Check digits between column

Trebor76,rylo : you are excellent! That it work! thanks for all the help!

UDF
Use in cell like
=If(iamrony(A1,B1)=0,"",iamrony(A1,B1))

Function iamrony(txt1 As String, txt2 As String) As Long
Dim i As Integer, flg As Boolean
If txt1 = txt2 Then Exit Function
i = 1
Do While i <= Application.Max(Len(txt1), Len(txt2))
    If (i > Application.Min(Len(txt1), Len(txt2))) + _
       (Mid$(txt1,i) <> Mid$(txt2,1)) Then
        flg = True
        Exit Do
    End If
    i = i + 1
Loop
If flg Then iamrony = i
End Function