5. A method for identifying at least one unique character string in an input document which is input into a computer system, said computer system being operable to search one or more documents stored in a storage medium, and using said unique character is used as a search string, the method comprising: extracting a partial input character string from said input document, and determining whether said partial input character string is a candidate character string; and evaluating an amount of feature of said candidate character string through comparison between appearance frequency information of at least a part of said candidate character string appearing in said input document and appearance frequency information of at least a part of said candidate character string appearing in said searchably stored documents to recognize said candidate character string as said unique character string.
6. A method for identifying at least one unique character string as the search string of claim 5, wherein in evaluating the amount of feature, said amount of feature is a point allocation for said candidate character string and points are allocated according to the equation: ##EQU13## where: count i=appearance frequency of an i-th candidate character string in the input document; Ncount i j=appearance frequency of a j-th partial input character string of the i-th candidate character string in the input document; Nsize i j=position data corresponding to the j-th partial input character string of the i-th candidate character string; Nnum i=a number of partial input character strings contained in an i-th candidate character string; Max (Nsize i j)=maximum Nsize i j for all searchably stored documents; Max (Ncount i j)=maximum Ncount i j for all searchably stored documents.