Highlight cell that doesn't have only letters - excel

Question

I'm currently trying to develop my first conditional formatting on VBA, but after hours of trial it still doesn't work.
I'm aiming for a formula that would change the background / highlight the text of a cell that contains something else than any alphabet letters (not sensitive to caps or not). Accents, numbers and special characters would be the trigger
Here is my current code
Thank you in advance for your help
Sub Highlight()
Dim MyRange As Range
Set MyRange = Selection
MyRange.FormatConditions.Delete
MyRange.FormatConditions.Add xlExpression, , Formula1:="=IsAlpha()=false"
MyRange.FormatConditions(1).Interior.Color = RGB(255, 0, 0)
End Sub
And IsAlpha would be a function like
Function IsAlpha(s) As Boolean
IsAlpha = Len(s) And Not s Like "*[!a-zA-Z ]*"
End Function

Answer 1

You need to pass an argument to IsAlpha. Try the following:
Sub Highlight()
Dim MyRange As Range
Set MyRange = Selection
MyRange.FormatConditions.Delete
Dim s As String
s = "=NOT(IsAlpha(" & MyRange.Cells(1).Address(False, False) & "))"
MyRange.FormatConditions.Add xlExpression, Formula1:=s
MyRange.FormatConditions(1).Interior.Color = vbRed 'Or use RGB...
End Sub
In action:

Answer 2

Highlight Not Pure Alpha
There is a delay of about 2s before the cells get colored (on my machine). I wonder if a worksheet change would handle this smoother (if the range contains values (not formulas)).
Option Explicit
Sub HighlightNotPureAlphaTEST()
If TypeOf Selection Is Range Then
HighlightNotPureAlpha Selection
End If
End Sub
Sub HighlightNotPureAlpha(ByVal rg As Range)
With rg
.FormatConditions.Delete
' To not highlight blanks...
.FormatConditions.Add xlExpression, , _
"=NOT(IsAlphaOrBlank(" & .Cells(1).Address(0, 0) & "))"
' To highlight blanks:
'.FormatConditions.Add xlExpression, , _
"=NOT(IsAlpha(" & .Cells(1).Address(0, 0) & "))"
.FormatConditions(1).Interior.Color = RGB(255, 0, 0)
End With
End Sub
Function IsAlphaOrBlank(ByVal S As String) As Boolean
Application.Volatile
IsAlphaOrBlank = Not S Like "*[!A-Za-z]*"
End Function
Function IsAlpha(ByVal S As String) As Boolean
Application.Volatile
If Len(S) > 0 Then IsAlpha = Not S Like "*[!A-Za-z]*"
End Function