1. A method for ad-hoc entity extraction, comprising: filtering a first set of documents to generate a second set of documents that includes documents of the first set based at least on a set of token sets that covers entity strings in a list of entity strings, the number of tokens in the set of token sets being less than the number of words of the entity strings in the list of entity strings, the set of tokens generated based on the entity strings in the list of the entity strings; and performing entity recognition on the second set of documents.