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; a storage device which stores the state transition table generated by the state transition table generator; an input character reader which extracts input characters one-by-one from an input document; and an automaton which, in the course of determining whether the matching condition exists for a string of one or more input characters already extracted from the input document, refers to the storage device to make a transition according to the stored state transition table and a current-state and input-character tuple, wherein if no next-transition destination state with respect to the 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, which is a state before reading the first input character, without proceeding to read the input character of the tuple for which no next-transition destination state exists.