Logical Foundations of Web Data Extraction
Several wrapper programming languages for extracting information from Web pages have been based on logic, specifically on fragments of datalog. This entry shows how logical languages can be used for Web information extraction, and surveys expressiveness and complexity aspects of a foundational logical wrapping language, monadic datalog.
A substantial amount of research has studied the problem of learning wrapper programs from examples (see Wrapper Induction). Unfortunately, it is now known that the expressive power of learnable wrappers is fundamentally limited. This has motivated further work on visual wrapper programming languages, which simplify and speed up wrapper definition. Visual wrapping is now supported by several implemented systems (cf. XWrap  and W4F ; Lixto , a commercial product). The Lixto project was the first to emphasize and study expressiveness of visual wrapper languages. Its approach is based on a datalog-like language,...
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