We survey the basic results on regular tree languages over unranked alphabets; that is, we use an unranked alphabet for the labels of nodes, we allow unbounded, yet regular, degree nodes and we treat sequences of trees that, following Courcelle, we call hedges. The survey was begun by the first and third authors. Subsequently, when they discovered that the second author had already written a summary of this view of tree automata and languages, the three authors decided to join forces and produce a consistent review of the area. The survey is still unfinished because we have been unable to find the time to finish it. We are making it available in this unfinished form as a research report because it has, already, been heavily cited in the literature.

this paper is limited in two ways. It assumes that the element names of XML documents are from a finite and known set and it ignores the data values in the leaf nodes and attributes of XML documents. For this reason, the work of Kaminski and Francez [11] on automata on infinite alphabets has been reexamined from an XML perspective [28, 23]

U.C. San Diego

We propose a new, highly scalable and efficient technique for evaluating node-selecting queries on XML trees which is based on recent advances in the theory of tree automata. Our query

We study the complexity of the inclusion, equivalence, and intersection problem for simple regular expressions arising in practical XML schemas. These basically consist of the concatenation of factors where each factor is a disjunction of strings possibly extended with ‘∗ ’ or ‘?’. We obtain lower and upper bounds for various fragments of simple regular expressions. Although we show that inclusion and intersection are already intractable for very weak expressions, we also identify some tractable cases. For equivalence, we only prove an initial tractability result leaving the complexity of more general cases open. The main motivation for this research comes from database theory, or more specifically XML and semi-structured data. We namely show that all lower and upper bounds for inclusion and equivalence, carry over to the corresponding decision problems for extended context-free grammars and single-type tree grammars, which are abstractions of DTDs and XML Schemas, respectively. For intersection, we show that the complexity only carries over for DTDs.

This article studies the problem of evaluating unary (or nodeselecting) queries on unranked trees compressed in a natural structure-preserving way, by the sharing of common subtrees. The motivation to study unary queries on unranked trees comes from the database field, where querying XML documents, which can be considered as unranked labelled trees, is an important task. We give complexity results for the evaluation of XPath and monadic datalog queries. Furthermore, we propose a new automata-theoretic formalism for querying trees and give algorithms for evaluating queries defined by such automata.

Abstract not found

Abstract not found

This paper studies the problem of evaluating unary (or nodeselecting) queries on unranked trees compressed in a natural structure-preserving way, by the sharing of common subtrees. The motivation to study unary queries on unranked trees comes from the database field, where querying XML documents, which can be considered as unranked labelled trees, is an important task. We give

We establish that regularly extended two-way nondeterministic tree automata with unranked alphabets have the same expressive power as regularly extended nondeterministic tree automata with unranked alphabets. We obtain this result by establishing regularly extended versions of a congruence on trees and of a congruence on, so called, views. Our motivation for the study of these tree models is the Extensible Markup Language (XML), a metalanguage for defining document grammars. Such grammars have regular sets of right-hand sides for their productions and tree automata provide an alternative and useful modeling tool for them. In particular, we believe that they provide a useful computational model for what we call caterpillar expressions. 1 Introduction We became interested in regularly extended two-way tree automata (tree automata that have a regular set of transitions instead of a finite set and, thus, unbounded degree nodes) because of our work [4] in which we show that tree l...