3. The method of claim 2 , further comprising: determining a weight of the extracted candidate unregistered word with respect to a type of the document, wherein the appearance frequency is weighted by the weight.
3. The method of claim 2 , further comprising: determining a weight of the extracted candidate unregistered word with respect to a type of the document, wherein the appearance frequency is weighted by the weight. 4. The method of claim 3 , wherein in determining the weight, a first weight, a second weight, and a third weight are allocated to candidate unregistered words retrieved from a table, a new article, and an ordinary document of the documents, respectively, and the first weight is greater than the second weight, and the second weight is greater than the third weight.