
'
'MsgBox "The value you entered is not valid."
'Application.Undo
'Exit For
End If
End If
Next xCell
End If
Letscontinue:
Application.EnableEvents = True
Exit Sub
Whoa:
MsgBox Err.Description
Resume Letscontinue
End Sub