The grammar for selectors:

[1]   Selector   ::=   Path ( '|' Path )*
[2]   Path       ::=   ('.//')? Step ( '/' Step )*
[3]   Step       ::=   '.' | NameTest
[4]   NameTest   ::=   QName | '*' | NCName ':' '*'

The grammar for fields:

[7]   Path   ::=   ('.//')? ( Step '/' )* ( Step | '@' NameTest )

Keep in mind that the default namespace does not affect XPath expressions--so don't use the default namespace in your schema!!!