Word phrases are stored in a phrase structure. Each word is stored as a keyword in a keyword structure. Each keyword is associated with usage attributes identifying use of a word in a word phrase. Any preceding words associated with a keyword, and a mapping from any preceding words to a word phrase, is stored for each word. A word string is input. Match attributes are updated in a match structure if a word in the word string matches any keyword and if any preceding words associated with any matching keyword includes a preceding word which precedes the word in the word string. The match attributes indicate use of the matching word in the word string and in a word phrase. Whether a word phrase is present in the word string is determined based on the usage attributes and the match attributes associated with multiple matching words.