We have studied a string rewriting system to improve the basic design of Tierra. Our system has three features. First, the Tierra instruction set is converted into a set of string rewriting rules using regular expressions. Second, every agent is composed of a string as the genome and a set of string rewriting rules as its own instruction set. Third, a genetic operation and selection of rewriting rules tire introduced to allow the agents to weed out the worst rules. We carried out experiments on how agents evolve through self-replication. The results have shown that our system can evolve not only the agents' genomes but also the rewriting rule sets.
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