>I cannot find any differences between these two expressions:
>
>([A-Za-z0-9]{1,20})-([A-Za-z0-9]{1,4})
>([A-Za-z0-9-]{1,20})-([A-Za-z0-9]{1,4})
>
>Is there one?

The second one accepts that it starts with a dash while the first one does not