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Finding an apartment is a lengthy and tedious process. Once decided, one can never be sure not to have missed an even better offer which would have been just one click away. Form understanding is key to automatically access and process all the relevant---and nowadays readily available---data.

We introduce opal (ontology-based web pattern analysis with logic), a novel, purely logical approach to web form understanding: opal labels, structures, and groups form fields according to a domain-specific ontology linked through phenomenological rules to a logical representation of a DOM. The phenomenological rules describe how ontological concepts appear on the web; the ontology formalizes and structures common patterns of web pages observed in a domain. A unique feature of opal is that all domain-independent assumptions about web forms are represented in rules, whereas domain-specific assumptions …