Logical aspects in the study of tree languages
Proc. of the conference on Ninth colloquium on trees in algebra and programming
Counter-Free Automata (M.I.T. research monograph no. 65)
Counter-Free Automata (M.I.T. research monograph no. 65)
Effective characterizations of tree logics
Proceedings of the twenty-seventh ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems
Tree Languages Defined in First-Order Logic with One Quantifier Alternation
ICALP '08 Proceedings of the 35th international colloquium on Automata, Languages and Programming, Part II
A Kleene Theorem for Forest Languages
LATA '09 Proceedings of the 3rd International Conference on Language and Automata Theory and Applications
Adjunct elimination in Context Logic for trees
Information and Computation
Adjunct elimination in context logic for trees
APLAS'07 Proceedings of the 5th Asian conference on Programming languages and systems
Finding your way in a forest: on different types of trees and their properties
FOSSACS'08/ETAPS'08 Proceedings of the Theory and practice of software, 11th international conference on Foundations of software science and computational structures
Simplifying XML schema: single-type approximations of regular tree languages
Proceedings of the twenty-ninth ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems
Simplifying XML Schema: Single-type approximations of regular tree languages
Journal of Computer and System Sciences
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We define regular expressions for unranked trees (actually, ordered sequences of unranked trees, called forests). These are compared to existing regular expressions for trees. On the negative side, our expressions have complementation, and do not define all regular languages. On the positive side, our expressions do not use variables, and have a syntax very similar to that of regular expressions for word languages. We examine the expressive power of these expressions, especially from a logical point of view. The class of languages defined corresponds to a form of chain logic [5,6]. Furthermore, the star-free expressions coincide with first-order logic. Finally, we show that a concatenation hierarchy inside the expressions corresponds to the quantifier prefix hierarchy for first-order logic, generalizing a result of Thomas.