14. A method of establishing a plain text document from a HTML document, comprising the steps of: (A) acquiring a HTML document defined by HTML elements, each composed of tags and content between the tags; (B) pre-processing the HTML document by omitting some of the HTML elements, whereby the rest of the HTML document comprises at least one target tag and at least one corresponding content; (C) using a data structure to store the remaining tags of the pre-processed HTML document; (D) grouping the remaining HTML elements with the remaining tags stored in the data structure of the pre-processed HTML document into at least one target group according to the target tag(s); (E) identifying the target group(s) most related to a title of the HTML document by comparing correlation(s) between the target group(s) and the title, and establishing a plain text document having the content of the identified target group, wherein the target group(s) most related to the title of the HTML document is identified by the steps: (E- 1 ) if there is no sub-group in the target group(s), identifying the target group most related to the title of the HTML document by comparing correlation(s) between the target group(s) and the title; (E- 2 ) calculating similarities of the target groups not be identified in the step (E- 1 ) to the most title-related target group based on a vector space model to identify the target groups having the similarities higher than a predetermined threshold, and establishing the plain text document having the content of the identified target groups; (E- 3 ) if there is (are) sub-group(s) in the target group(s), identifying the sub-group most related to the title of the HTML document by comparing correlation(s) between the sub-groups and the title; (E- 4 ) if there is only one sub-group, establishing the plain text document having the content of the identified sub-group; and (E- 5 ) if there are more than one sub-groups, calculating similarities of the other sub-groups to the most title-related sub-group based on a vector space model to identify the sub-groups having the similarities higher than a predetermined threshold, and establishing the plain text document having the content of the identified sub-groups.
14. A method of establishing a plain text document from a HTML document, comprising the steps of: (A) acquiring a HTML document defined by HTML elements, each composed of tags and content between the tags; (B) pre-processing the HTML document by omitting some of the HTML elements, whereby the rest of the HTML document comprises at least one target tag and at least one corresponding content; (C) using a data structure to store the remaining tags of the pre-processed HTML document; (D) grouping the remaining HTML elements with the remaining tags stored in the data structure of the pre-processed HTML document into at least one target group according to the target tag(s); (E) identifying the target group(s) most related to a title of the HTML document by comparing correlation(s) between the target group(s) and the title, and establishing a plain text document having the content of the identified target group, wherein the target group(s) most related to the title of the HTML document is identified by the steps: (E- 1 ) if there is no sub-group in the target group(s), identifying the target group most related to the title of the HTML document by comparing correlation(s) between the target group(s) and the title; (E- 2 ) calculating similarities of the target groups not be identified in the step (E- 1 ) to the most title-related target group based on a vector space model to identify the target groups having the similarities higher than a predetermined threshold, and establishing the plain text document having the content of the identified target groups; (E- 3 ) if there is (are) sub-group(s) in the target group(s), identifying the sub-group most related to the title of the HTML document by comparing correlation(s) between the sub-groups and the title; (E- 4 ) if there is only one sub-group, establishing the plain text document having the content of the identified sub-group; and (E- 5 ) if there are more than one sub-groups, calculating similarities of the other sub-groups to the most title-related sub-group based on a vector space model to identify the sub-groups having the similarities higher than a predetermined threshold, and establishing the plain text document having the content of the identified sub-groups. 15. The method of establishing a plain text document from a HTML document as claimed in claim 14 , wherein the predetermined threshold is 0.6.