Написать функцию-аналог стандартной функции INSTR

Function IstrNew(start, string1 As String, ByVal string2 As String, compare)
'Part Description
'start Required. Numeric expression that sets the starting position for each search. If omitted, search begins at the first character position. If start contains Null, an error occurs. The start argument is required if compare is specified.
'string1 Required. String expression being searched.
'string2 Required. String expression sought.
'compare Optional. Specifies the type of string comparison. If compare is Null, an error occurs. If compare is omitted, the Option Compare setting determines the type of comparison. Specify a valid LCID (LocaleID) to use locale-specific rules in the comparison.
'The compare argument settings are:
'        Constant Value Description
'        vbBinaryCompare 0 Performs a binary comparison.
'        vbTextCompare 1 Performs a textual comparison.
'IstrNew Return Values
'    string1 is zero-length 0
'    string1 is Null Null
'    string2 is zero-length start
'    string2 is Null Null
'    string2 is not found 0
'    string2 is found within string1 Position at which match is found
'    start > string2 0
 
    Dim i As Integer, Dlina1 As Byte, Dlina2 As Byte, CurString As String
    Dlina1 = Len(string1)
    Dlina2 = Len(string2)
    IstrNew = 0
    For i = 1 To Dlina1
        CurString = Mid(string1, i, Dlina2)
        Select Case compare
            Case 0
            Case 1
                 CurString = UCase(CurString)
                 string2 = UCase(string2)
        End Select
        If CurString = string2 Then
            IstrNew = i
            Exit For
        End If
    Next i
End Function