17. The non-transitory recording medium according to claim 16 , wherein of the N-gram strings extracted by the extractor, the N-gram strings containing the delimiting characters each include two or more characters.
17. The non-transitory recording medium according to claim 16 , wherein of the N-gram strings extracted by the extractor, the N-gram strings containing the delimiting characters each include two or more characters. 20. The non-transitory recording medium according to claim 17 , wherein of the N-gram strings extracted by the extractor, the N-gram strings not containing the delimiting characters are each composed of one character.