Query languages for XML often use path expressions to locate elements
in XML documents. Path expressions are regular expressions such that
underlying alphabets represent conditions on nodes. Path expressions
represent conditions on paths from the root, but do not represent
conditions on siblings, siblings of superiors, and descendants of such
siblings. In order to capture such conditions, we propose to extend
underlying alphabets. Each symbol in an extended alphabet is a
triplet (e1, a, e2), where "a" is a condition on nodes, and e1 (e2) is
a condition on elder (resp. younger) siblings and their descendants;
e1 and e2 are represented by hedge regular expressions, which are as
expressive as hedge automata (hedges are ordered sequences of trees).
Such an extended path expression can be evaluated for every element by
traversing the XML document three times. Furthermore, given an input
schema and a query operation controlled by an extended path
expression, it is possible to construct an output schema. This is
done by identifying where in the input schema the given pointed hedge
representation is satisfied.
By: MURATA Makoto
Published in: RT0389 in 2002
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