Abstract: The invention provides a rapid phrase identification method, especially for Spanish derivatives. The main concern of the present invention is to provide for an electronic Spanish dictionary device a means for quickly identifying a derivative without demanding large memory space for storing all the Spanish vocabulary, etymons and derivatives. All the transformation rules from etymons to derivatives have been collected, classified and reversed to give reversion rules. These reversion rules are sorted and indexed into a look-up table for searching. A rapid searching method pursuant to this look-up table is proposed.