1. A string matching system, comprising: a state transition table generator for generating a state transition table based on a matching condition described by a regular expression; and an automaton for making a transition according to the state transition table generated by the state transition table generator; wherein if no next-transition destination state with respect to a current state and input-character tuple exists in the state transition table generated based on the matching condition, then the automaton makes a transition back to its initial state without proceeding to read the input character.