Nowadays online monitoring of data streams is essential in many real life applications, like sensor network monitoring, manufacturing process control, and video surveillance. One major problem in this area is the online identification of streaming sequences similar to a predefined set of pattern-sequences. In this paper, we present a novel solution that extends the state of the art both in terms of effectiveness and efficiency. We propose the first online similarity matching algorithm based on

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... ongest Common SubSequence that is specifically designed to operate in a streaming context, and that can effectively handle time scaling, as well as noisy data. In order to deal with high stream rates and multiple streams, we extend the algorithm to operate on multilevel approximations of the streaming data, therefore quickly pruning the search space. Finally, we incorporate in our approach error estimation mechanisms in order to reduce the number of false negatives. We perform an extensive experimental evaluation using forty real datasets, diverse in nature and characteristics, and we also compare our approach to previous techniques. The experiments demonstrate the validity of our approach.