1. An information processing system comprising: at least one memory operable to store program instructions; at least one processor operable to read said program instructions and operate according to said program instructions, said program instructions including: acquiring instructions configured to cause at least one of said at least one processor to acquire a list of onomatopoeia by referring to an onomatopoeia database that stores a plurality of onomatopoeia, divide a posted review among a plurality of posted reviews stored in a storage unit into one or more posted sentences, and determine whether each posted sentence contains any onomatopoeia included in the list of onomatopoeia; specifying instructions configured to cause at least one of said at least one processor to: obtain a degree of co-occurrence with onomatopoeia, indicating a probability that a given word included in the posted review appears with any onomatopoeia included in the list of onomatopoeia within one posted sentence, by using a ratio of a probability of a presence of a posted sentence that contains both of the given word and at least one onomatopoeia among all of the one or more posted sentences to a product of a probability of a presence of a posted sentence that contains the given word among all of the one or more posted sentences and a probability of a presence of a posted sentence that contains at least one onomatopoeia among all of the one or more posted sentences, and specify a content word, among the given word, having a certain degree of co-occurrence with onomatopoeia or higher; and extraction instructions configured to cause at least one of said at least one processor to extract a posted sentence containing the content word from the plurality of posted reviews and output, on a display of a user terminal, via a communication network, the extracted posted sentence containing the content word.