Ontology is one of the most popular representation model used for knowledge representation, sharing and reusing. In light of the importance of ontology, different methodologies for building ontologies have been proposed. Ontology construction is a difficult and time-consuming process. Many efforts have been made to help ontology engineers to construct ontologies and to overcome the bottleneck of knowledge acquisition. The aim of this paper is to give a brief overview of ontology learning approaches and to review some of ontology extraction systems and tools followed by a summarizing comparison of them. Also some of the current issues and main trends of ontology construction from texts will be discussed.
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