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Introduction to Web Crawling and Regular Expression

Introduction to Web Crawling and Regular Expression. CSC4170 Web Intelligence and Social Computing Tutorial 1 Tutor: Tom Chao Zhou Email: czhou@cse.cuhk.edu.hk. Outline. Course & Tutors Information Introduction to Web Crawling Utilities of a crawler Features of a crawler

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Introduction to Web Crawling and Regular Expression

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  1. Introduction to Web Crawling and Regular Expression CSC4170 Web Intelligence and Social Computing Tutorial 1 Tutor: Tom Chao Zhou Email: czhou@cse.cuhk.edu.hk

  2. Outline • Course & Tutors Information • Introduction to Web Crawling • Utilities of a crawler • Features of a crawler • Architecture of a crawler • Introduction to Regular Expression • Appendix

  3. Course and Tutors Information • Course homepage: • http://wiki.cse.cuhk.edu.hk/irwin.king/teaching/csc4170/2009 • Tutors: • Xin Xin • Email: xxin@cse.cuhk.edu.hk • Venue: Room 101 • Tom (me) • Email: czhou@cse.cuhk.edu.hk • Venue: Room 114A

  4. Utilities of a crawler • Web crawler, spider. • Definition: • A Web crawler is a computer program that browses the World Wide Web in a methodical, automated manner. (Wikipedia) • Utilities: • Gather pages from the Web. • Support a search engine, perform data mining and so on. • Object: • Text, video, image and so on. • Link structure.

  5. Features of a crawler • Must provide: • Robustness: spider traps • Infinitely deep directory structures: http://foo.com/bar/foo/bar/foo/... • Pages filled a large number of characters. • Politeness: which pages can be crawled, and which cannot • robots exclusion protocol: robots.txt • http://blog.sohu.com/robots.txt • User-agent: * • Disallow: /manage/

  6. Features of a crawler (Cont’d) • Should provide: • Distributed • Scalable • Performance and efficiency • Quality • Freshness • Extensible

  7. Doc Fingerprint Robots templates URL set www DNS Parse Content Seen? URL Filter Dup URL Elim Fetch URL Frontier Architecture of a crawler

  8. Doc Fingerprint Robots templates URL set www DNS Parse Content Seen? URL Filter Dup URL Elim Fetch URL Frontier Architecture of a crawler (Cont’d) • URL Frontier: containing URLs yet to be fetches in the current crawl. At first, a seed set is stored in URL Frontier, and a crawler begins by taking a URL from the seed set. • DNS: domain name service resolution. Look up IP address for domain names. • Fetch: generally use the http protocol to fetch the URL. • Parse: the page is parsed. Texts (images, videos, and etc.) and Links are extracted.

  9. Doc Fingerprint Robots templates URL set www DNS Parse Content Seen? URL Filter Dup URL Elim Fetch URL Frontier Architecture of a crawler (Cont’d) • Content Seen?: test whether a web page with the same content has already been seen at another URL. Need to develop a way to measure the fingerprint of a web page. • URL Filter: • Whether the extracted URL should be excluded from the frontier (robots.txt). • URL should be normalized (relative encoding). • en.wikipedia.org/wiki/Main_Page • <a href="/wiki/Wikipedia:General_disclaimer" title="Wikipedia:General disclaimer">Disclaimers</a> • Dup URL Elim: the URL is checked for duplicate elimination.

  10. Architecture of a crawler (Cont’d) • Other issues: • Housekeeping tasks: • Log crawl progress statistics: URLs crawled, frontier size, etc. (Every few seconds) • Checkpointing: a snapshot of the crawler’s state (the URL frontier) is committed to disk. (Every few hours) • Priority of URLs in URL frontier: • Change rate. • Quality. • Politeness: • Avoid repeated fetch requests to a host within a short time span. • Otherwise: blocked 

  11. Regular Expression • Usage: • Regular expressions provide a concise and flexible means for identifying strings of text of interest, such as particular characters, words or patterns of characters. • Today’s target: • Introduce the basic principle. • A tool to verify the regular expression: Regex Tester • http://www.dotnet2themax.com/blogs/fbalena/PermaLink,guid,13bce26d-7755-441e-92b3-1eb5f9e859f9.aspx

  12. Regular Expression • Metacharacter • Similar to the wildcard in Windows, e.g.: *.doc • Target: Detect the email address

  13. Regular Expression • \b: stands for the beginning or end of a Word. • E.g.: \bhi\b find hi accurately • \w: matches letters, or numbers, or underscore. • .: matches everything except the newline • *: content before * can be repeated any number of times • \bhi\b.*\bLucy\b • +: content before + can be repeated one or more times • []: match characters in it • E.g: \b[aeiou]+[a-zA-Z]*\b • {n}: repeat n times • {n,}: repeat n or more times • {n,m}: repeat n to m times

  14. Regular Expression • Target: Detect the email address • Specifications: • A@B • A: combinations English characters a to z, or digits, or . or _ or % or + or – • B: cse.cuhk.edu.hk or cuhk.edu.hk (English characters) • Answer: • \b[a-z0-9._%+-]+@[a-z.]+\.[a-z]{2}\b

  15. Appendix • Mercator Crawler: • http://mias.uiuc.edu/files/tutorials/mercator.pdf • Regular Expression tutorial: • http://www.regular-expressions.info/tutorial.html

  16. Questions?

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