automatically learn Stalker **rules for these Web sources. The new extraction rules were**
validated by using them to successfully extract data from sets of test pages.

There remains work to be done on wrapper maintenance. Our current algorithms are not sufficient to automatically re-generate Stalker **rules for sources that return lists of tuples.** However, preliminary results indicate (Lerman et al., 2001) that it is feasible to combine information about the structure of data with *a priori* **expectations about the structure of** Web pages containing lists to automatically extract data from lists and assign it to rows and columns. We believe that these techniques will eventually eliminate the need for the user to mark up Web pages and enable us to automatically generate wrappers for Web sources. Another exciting direction for future work is using the DataProG **algorithm to automatically**
create wrappers for new sources in some domain given existing wrappers for other sources in the same domain. For example, we can learn the author, title and price fields for the AmazonBooks source, and use them to extract the same fields on the Barnes&**NobleBooks** source. Preliminary results show that this is indeed feasible. Automatic wrapper generation is an important cornerstone of information-based applications, including Web agents.

## 6. Acknowledgments

We would like to thank Priyanka Pushkarna for carrying out the wrapper verification experiments.

The research reported here was supported in part by the Defense Advanced Research Projects Agency (DARPA) and Air Force Research Laboratory under contract/agreement numbers F30602-01-C-0197, F30602-00-1-0504, F30602-98-2-0109, in **part by the Air Force** Office of Scientific Research under grant number F49620-01-1-0053, **in part by the Integrated**
Media Systems Center, a National Science Foundation (NSF) Engineering Research Center, cooperative agreement number EEC-9529152 and in part by the NSF under award number DMI-0090978. The U.S. Government is authorized to reproduce **and distribute reports for** Governmental purposes notwithstanding any copy right annotation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of any of the above organizations or any person connected with them.

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