Discovery of maximally frequent tag tree patterns with contractible variables from semistructured documents

In order to extract meaningful and hidden knowledge from semistructured documents such as HTML or XML files, methods for discovering frequent patterns or common characteristics in semistructured documents have been more and more important. We propose new methods for discovering maximally frequent tree structured patterns in semistructured Web documents by using tag tree patterns as hypotheses. A tag tree pattern is an edge labeled tree which has ordered or unordered children and structured variables. An edge label is a tag or a keyword in such Web documents, and a variable can match an arbitrary subtree, which represents a field of a semistructured document. As a special case, a contractible variable can match an empty subtree, which represents a missing field in a semistructured document. Since semistructured documents have irregularities such as missing fields, a tag tree pattern with contractible variables is suited for representing tree structured patterns in such semistructured documents. First, we present an algorithm for generating all maximally frequent ordered tag tree patterns with contractible variables. Second, we give an algorithm for generating all maximally frequent unordered tag tree patterns with contractible variables.