VBA Function - show and replace string inside brackets

'Function - show and replace string inside brackets - v1.xls
'This example replaces the index inside brackets.
'Useful when a command must read more components
'of what is the default in the Export file
'============================================================
Function ReplaceIndex(str As String, iDel As String, fDel As String, newIdx As String)
'this function replaces the index inside brackets and replace is with
Dim midStr As String
    On Error GoTo error1
    midStr = Mid(str, WorksheetFunction.Find(iDel, str) + 1, _
             WorksheetFunction.Find(fDel, str) _
           - WorksheetFunction.Find(iDel, str) - 1)
    ReplaceIndex = WorksheetFunction.Replace(str, WorksheetFunction.Find(iDel, str) _
                    + 1, Len(midStr), newIdx)
    Exit Function
error1:
    ReplaceIndex = "Incorrect delimiter"
    Application.Calculate
End Function