“…The large amount of textual information, mostly unstructured and without any semantic description, available on the Internet requires the development of tools that help end users to efficiently process such amount of data and quickly search, classify, and assess natural language documents according to the end users' needs. The keyword extraction is a main underlying task of many NLP tools as it is often an important step of many text mining applications, for instance, relevance assessment, index generation, query refinement, document categorization, named-entity recognition, document similarity measurements, text summarization, and so on (Beliga, 2014;Siddiqi and Sharan, 2015). Therefore, in this paper we have selected the keyword extraction problem due to its complexity and also as a good example from a wide range of NLP applications or their relevant parts.…”