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This article proposes a framework P3E for evaluating entity extraction problem over cyber-physical system data. As known by us, this article is the first work focusing on this problem, which is an important step for identifying entities in cyber-physical system data. Directed by the initial entities, utilizing the relaxation and verification techniques, P3E provides a path expression-based solution for entity extraction problem, which has following characteristics. First, using path expressions, P3E provides a condensed presentation for entity locations whose size may get very large when scaling up the data size. Second, requiring only one entity example to indicate the interests, using relaxation technique, P3E can discover other similar entities automatically. Third, by adjusting the example given to P3E, users can specify their own interesting entities and control the entities discovered by P3E. Besides, utilizing the idea of sharing computations, by extending previous automaton techniques, an efficient implementation of P3E is provided. Experimental results are reported, which show that P3E can provide an effective and efficient solution to the entity extraction problem.