A template or wrapper tree for a document such as a web page is generalized from the bottom up (from leaf toward root of a logical tree structure of the template). At a given level in the tree, sub-trees are clustered and the clustered sub-trees are generalized, and the process is repeated at a next higher level in the tree, resulting in a generalized template or wrapper tree. This can be done by generating a nested pattern regular expression based on the sub-tree clusters, merging sub-trees based on the nested pattern regular expression, and then replacing sub-trees in a tree-based regular expression of the template or wrapper at the given level with the merged sub-trees. This process is repeated at a next higher level of the tree (progressing from leaf towards root) until the wrapper or tree-based regular expression that represents the template is fully generalized.