Monadic Datalog and the Expressive Power of Languages for Web Information Extraction

Research on information extraction from Web pages (wrapping) has seen much activity in recent times (particularly systems implementations), but little work has been done on formally studying the expressiveness of the formalisms proposed or on the theoretical foundations of wrapping. In this paper, we first study monadic datalog as a wrapping language (over ranked or unranked tree structures). Using previous work by Neven and Schwentick, we show that this simple language is equivalent to full monadic second order logic (MSO) in its ability to specify wrappers. We believe that MSO has the right expressiveness required for Web information extraction and thus propose MSO as a yardstick for evaluating and comparing wrappers. Using the above result, we study the kernel fragment Elogof the Elog wrapping language used in the Lixto system (a visual wrapper generator). The striking fact here is that Elogexactly captures MSO, yet is easier to use. Indeed, programs in this language can be entirel...

• Type: Conference
• Year: 2002
• Where: PODS
• Authors: Georg Gottlob, Christoph Koch
• Keywords: Database, Elog Wrapping Language, Monadic Datalog, PODS 2002, Wrapping Languages