The attribute axis selects an attribute node contained by the
context node. For example, the XPointer
/descendant::SPOUSE/attribute::IDREF selects all IDREF
attributes of all SPOUSE elements in the document. The attribute
axis can be abbreviated by an @ sign. Thus
//SPOUSE/@IDREF selects all IDREF attributes of all
SPOUSE elements in the document. @* is a general
abbreviation for an attribute with any name. Thus //SPOUSE/@*
indicates all attributes of all SPOUSE elements.
For another example, to find all PERSON elements in the document
http://www.theharolds.com/genealogy.xml whose FATHER
attribute is Jean Francois Bellau (ID p2), you could write
//PERSON[@FATHER="p2"].