When samba is used in evironments with Active Directory, e.g. as a AD domain member or via trusts in an IPA domain, as a file server for users from the AD domaina corresponding local user must exist. The sssd AD or IPA provider take care that the user is available but samba expect the user names to be 'DOMNAME\username' where DOMNAME is the NetBIOS domain name of the trusted domain and username the user name in the trusted domain.

By default sssd uses 'username@DNS.DOMAIN.NAME' as format for fully qualified user names. To allow both types of fully qualified user names and short names for the local domain the regular expression to match user and domain names can be extended like:

re_expression = (((?P<domain>[^\\]+)\\(?P<name>.+$))|((?P<name>[^@]+)@(?P<domain>.+$))|(^(?P<name>[^@\\]+)$))

Since the samba fileserver use case will be a typical use case for clients where either the AD or IPA provider with trusts is used it would be lower the configuration effort if a regular expression similar to the one above is the default for the AD and IPA provider.