Abstract:
The present invention relates to a structured document classification device for classifying structured documents by types, comprising: a structural feature extracting section for extracting a structural feature or an incidental feature from each structured document; a structural feature rule base for storing a rule dedicated to the extracted structural feature or incidental feature; and a verifier for verifying each feature, which is extracted by the structural feature extracting section, according to the rule stored in the structured rule base, calculating relevance to each type.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a structured document classification device, a structured document search system, and a computer-readable memory causing a computer to function as a structured document classification device and a structured document search system. 
     This application is based on Japanese Patent Application No. Hei 10200171, the contents of which are incorporated herein by reference. 
     2. Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98. 
     One of the conventional processes for searching for a desired document in a database of structured documents, which is principally the WWW (World Wide Web), includes collecting published documents on the WWW by a robot, which is called “crawler”, and converting the documents into the database to allow full-text retrieval. “Goo” (http://www.goo.ne.jp) is an example of the service providing this retrieval. 
     The database includes more than one million documents, and as the WWW further expands, the number of the documents will increase. Therefore, in response to a small number of keywords input by a user initiating a retrieval for a document, a great number of results are returned. The user must find a target document among a great number of results, which takes much time and labor. Therefore, the conventional retrieval processes are of little practical use. 
     A conventional process for performing retrieval using the structural features in structured documents, for example, of SGML (Standard Generalized Markup Language) is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 7-225771. This system prepares a retrieval expression which includes the structural features of the structured documents, and enables a precise retrieval when the type of a retrieval target document (for example, a patent document, a study, or a specification) is clear. 
     The conventional system of Japanese Unexamined Patent Application, First Publication No. Hei 7-225771, can perform an accurate retrieval by specifying a target document in the SGML document database by keyword and type of the target document, but is not applicable to the structured documents (HTML: Hypertext Markup Language) on the WWW whose structure is not clearer than that of the SGML. 
     Further, because the process of Japanese Unexamined Patent Application, First Publication No. Hei 7-225771 requires examples of the structured documents, the conventional process is not applicable to the WWW. 
     Further, Japanese Unexamined Patent Application, First Publication No. Hei 9-311869 discloses a search server which, in response to an input of search parameters, searches for a target information from a number of URLs. Japanese Unexamined Patent Application, First Publication No. Hei 10-124519 discloses an information display device which automatically arrange keywords in a hierarchical structure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a structured document classification device which enhances the accuracy of a search and reduces the labor of a searcher searching for a target document by classifying the target HTML documents according to types beforehand. 
     In one aspect of the present invention, the structured document classification device for classifying structured documents by types, comprises: a structural feature extracting section for extracting a structural feature or an incidental feature from each structured document; a structural feature rule base for storing a rule dedicated to the extracted structural feature or incidental feature; and a verifier for verifying each feature, which is extracted by the structural feature extracting section, according to the rule stored in the structured rule base, calculating relevance to each type. 
     The structural feature extracting section includes a keyword feature extractor for extracting a tag and keyword pair from each structured document. The structural feature extracting section may include a image file feature extractor for extracting a feature of an image file from each structured document. The structural feature extracting section may include a link feature extractor for extracting a feature of a link from each structured document. The structural feature extracting section may include a tag structural feature extractor for extracting a feature of a tag structure from each structured document. The structural feature extracting section may include a URL feature extractor for extracting a feature of URL information from each structured document. The structural feature extracting section may include a plugin feature extractor for extracting a feature of a plugin from each structured document. The structural feature extracting section may include an upper-lower level feature extractor for extracting structural features of an upper level document and of a lower level document from each structured document. Further, the structural feature extracting section may extract any combination of features of a tag and keyword pair, an image file, a link, a tag structure, URL information, and a plugin. 
     The structured document classification device of the present invention further comprises: a score controller for controlling the relevance of each structured document according to a control rule which finely controls the relevance in consideration of relationships between the types and of the context as a whole. 
     In another aspect of the present invention, the structured document search system using the structured document classification device, comprises: a input/output device for inputting a search parameter and a type of a target structured document and for outputting search results; a search engine for performing a search in a database storing structured documents by the input search parameter; a type searcher for searching for the relevance to the input type found by the search engine, the relevance being calculated by the structured document classification device; and a restrictor for receiving the search results from the search engine, receiving the relevance of the structured document found by the type searcher, restricting the search results by consulting the relevance to the input type, and outputting the narrowed search results to the input/output device. 
     Instead of the restrictor, the system may have a separator for receiving the search results from the search engine, receiving the relevance of the structured document found by the type searcher, grouping the found documents according to their relevance to the input type, and outputting the search results to the input/output device. 
     The first advantage of the present invention is that the classification of the structured documents, for example, HTML documents, is made accurate because this invention extracts the features of a tag and keyword pair, an image file, a link information, a tag structure, URL information, a plugin information, any combination of these, or upper and lower level documents. 
     The second advantage of the present invention is that the classification is made consistent because this invention finely controls the search results in consideration of the relationships between the groups in the classification and the context as a whole. 
     The third advantage of the present invention is that a target HTML document can be found efficiently, because this invention calculates the relevance to the types accurately beforehand and narrows the search results based on the relevance, or because this invention calculates the relevance to the types accurately beforehand and displays the search results by groups. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the first embodiment of the present invention. 
     FIG. 2 is a diagram showing an example of an HTML document for the first embodiment of the present invention. 
     FIG. 3 is a diagram showing an example of a rule for the first embodiment of the present invention. 
     FIG. 4 is a block diagram of the second embodiment of the present invention. 
     FIG. 5 is a diagram showing an example of an HTML document for the second embodiment of the present invention. 
     FIG. 6 is a diagram showing an example of a rule for the second embodiment of the present invention. 
     FIG. 7 is a block diagram showing the third embodiment of the present invention. 
     FIG. 8 is a diagram showing an example of an HTML document for the third embodiment of the present invention. 
     FIG. 9 is a diagram showing an example of a rule for the third embodiment of the present invention. 
     FIG. 10 is a block diagram showing the fourth embodiment of the present invention. 
     FIG. 11 is a diagram showing an example of an HTML document for the fourth embodiment of the present invention. 
     FIG. 12 is a diagram showing an example of a rule for the fourth embodiment of the present invention. 
     FIG. 13 is a block diagram showing the fifth embodiment of the present invention. 
     FIG. 14 is a diagram showing an example of a rule for the fifth embodiment of the present invention. 
     FIG. 15 is a block diagram showing the sixth embodiment of the present invention. 
     FIG. 16 is a diagram showing an example of an HTML document for the sixth embodiment of the present invention. 
     FIG. 17 is a diagram showing an example of a rule for the sixth embodiment. 
     FIG. 18 is a block diagram showing the seventh embodiment of the present invention. 
     FIG. 19 is a diagram showing an example of a rule for the seventh embodiment of the present invention. 
     FIG. 20 is a block diagram showing the eighth embodiment of the present invention. 
     FIG. 21 is a diagram showing an example of a rule for the eight embodiment of the present invention. 
     FIG. 22 is a block diagram showing the ninth embodiment of the present invention. 
     FIG. 23 is a diagram showing an example of a rule for the ninth embodiment. 
     FIG. 24 is a block diagram showing the tenth embodiment of the present invention. 
     FIG. 25 is a table showing a list in the database in the tenth embodiment of the present invention. 
     FIG. 26 is a block diagram showing the eleventh embodiment of the present invention. 
     FIG. 27 is a block diagram showing a modification of the eleventh embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [First Embodiment] 
     The first embodiment of the present invention will be explained with reference to the Figures. 
     Referring to FIG. 1, the first embodiment includes a document database  200  for storing HTML document data, a document classifier  100  controlled by a computer program, and a type index  210  for storing the types of the HTML documents generated from the document classifier  100 . 
     The document classifier  100  includes a structural feature extracting section  110  which includes a keyword feature extractor  111  for extracting features by a tag and keyword pair in the HTML document, a structural feature rule base  130  for scoring the points of the extracted structural features, and a verifier  120  for verifying the structural features according to the rules and adding the score to obtain the relevance to the HTML document. 
     The operation of the first embodiment will now be explained with reference to FIG.  1 . 
     The document classifier  100  extracts one HTML document at a time from the document database  200  storing the HTML document data, and inputs the HTML documents to the structural feature extracting section  110 . The structural feature extracting section  110  starts the keyword feature extractor  111 , extracts a tag and keyword pair from the HTML document, and send them to the verifier  120 . The structural feature rule base  130  stores a rule which defines conditions and scores for each type of document. The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules of the structural feature rule base  130 . The verifier adds the corresponding scores described by the rule, and obtains the relevance to each type of document. The verifier stores the obtained results in the type index  210 . 
     According to the first embodiment, the structural feature extracting section extracts a tag and keyword pair from the HTML document and determines the type of the document, allowing more detailed classification than that by the conventional process using only keyword information. 
     The operation of the first embodiment will now be described in greater detail, by way of example. 
     The document database  200  stores the HTML documents which a robot automatically collects from the Internet. The save format may be a database format or a file format in a file system. 
     FIG. 2 shows an example of the HTML document stored in the document database  200 . The type of the HTML document is “product catalog”. 
     The structural feature rule base  130  includes the rule shown in FIG.  3 . 
     The rule, forms, and scores shown in FIG. 3 are only examples, the present invention is not limited to this. 
     The meaning of the rule of FIG. 3 will be explained. The forms are marked off by colons (:). In this example, the basic form of the rule is “keyword: type: score: tag: keyword list.” The first item in this style is a label indicating that the rule matches the keyword feature extractor. The second item indicates that the rule is particular to the type of the document. The third item indicates the score to be added when the type of the document is verified. The fourth item indicates a tag which includes the keyword. The fifth item is a list of keywords in the tag. 
     For example, when the tag &lt;h 1 &gt; includes keywords “specification” or “spec.” with the tag &lt;h 1 &gt;, the rule “keyword: product catalog:  3 : &lt;h 1 &gt;: specification | spec.” adds three for the type of “product catalog”. FIG. 3 describes the rules for “product catalog”, “laboratory”, and “link collection”. 
     The structural feature extracting section  110  starts the keyword feature extractor  111 , extracts tag and keyword pairs from the HTML document, and sends them to the verifier  120 . The keyword feature extractor  111  extracts the tag and keyword pair of the HTML document. 
     Specifically, for the underlined portion (“equipment”) in the HTML document of FIG. 2, “&lt;HTML&gt;&lt;BODY&gt;&lt;CENTER&gt;&lt;FONT&gt;&lt;B&gt;&lt;A&gt;standard &lt;/A&gt;&lt;/B&gt;&lt;/FONT&gt;&lt;/CENTER&gt;&lt;/BODY&gt;&lt;/HTML&gt;” is extracted. This pair structure is an example, and the feature structure may employ a tree structure in which the branches correspond to the tags and the leaves correspond to the keywords. The form of the feature is not limited to the first embodiment. Further, the first embodiment extracts only keywords, but it may also extract phrases (such as “is”, or “is/are”). 
     The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules of the structural feature rule base  130 . One of the rules in FIG. 3, “keyword: product catalog:  1 : &lt;body&gt;: loading | possible | equipment”, is used to verify the features extracted by the keyword feature extractor  111 . That is, the verifier finds the tag of the fourth item in the rule from the tag and keyword pair structure, and finds the keyword “standard” in the same pair structure. Thus, the verification according to the rule is successful, and the score defined in the rule is added to obtain the relevance to the type of the HTML document (the product catalog in this example). 
     When the HTML document of FIG. 2 is verified according to the rules for three types in FIG. 3 (“product catalog”, “laboratory”, and “link collection”), only the above rule matches the HTML document. As the result, “product catalog” scores one point, “laboratory” scores zero points, and “link collection” scores zero points. Because the middle part of the HTML document is omitted, the points of “product catalog” through the entire document may in fact be higher. 
     If all the verifications according to the rules for the types in the structural feature rule base  130  are successful, the verifier  120  scores a full mark, which is defined as 100%. The verifier  120  outputs the relevance to each type. According to the rules shown in FIG. 3, the full mark of “product catalog” is at 11 points, the full mark of “laboratory” is at 19 points, and the full mark of “link collection” is at 13 points. Therefore, the relevance to “product catalog” is 9% (1/11×100), the relevance to “laboratory” is 0% (0/19×100), and the relevance to “link collection” is 0% (0/13×100). This calculation process is an example, is not limited to this embodiment, and may include averaging weights for the points of types. 
     The verifier  120  writes the results of the relevance in the type index  210 . The type index  210  stores the relevances to the types as they are, or may attach flags which are turned on for the type of the highest relevance and which are turned off for the other types. 
     [Second Embodiment] 
     The second embodiment of the present invention will now be explained with reference to Figures. 
     Referring to FIG. 4, as compared with the first embodiment, the second embodiment includes an image feature extractor  112 , which is provided in the structural feature extracting section  110 , for extracting an image feature from the HTML documents, instead of the keyword feature extractor  111 . 
     The operation of the second embodiment will be explained with reference to FIG.  4 . 
     In a manner similar to the first embodiment, the HTML document is input to the structural feature extracting section  110 . The structural feature extracting section  110  starts the image feature extractor  112 , extracts image files from the HTML document, and sends the image files to the verifier  120 . The structural feature rule base  130  stores a rule which defines conditions and scores for each type of document. 
     The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules in the structural feature rule base  130 . The verifier  120  adds the scores described in the rule, and obtains relevance of the document to each type. The calculated results are stored in the type index  210 . 
     According to the present invention, the structural feature extracting section extracts the image file from the HTML document and determines the type of the document, allowing more detailed classification than that by the conventional process using only keyword information. 
     The operation of the second embodiment will now be described in greater detail, by way of example. 
     Except for the structural feature extracting section  110 , the second embodiment is identical to the first embodiment, and therefore mainly the structural feature extracting section and the structural feature rule base  130  are described in the following. 
     FIG. 5 shows the middle part of the HTML document which is omitted in FIG.  2 . The type of the HTML document is “product catalog”. 
     The structural feature rule base  130  includes the rule shown in FIG.  6 . The basic form of this rule is “image: type: score: conditional expression.” The first item is a label indicating that the rule matches the feature extractor. The second item indicates a type of the rule, and the third item indicates a score. The fourth item indicates a conditional expression for an image file. 
     The rule “image: product catalog:  5 : over(30000)&gt;=2” means adding five points to the score of the type “product catalog” when the HTML document includes more than two images, each of which is more than 30000 bytes (=30 kilobytes). FIG. 6 describes the rules of the three types for “product catalog”, “laboratory”, and “link collection.” 
     The structural feature extracting section  110  starts the image feature extractor  112 , extracts image files from the HTML document, and sends the features of the image files to the verifier  120 . 
     For example, in the HTML document shown in FIG. 5, the byte sizes, the sizes of the images, and the formats are extracted from the image files of the five underlined portions (banner.gif, win 2 .gif, r 13  tit.gif, r500.gif, and r300.gif). If necessary, the images may be read by an OCR (Optical Character Reader), and the extracted character strings may be sent to the verifier. In FIG. 6, the rule “image: laboratory:  4 : ocr=laboratory” matches the character strings read by the OCR, and means adding four points when the character string “laboratory” is extracted after the process of the OCR. 
     The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules in the structural feature rule base  130 . In the HTML document of FIG. 5, the byte sizes of the five image files are 7722 bytes, 4490 bytes, 1947 bytes, 47804 bytes, and 56573 bytes, respectively. Then, the verification according to the rule in FIG. 6 “image: product catalog:  5 : over (30000)&gt;=2” is successful. According to the calculation process similar to the first embodiment, “product catalog” scores five points, “laboratory” scores two points, and “link collection” scores zero points. The relevance to “product catalog” is 83% (5/6×100), the relevance to “laboratory” is 22% (2/9×100), and the relevance to “link collection” is 0% (0/6×100). 
     [Third Embodiment] 
     The third embodiment of the present invention will be explained. 
     Referring to FIG. 7, as compared with the first embodiment, the third embodiment includes a link feature extractor  113 , provided in a structural feature extracting section  110 , for extracting a link feature from the HTML document, instead of the keyword feature extractor  111 . 
     In a manner similar to the first embodiment, the HTML document is input to the structural feature extracting section  110 . The structural feature extracting section  110  starts the link feature extractor  113 , extracts link features from the HTML document, and sends the link features to the verifier  120 . The structural feature rule base  130  stores a rule which defines conditions and scores for each type of document. 
     The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules in the structural feature rule base  130 . The verifier adds the scores described in the rule, and obtains the relevance of the document to each type. The verifier stores the calculation results in the type index  210 . 
     According to the third embodiment, the structural feature extracting section extracts the link information from the HTML document and determines the type of the document, allowing more detailed classification than that by the conventional process using only keyword information. 
     The operation of the third embodiment will now be described in greater detail, by way of example. 
     Except for the structural feature extracting section  110 , the third embodiment is identical to the first embodiment, and therefore mainly the structural feature extracting section and the structural feature rule base  130  are described in the following. 
     FIG. 8 shows an example of the HTML document whose type is “link collection”. 
     The structural feature rule base  130  stores the rule shown in FIG.  9 . The basic form of the rule is “link: type: score: conditional expression.” The first item is a label indicating that the rule matches the link feature extractor. The second item indicates a type of the rule, and the third item indicates a score. The fourth item indicates a conditional expression for a link. 
     For example, the rule “link: link collection:  4 : 20&gt;external&gt;=10” means adding four points to the score of the type “link collection” when the number of links to external sites is above 10 and below 20. The variable “external” indicates the number of links to the external sites, and the variable “internal” indicates the number of links to internal sites. FIG. 9 shows the rules for the three types for “product catalog”, “laboratory”, and “link collection”. 
     The structural feature extracting section  110  starts the link feature extractor  113 , extracts link information from the HTML document, and sends the features to the verifier  120 . For example, the thirteen underlined portions in the HTML document shown in FIG. 8 are extracted as external links. The verifier  120  verifies the features, which are sent from the structural feature extractor  110 , according to the rule of the structural rule base  130 . 
     The HTML document shown in FIG. 8 includes  21  external links and no internal links. A number of external and internal links are in fact included in the omitted middle portion of the document. Then, the verification according to the rule in FIG. 9 “link: link collection:  8 : external&gt;=20” is successful. According to the calculation process similar to the first embodiment, “product catalog” scores zero points, “laboratory” scores zero points, and “link collection” scores 8 points. That is, the relevance to “product catalog” is 0% (0/5×100), the relevance to “laboratory” is 0% (0/7×100), and the relevance to “link collection” is 57% (8/14×100). 
     [Fourth Embodiment] 
     The fourth embodiment of the present invention will be explained. 
     Referring to FIG. 10, as compared with the first embodiment, the fourth embodiment includes a tag structural feature extractor  114 , which is provided in the structural feature extracting section  110 , for extracting a tag feature from the HTML documents, instead of the keyword feature extractor  111 . 
     In a manner similar to the first embodiment, the HTML document is input to the structural feature extracting section  110 . The structural feature extracting section  110  starts the tag structural feature extractor  114 , extracts tag structure information from the HTML document, and sends the information to the verifier  120 . The structural feature rule base  130  stores a rule which defines conditions and scores for each type of document. The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules in the structural feature rule base  130 . The verifier adds the points described in the rule, and calculates the relevance to each type of document. The verifier stores the calculation results in the type index  210 . 
     According to the first embodiment, the structural feature extracting section extracts the tag information from the HTML document and determines the type of the document, allowing more detailed classification than that by the conventional process using only keyword information. 
     The operation of the fourth embodiment will now be described in greater detail, by way of example. 
     Except for the structural feature extracting section  110 , the fourth embodiment is identical to the first embodiment, and therefore mainly the structural feature extracting section and the structural feature rule base  130  are described in the following. 
     FIG. 11 shows a portion of the HTML document whose type is “product catalog.” 
     The structural feature rule base  130  stores a rule shown in FIG.  12 . The basic form of the rule is “structure: type: score: tag: [conditional expression].” The first item is a label indicating that the rule matches the tag structural feature extractor. The second item indicates a type of the rule, and the third item indicates a score. The fourth item is a tag portion structure to be extracted, and the fifth item indicates a conditional expression for variables (%1, %2, . . . ) in the tag structure or for the number (%n) of the tag structures. The fifth item is an option, and may not be necessarily filled when a conditional expression for the variables is unnecessary. 
     For example, the rule “structure: product catalog:  3 : &lt;table border=%1&gt;:%1&gt;=1” means adding three points to the score of “product catalog” when a &lt;table&gt; tag is found, a “border” attribute is detected, and the value of the “border” attribute is above 1. The rule “structure: product catalog:  2 : &lt;select&gt;:” means adding one point to the score of “product catalog” when a “&lt;select&gt; tag is found. The rule “structure: product catalog:  3 : &lt;sup&gt;: %n&gt;=3” means adding three points to the score of “product catalog” when three &lt;sup&gt; tags are found. Thus, FIG. 9 shows rules for the three types for “product catalog”, “laboratory”, and “link collection.” 
     The structural feature extracting section  110  starts the tag structural feature extractor  114 , extracts tag structure information from the HTML document, and sends the tag features to the verifier  120 . For example, the underlined portions in the HTML document shown in FIG. 11 are extracted as the &lt;table&gt; tag structure information and as the &lt;sup&gt;tag structure information. The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules in the structural feature rule base  130 . The HTML document in FIG. 11 includes the &lt;table&gt; tag whose “border” value is 1 and four &lt;sup&gt; tags. 
     In the rules listed in FIG. 12, the verifications according to two rules described above are successful. Because this verification allows the partial match for the &lt;table&gt; tag, “&lt;table border=1 width=100%&gt;”, extracted by the tag structural feature extractor  114 , matches the condition in the rule “&lt;table border=%1&gt;”. The rule may be a exact matching rule for detecting an item whose all attributes match the rule. An example of the verification using the exact matching rule is “structure: product catalog:  3 : &lt;table border=%1 width=%2&gt;: %1&gt;=1&amp;%2&gt;=50”. This rule must describe the attribute “width”. According to the calculation process similar to that in the first embodiment, “product catalog” scores 6 points, “laboratory” scores zero points, and “link collection” scores 2 points. The relevance to “product catalog” is 67% (6/9×100), the relevance to “laboratory” is 0% (0/1××100), and the relevance to “link collection” is 22% (2/9×100). 
     [Fifth Embodiment] 
     The fifth embodiment of the present invention will be explained. 
     Referring to FIG. 13, as compared with the first embodiment, the fifth embodiment includes a URL feature extractor  115 , which is provided in the structural feature extracting section  110 , for extracting URL feature information from the HTML documents, instead of the keyword feature extractor  111 . 
     In a manner similar to the first embodiment, the HTML document is input to the structural feature extracting section  110 . The structural feature extracting section  110  starts the URL feature extractor  115 , extracts the URL information of the HTML document, and sends it to the verifier  120 . The structural feature rule base  130  stores a rule which defines conditions and scores for each type of document. The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules of the structural feature rule base  130 . The verifier adds the corresponding scores described in the rule, and obtains the relevance to each type of document. The verifier stores the obtained results in the type index  210 . 
     According to the fifth embodiment, the structural feature extracting section extracts the URL information of the HTML document and determines the type of the document, allowing more detailed classification than that by the conventional process using only keyword information. 
     The operation of the fifth embodiment will now be described in greater detail, by way of example. 
     Except for the structural feature extracting section  110 , the fifth embodiment is identical to the first embodiment, and therefore mainly the structural feature extracting section and the structural feature rule base  130  are described in the following. 
     The structural feature rule base  130  includes the rule shown in FIG.  14 . The basic form of this rule is “url: type: score: [conditional expression].” The first item is a label indicating that the rule matches the URL feature extractor. The second item indicates a type of the rule, and the third item indicates a score. The fourth item indicates a conditional expression for a URL character string. 
     For example, the rule “url: product catalog:  2 : organization=co” means adding two points to the score of “product catalog” when the organization of the URL in the HTML document is a company (co). In addition to “organization”, the variables “nation”, name”, “path”, and “file” are employed. 
     When the url is “http://www.ΔΔ.co.jp/Products/PC××NX/index.html”, “ΔΔ” is bound to “name”, “co” is bound to “organization”, “jp” is bound to “nation”, “Products/PC××NX” is bound to “path”, and “index.html” is bound to “file”. Thus, FIG. 14 shows the rules for the three types, that is, “product catalog”, “laboratory”, and “link collection.” 
     The structural feature extracting section  110  starts the URL structural feature extracting section  115 , extracts the URL information of the HTML document, and sends it to the verifier  120 . For example, when the URL of the HTML document is the example described above, the bound items are extracted as the URL information. The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules in the structural feature rule base  130 . In the rules listed in FIG. 14, the verifications according to three rules are successful. According to the calculation process similar to that in the first embodiment, “product catalog” scores 8 points, “laboratory” scores zero points, and “link collection” scores zero points. The relevance to “product catalog” is 100% (8/8×100), the relevance to “laboratory” is 0% (0/2×100), and the relevance to “link collection” is 0% (0/10×100). 
     [Sixth Embodiment] 
     The sixth embodiment of the present invention will be explained. 
     Referring to FIG. 15, as compared with the first embodiment, the sixth embodiment includes a plugin feature extractor  116 , which is provided in the structural feature extracting section  110 , for extracting a plugin information from the HTML documents, instead of the keyword feature extractor  111 . 
     In a manner similar to the first embodiment, the HTML document is input to the structural feature extracting section  110 . The structural feature extracting section  110  starts the plugin feature extractor  116 , extracts the plugin information from the HTML document, and sends it to the verifier  120 . The structural feature rule base  130  stores a rule which defines conditions and scores for each type of document. The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules of the structural feature rule base  130 . The verifier adds the corresponding scores described in the rule, and obtains the relevance to each type of document. The verifier stores the obtained results in the type index  210 . 
     According to the present invention, the structural feature extracting section extracts the plugin information from the HTML document and determines the type of the document, allowing more detailed classification than that by the conventional process using only keyword information. 
     The operation of the sixth embodiment will now be described in greater detail, by way of example. 
     Except for the structural feature extracting section  110 , the sixth embodiment is identical to the first embodiment, and therefore mainly the structural feature extracting section and the structural feature rule base  130  are described in the following. 
     FIG. 16 shows an example of the HTML document whose type is “game.” 
     The structural feature rule base  130  stores a rule shown in FIG.  17 . The basic form of the rule is “plugin: type: score: [conditional expression].” The first item is a label indicating that the rule matches the tag structural feature extractor. The second item indicates a type of the rule, and the third item indicates a score. The fourth item indicates a conditional expression to determine the type of the plugin. 
     For example, the rule “plugin: game:  3 : type=swf” means adding 3 points to the score of the type “game” when the type of the plugin source is Shockwave Flash (swf) (the trademark of Macromedia, Inc.). FIG. 17 describes the rules for the three types for “game”, “product catalog”, and “personal page.” 
     The structural feature extracting section  110  starts the plugin structural feature extracting section  116 , extracts the plugin information of the HTML document, and sends it to the verifier  120 . For example, in the HTML document in FIG. 16, the underlined portions are extracted as the plugin information. The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules in the structural feature rule base  130 . In the rules listed in FIG. 17, the verifications according to the rules at lines  1  and  7  are successful. According to the calculation process similar to that in the first embodiment, “game” scores 3 points, “product catalog” scores zero points, and “personal page” scores 3 points. The relevance to “game” is 60% (3/5×100), the relevance to “product catalog” is 0% (0/10×100), and the relevance to “personal page” is 38% (3/8×100). 
     [Seventh Embodiment] 
     The seventh embodiment of the present invention will be explained. 
     Referring to FIG. 18, as compared with the first embodiment, the structural feature extracting section  110  further comprises an image feature extractor  112 , a link feature extractor  113 , and a tag structural feature extractor  114 . 
     In a manner similar to the first embodiment, the HTML document is input to the structural feature extracting section  110 . The structural feature extracting section  110  starts the keyword feature extractor  111 , the image feature extractor  112 , the link feature extractor  113 , and the tag structural feature extractor  114 , extracts various information from the HTML document, and sends it to the verifier  120 . The structural feature rule base  130  stores a rule which defines conditions and scores for each type of document. The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules of the structural feature rule base  130 . The verifier adds the corresponding scores described in the rule, and obtains the relevance to each type of document. The verifier stores the obtained results in the type index  210 . 
     According to the seventh embodiment, the structural feature extracting section extracts the various information from the HTML document and determines the type of the document, allowing more detailed classification than that by the conventional process using only keyword information. 
     The operation of the seventh embodiment will now be described in greater detail, by way of example. 
     Except that the structural feature extracting section  110  further comprises the image feature extractor  112 , the link feature extractor  113 , and the tag structure extractor  114 , the seventh embodiment is identical to the first embodiment, and therefore mainly the structural feature extracting section and the structural feature rule base  130  are described in the following. 
     The rules in the structural feature rule base  130  are shown in FIG.  19 . The forms of the rules are identical to the forms in the first, second, third, and fourth embodiments. 
     The structural feature extracting section  110  starts the keyword feature extractor  111 , the image feature extractor  112 , the link feature extractor  113 , and the tag structural feature extractor  114 , extracts various information, of which the feature extractors control, from the HTML document, and sends it to the verifier  120 . The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules of the structural feature rule base  130 . 
     While in the seventh embodiment the structural feature extracting section  110  includes the keyword feature extractor  111 , the image feature extractor  112 , the link feature extractor  113 , and the tag feature extractor  114 , one or some of the feature extractors  111  to  116  may be combined in the structural feature extracting section  110 . 
     [Eighth Embodiment] 
     The eight embodiment of the present invention will be explained. 
     Referring to FIG. 20, as compared with the first embodiment, the structural feature extractor  110  in the eighth embodiment includes an upper-lower level feature extractor  117  for extracting features of HTML documents which are linked with the target HTML document and of HTML documents with which the target HTML document is linked, instead of the keyword feature extractor  111 . The structural feature extractor  110  includes another structural feature extractor  140  which is started through the upper-lower level feature extractor  117 . 
     In a manner similar to the first embodiment, the HTML document is input to the structural feature extracting section  110 . The structural feature extracting section  110  starts the upper-lower level feature extractor  117 , extracts the information of the upper level documents (the HTML documents linked with the target HTML document) and of the lower level document (the HTML documents with which the target HTML document is linked), and sends the information to the verifier  120 . To extract the structural features from the upper- and lower-level documents, the upper-lower level extracting section  117  starts the structural feature extracting section  140  which is identical to one of the structural feature extracting sections  110  in the first to seventh embodiments. The structural feature rule base  130  stores a rule which defines conditions and scores for each type of document. The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules in the structural feature rule base  130 . The verifier  120  adds the scores described in the rule, and obtains relevance of the document to each type. The calculated results are stored in the type index  210 . 
     According to the eighth embodiment, the structural feature extracting section extracts the information of the upper and lower level documents and determines the type of the document, allowing more detailed classification than that by the conventional process using only keyword information. 
     The operation of the eighth embodiment will now be described in greater detail, by way of example. 
     Except for the structural feature extracting section  110 , the eighth embodiment is identical to the first embodiment, and therefore mainly the structural feature extracting section and the structural feature rule base  130  are described in the following. 
     The structural feature rule base  130  includes the rule shown in FIG.  21 . The basic form of this rule is “relation: type: score: up-down: rule.” The first item is a label indicating that the rule matches the upper-lower level feature extractor. The second item indicates a type of the rule, and the third item indicates a score. The fourth item indicates which upper or lower document is to be extracted. The fifth item is a rule depending on the feature extractor in the structural feature extracting section  140 , but the rule does not include “type” and “score.” 
     For example, the rule “relation: product catalog:  5 : up: keyword: &lt;a&gt;: specification | spec.” means adding 5 points to the score of “product catalog” when a “&lt;a&gt;” tag in the upper level document includes the keywords “specification” or “spec.” FIG. 21 describes the rules of the three types for “product catalog”, “laboratory”, and “link collection.” 
     The structural feature extracting section  110  starts the upper-lower level feature extractor  117 , which then starts the structural feature extracting section  140 . The structural feature extracting section  110  collects the features obtained from the feature extractors in the structural feature extracting section  140 , and sends them to the verifier  120 . Supposing that the structural feature extracting section  140  includes the keyword feature extractor  111 , the image feature extractor  112 , the link feature extractor  113 , and the tag structural feature extractor  114 , the rule of FIG. 21 is prepared. 
     The verifier  120  verifies the features, which are sent from the structural feature extracting section  110 , according to the rules in the structural feature rule base  130 . While in this embodiment the keyword feature extractor  111 , the image feature extractor  112 , the link feature extractor  113 , and the tag structural feature extractor  114  are combined in the structural feature extracting portion  140 , any or all of the feature extractors  111  to  116  may be combined. 
     [Ninth Embodiment] 
     The ninth embodiment of the present invention will be explained. 
     Referring to FIG. 22, as compared with the seventh embodiment, the ninth embodiment includes a score controller  150  for performing fine control of the relevance calculated by the verifier  120  based on a control rule  151 . 
     In a manner similar to the seventh embodiment, the verifier  120  calculates the relevance to the types. Then, the score controller  150  performs the fine control for the relevance to the types according to control rules  151  in consideration of the relationships between the types and of the whole situation. The relevance to the types controlled by the score controller  150  are stored in the type index  210 . 
     According to the ninth embodiment, the determination results, that is, relevance of the type of the HTML document made by the structural feature extracting section  110  and the structural feature rule base  130 , are finely controlled in consideration of the relationships between the types and of the whole situation, allowing more detailed classification than that by the conventional process using only keyword information. 
     Except for the score controller  150  and the control rule  151 , the ninth embodiment is identical to the seventh embodiment, and therefore mainly the verifier  120 , the score controller  150 , and the control rule  151  are described in the following. 
     An example of the control rule  151  is shown in FIG.  23 . The first control rule in FIG. 23 is an “if-then” form in which “if” represents a condition and “then” represents execution. For example, the rule at line  1  in FIG. 23 means increasing the relevance to “product catalog” by 10% and reducing the relevance to “personal page” by 10% when the difference between the relevance to “product catalog” and “personal page” is equal to or above 0% and equal to or below 10%, when the relevance to “persona page” is equal to or above 0%, and when the relevance to “product catalog” is equal to or below 90%. In FIG. 23, “max”, “min”, and “ave” are variable. “Max” represents a type showing the highest relevance, “min” represents a type showing the lowest relevance, and “ave” represents the average of all relevances of the types. The fourth control rule in FIG. 23 means increasing the highest relevance of the type by 20% when the highest relevance is equal to or below 30% and when the average of the relevances is equal to or below 20%. 
     The verifier  120  stores the scores, which are controlled by the score controller  150 , in the type index  210 . 
     [Tenth Embodiment] 
     The tenth embodiment of the present invention will be explained. 
     Referring to FIG. 24, as compared with the first embodiment, the tenth embodiment further comprises a document classifier  100 , a document data base  200 , a type index  210 , a searcher  300 , and an input/output device  400 . 
     The document classifier  100  may be one of the first to ninth embodiments. The searcher  300  comprises a full text search engine  310  for performing a search in the document database  200  by a keyword, a type searcher  320  for finding the relevance of a document to each type, a restrictor  330  for starting the full text search engine  310  and the type searcher  320  by a keyword and a type input from the input/output device  400 , and for restricting the search results. 
     The document classifier  100  calculates the relevance of all HTML documents in the document database  200  to the types, and the type index  210  stores the relevance with the same IDs as the HTML documents. From the input/output device  400 , a keyword and a type are sent to the restrictor  330 . The restrictor  330  sends the keyword to the full text search engine  310 . The full text search engine  310  performs the full text search in the document database  200 , and sends the IDs of the HTML documents matching the keyword to the restrictor  330 . The restrictor  330  sends the IDs and types of the HTML documents to the type searcher  320 . Based on the IDs and types of the HTML documents, the type searcher  320  performs a search of the relevance of the documents to the types, and sends the results to the restrictor  330 . The restrictor  330  sends the search results to the input/output device  400 . 
     According to the tenth embodiment, the document classifier classifies the documents by types, and the full text search engine narrows the enormous search results based on the index and indicates the narrowed results, allowing a user to find a target HTML document easily. 
     The operation of the tenth embodiment will now be described in greater detail, by way of example. 
     For example, the document database  200  stores 1,000,000 HTML documents. The ID numbers 1 to 1,000,000 are assigned to the HTML documents. The document classifier  100  has calculated the relevance to seven types and stores the relevance in the type index  210  beforehand. 
     FIG. 25 shows a part of the type index  210 . From the input/output device  400 , a user inputs, for example, a keyword “mobile xx” and a type “product catalog”. The restrictor  330  sends the keyword “mobile xx” to the full text search engine  310 . The full text search engine  310  performs the full text search in the document database  200 , and obtains results which include, for example, 10,000 documents. While in this embodiment the full text search engine is used to make a search in the HTML documents, the present invention is not limited to this, and may perform a search by sample phrases, example sentences, and similar documents. The restrictor  330  sends the IDs of the 10,000 HTML documents and the type “product catalog” to the type searcher  320 . 
     The type searcher  320  consults the type index  210  by the IDs, and obtains the relevance to the type “product catalog” from the records of the product catalog fields. For example, when the ID of the HTML document shown in FIG. 25 is  2 , the relevance to “product catalog” is 88%. 
     The restrictor  330  returns the ID of the HTML document, whose relevance is above a threshold value, to the input/output device  400 . When the threshold value is above 80%, there are three HTML documents  2 ,  6 , and  11  whose relevance to “product catalog” is above 80%. When the full text search engine  310  performs the full text search through the documents of IDs 1 to 17 by the keyword “mobile xx”, a conventional search system having only the full text search engine may display the results which include 17 documents, but the present invention narrows the results to the three documents. When, for example, the type index  210  as shown in FIG. 25, in which three documents are chosen from 17 documents, is applied to all the documents, the search can narrows the 10,000 documents to approximately 1765 documents. While in the embodiment the threshold value is above 80%, the threshold value may be changed depending on the circumstances. While the embodiment ignores the documents below the threshold value, the 17 documents may be displayed according to the order from the highest relevance to the lowest relevance. 
     [Eleventh Embodiment] 
     The eleventh embodiment will be explained with reference to Figures. 
     Referring to FIG. 26, the eleventh embodiment has a separator  340  for separating the search results by types, instead of the restrictor  330  in the tenth embodiment. 
     The input/output device  400  sends a keyword to the separator  340 . The separator  340  sends the keyword to the full text search engine  310 . The full text search engine  310  performs a full text search, and sends the IDs of the HTML document matching the keyword to the separator  340 . The separator  340  sends the IDs of the HTML documents to the type searcher  320 . The type searcher  320  finds the relevance to each type based on the IDs of the HTML documents, and sends the results back to the separator  340 . The separator  340  determines which type has the highest relevance in each HTML document, groups all the HTML documents by the type of the highest relevance, and sends the search results to the input/output device  400 . 
     According to the eleventh embodiment, the document classifier classifies the documents by types, and the full text search engine narrows the enormous search results based on the index and indicates the narrowed results, allowing a user to find a target HTML document easily. 
     The operation of the first embodiment will now be described in greater detail, by way of example. 
     The document database  200  and the type index  210  include the contents identical to those in the tenth embodiment. 
     From the input/output device  400 , the user inputs, for example, a keyword “mobile xx”. The separator  340  sends the keyword “mobile xx” to the full text search engine  310 . The full text search engine  310  performs the full text search in the document database  200 , and obtains the search results which include 10,000 hits. 
     The separator  340  sends the IDs of the 10,000 HTML documents to the type searcher  320 , determines which type has the highest relevance in each HTML document, and groups all the HTML documents by the type of the highest relevance. In FIG. 25, the HTML document whose ID is  1  is classified as “product catalog”, and the HTML document whose ID is  3  is classified as “laboratory.” 
     The grouped search results are sent back to the input/output device  400  for display to the user. 
     While in this embodiment all the HTML documents are grouped by the seven types, the HTML documents whose highest relevance are below 50% may be classified into a group “other” on the display. 
     As shown in FIG. 27, a WWW server engine  500  may be provided instead of the input/output device  400 , and a www browser  600  may allow the data input/output. The WWW server engine  500  and the WWW browser  600  may be provided in the tenth embodiment. 
     The embodiments described above may be provided by computer programs. In the inventions of the first to ninth embodiments, the document classifier  100  may be achieved by a computer-readable memory recording the programs which includes computer instructions. In the tenth and eleventh embodiments, the search engine  300  may be provided by a computer-readable memory recording the program which includes computer instructions. 
     This invention may be embodied in other forms or carried out in other ways without departing from the spirit thereof The present embodiments are therefore to be considered in all respects illustrative and not limiting, the scope of the invention being indicated by the appended claims, and all modifications falling within the meaning and range of equivalency are intended to be embraced therein.