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The location path language XPath is of particular importance for XML applications since it is a core component of many XML processing standards such as XSLT or XQuery. In this paper, based on axis symmetry of XPath, equivalences of XPath 1.0 location paths involving reverse axes, such as ancestor and preceding, are established. These equivalences are used(More)
The evolution of the web has outpaced itself: A growing wealth of information and increasingly sophisticated interfaces necessitate automated processing, yet existing automation and data extraction technologies have been overwhelmed by this very growth. To address this trend, we identify four key requirements for web data extraction, automation, and(More)
Data streams are an emerging technology for data dissemination in cases where the data throughput or size makes it unfeasible to rely on the conventional approach based on storing the data before processing it. SPEX evaluates XPath queries against XML data streams. SPEX is built upon formal frameworks for (1) rewriting XPath queries into equivalent XPath(More)
Search engines are the sinews of the web. These sinews have become strained, however: Where the web's function once was a mix of library and yellow pages, it has become the central marketplace for information of almost any kind. We search more and more for objects with specific characteristics, a car with a certain mileage, an affordable apartment close to(More)
A number of techniques have been developed to facilitate powerful data retrieval on the Web and Semantic Web. Three categories of Web query languages can be distinguished, according to the format of the data they can retrieve: XML, RDF and Topic Maps. This article introduces the spectrum of languages falling into these categories and summarises their(More)
Querying XML streams is receiving much attention due to its growing range of applications from traffic monitoring to routing of media streams. Existing approaches to querying XML streams consider restricted query language fragments, in most cases with exponential worstcase complexity in the size of the query. This paper gives correctness and complexity(More)
The evolution of the web has outpaced itself: The growing wealth of information and the increasing sophistication of interfaces necessitate automated processing. Web automation and extraction technologies have been overwhelmed by this very growth. To address this trend, we identify four key requirements of web extraction: (1) Interact with sophisticated web(More)
Forms are our gates to the web. They enable us to access the deep content of web sites. Automatic form understanding unlocks this content for applications ranging from crawlers to meta-search engines and is essential for improving usability and accessibility of the web. Form understanding has received surprisingly little attention other than as component in(More)
This article is firstly an introduction into query languages for the Semantic Web, secondly an in-depth comparison of the languages introduced. Only RDF query languages are considered because, as of the writing of this paper, query languages for other Semantic Web data modeling formalisms, especially OWL, are still an open research issue, and only a very(More)
This survey article introduces into the essential concepts and methods underlying rule-based query languages. It covers four complementary areas: declarative semantics based on adaptations of mathematical logic, operational semantics, complexity and expressive power, and optimisation of query evaluation. The treatment of these areas is foundation-oriented,(More)