Dag Hovland

Regular Expressions with Numerical Constraints and Automata with Counters

Regular expressions with numerical constraints are an extension of regular expressions, allowing to bound numerically the number of times that a subexpression should be matched. Expressions in this extension describe the same languages as the usual regular expressions, but are exponentially more succinct. We define a class of finite automata with counters and a deterministic subclass of these. Deterministic finite automata with counters can recognize words in linear time. Furthermore, we describe a subclass of the regular expressions with numerical constraints, a polynomial-time test for this subclass, and a polynomial-time construction of deterministic finite automata with counters from expressions in the subclass.

Ontology-Based Integration of Cross-Linked Datasets

In this paper we tackle the problem of answering SPARQL queries over virtually integrated databases. We assume that the entity resolution problem has already been solved and explicit information is available about which records in the different databases refer to the same real world entity. Surprisingly, to the best of our knowledge, there has been no attempt to extend the standard Ontology-Based Data Access OBDA setting to take into account these DB links for SPARQL query-answering and consistency checking. This is partly because the OWL built-in owl:sameAs property, the most natural representation of links between data sets, is not included in OWLi¾?2i¾?QL, the de facto ontology language for OBDA. We formally treat several fundamental questions in this context: how links over database identifiers can be represented in terms of owl:sameAs statements, how to recover rewritability of SPARQL into SQL lost because of owl:sameAs statements, and how to check consistency. Moreover, we investigate how our solution can be made to scale up to large enterprise datasets. We have implemented the approach, and carried out an extensive set of experiments showing its scalability.

The membership problem for regular expressions with unordered concatenation and numerical constraints

We study the membership problem for regular expressions extended with operators for unordered concatenation and numerical constraints. The unordered concatenation of a set of regular expressions denotes all sequences consisting of exactly one word denoted by each of the expressions. Numerical constraints are an extension of regular expressions used in many applications, e.g. text search (e.g., UNIX grep), document formats (e.g. XML Schema). Regular expressions with unordered concatenation and numerical constraints denote the same languages as the classical regular expressions, but, in certain important cases, exponentially more succinct. We show that the membership problem for regular expressions with unordered concatenation (without numerical constraints) is already NP-hard. We show a polynomial-time algorithm for the membership problem for regular expressions with numerical constraints and unordered concatenation, when restricted to a subclass called strongly 1-unambiguous.

Engineering ontology-based access to real-world data sources

The preparation of existing real-world datasets for publication as high-quality semantic web data is a complex task that requires the concerted execution of a variety of processing steps using a range of different tools. Faced with both changing input data and evolving requirements on the produced output, we face a significant engineering task for schema and data transformation. We argue that to achieve a robust and flexible transformation process, a high-level declarative description is needed, that can be used to drive the entire tool chain. We have implemented this idea for the deployment of ontology-based data access (OBDA) solutions, where semantically annotated views that integrate multiple data sources on different formats are created, based on an ontology and a collection of mappings. Furthermore, we exemplify our approach and show how a single declarative description helps to orchestrate a complete tool chain, beginning with the download of datasets, and through to the installation of the datasets for a variety of tool applications, including data and query transformation processes and reasoning services. Our case study is based on several publicly available tabular and relational datasets concerning the operations of the petroleum industry in Norway. We include a discussion of the relative performance of the used tools on our case study, and an overview of lessons learnt for practical deployment of OBDA on real-world datasets.

The inclusion problem for regular expressions

This paper presents a polynomial-time algorithm for the inclusion problem for a large class of regular expressions. The algorithm is not based on construction of finite automata, and can therefore be faster than the lower bound implied by the Myhill-Nerode theorem. The algorithm automatically discards irrelevant parts of the right-hand expression. The irrelevant parts of the right-hand expression might even be 1-ambiguous. For example, if r is a regular expression such that any DFA recognizing r is very large, the algorithm can still, in time independent of r, decide that the language of ab is included in that of (a+r)b. The algorithm is based on a syntax-directed inference system. It takes arbitrary regular expressions as input. If the 1-ambiguity of the right-hand expression becomes a problem, the algorithm will report this. Otherwise, it will decide the inclusion problem for the input.

The SHIP Validator: An Annotation-Based Content-Validation Framework for Java Applications

In this paper, we investigate the use of Java annotations for software security purposes. In particular, we implement a framework for content validation where the validation tests are specified by annotations. This approach allows to tag what properties to validate directly in the application code and eliminates the need for external XML configuration files. Furthermore, the testing code is still kept separate from the application code, hence facilitating the creation and reuse of custom tests. The main novelty of this framework consists in the possibility of defining tests for the validation of multiple and interdependent properties. The flexibility and reusability of tests are also improved by allowing composition and boolean expressions. The main result of the paper is a flexible framework for content-validation based on Java annotations.

Investigating the Limitations of Java Annotations for Input Validation

Recently Java annotations have received a lot of attention as a possible way to simplify the usage of various frameworks, ranging from persistence and verification to security. In this paper we discuss our experiences in implementing an annotation framework for input validation purposes. We investigate the advantages and more importantly their limitations in the design of validation tests. We conclude that annotations are a good choice for specifying common validation tests. However, the limitations of annotations have an impact on creating and using generic tests and tests involving multiple properties.

Deciding twig-definability of node selecting tree automata

Node selecting tree automata (NSTAs) constitute a general formalism defining unary queries over trees. Basically, a node is selected by an NSTA when it is visited in a selecting state during an accepting run. We consider twig patterns as an abstraction of XPath. Since the queries definable by NSTAs form a strict superset of twig-definable queries, we study the complexity of the problem to decide whether the query by a given NSTA is twig-definable. In particular, we obtain that the latter problem is EXPTIME-complete. In addition, we show that it is also EXPTIME-complete to decide whether the query by a given NSTA is definable by a node selecting string automaton.

A Type System for Usage of Software Components

The aim of this article is to support component-based software engineering by modelling exclusive and inclusive usage of software components. Truong and Bezem describe in several papers abstract languages for component software with the aim to find bounds of the number of instances of components. Their language includes primitives for instantiating and deleting instances of components and operators for sequential, alternative and parallel composition and a scope mechanism. The language is here supplemented with the primitives use , lock and free . The main contribution is a type system which guarantees the safety of usage, in the following way: When a well-typed program executes a subexpression use [x] or lock [x ], it is guaranteed that an instance of x is available.

Efficient Ontology-Based Data Integration with Canonical IRIs

In this paper, we study how to efficiently integrate multiple relational databases using an ontology-based approach. In ontology-based data integration (OBDI) an ontology provides a coherent view of multiple databases, and SPARQL queries over the ontology are rewritten into (federated) SQL queries over the underlying databases. Specifically, we address the scenario where records with different identifiers in different databases can represent the same entity. The standard approach in this case is to use sameAs to model the equivalence between entities. However, the standard semantics of sameAs may cause an exponential blow up of query results, since all possible combinations of equivalent identifiers have to be included in the answers. The large number of answers is not only detrimental to the performance of query evaluation, but also makes the answers difficult to understand due to the redundancy they introduce. This motivates us to propose an alternative approach, which is based on assigning canonical IRIs to entities in order to avoid redundancy. Formally, we present our approach as a new SPARQL entailment regime and compare it with the sameAs approach. We provide a prototype implementation and evaluate it in two experiments: in a real-world data integration scenario in Statoil and in an experiment extending the Wisconsin benchmark. The experimental results show that the canonical IRI approach is significantly more scalable.