NAME

WWW::Search::Scraper - framework for scraping results from search engines.

SYNOPSIS

WWW::Search::Scraper('engineName');

DESCRIPTION

"Scraper" is a framework for issuing queries to a search engine, and scraping the data from the resultant multi-page responses.

As a framework, it allows you to get these results using only slight knowledge of HTML and Perl. (All you need to know you can learn by reading this document.)

A Perl script, "Scraper.pl", uses Scraper.pm to investigate the "advanced search page" of a search engine, issue a user specified query, and parse the results. (Scraper.pm can be used by itself to support more elaborate searching Perl scripts.) Scraper.pl and Scraper.pm have a limited amount of intelligent to figure out how to interpret the search page and its results. That's where your human intelligence comes in. You need to supply hints to Scraper to help it find the right interpretation. And that is why you need some limited knowledge of HTML and Perl.

FRONT-END

The front-end of Scraper is the part that figures out the search page and issues a query. There are three ways to implement this end.

Use Sherlock

Apple Computer and Mozdev have established a standard format for accessing search engines named "Sherlock". Via a "plugin" that describes the search engine's submission form and results pages, both Mac's and Mozilla can access many search engines via a common interface. The Perl package WWW::Search::Sherlock can read Sherlock plugins and access those sites the same way.

This is simplest way to get up and running on a search engine. You just name the plugin, provide your query, and watch it fly! You do not even need a sub-module associated with the search engine for this approach. See WWW::Search::Sherlock for examples of how this is done.

There are about a hundred plugins available at http://sherlock.mozdev.org/source/browse/sherlock/www/, contributed by many in the Open Source community.

There are a few drawbacks to this approach.

Not all search engines have plugins.

Obviously, you are limited in this approach to those search engines that have plugins already built. (You can build a plugin yourself, which is remarkably easy. See http://www.apple.com/sherlock/plugindev.html.)

The Sherlock standard is somewhat limited.

Two items in particular are lacking in the Sherlock standard. One, you can supply the value for only one of the fields in the submission form. This still makes Sherlock pretty valuable, since it's rare that a search engine will have more than one interesting field to fill in (and WWW::Search::Sherlock will allow you to set other field values, even though that's not in the Sherlock standard.) Secondly, Sherlock makes no effort to load NEXT pages, so your results will be limited to those appearing on the first response page.

The Sherlock standard does not parse all the interesting data from the response.

Sherlock parses only a half-dozen interesting fields from the data. Your client is left with the burden of parsing the rest from the 'result' data field.

Not all Sherlock plugins are created equal.

You'll find that many of the plugins simply do not work. This is in spite of the fact that Sherlock includes an automatic updating feature. But if you're updating to a version that doesn't work either, then you're kind of stuck!

If you run into these limitations, then you may want to use one of the following approaches.

Load-and-parse the submission <FORM>

Scraper has the capability to automatically load the submission form, parse it, and create the query request. All you do is supply the URL for the submission form, and the query parameter(s). Scraper loads and parses the form, fills in the fields, and pushes the submit button for you. All you need to do is parse the results in the back-end.

Parse the <FORM> manually.

Go get the form yourself, and view the source. Editing the native_setup_search() method of your search module, find the URL of the ACTION= in the <FORM>, and plug that into your search module's {_option}{_base_url} attribute. Also provide the METHOD= value from the <FORM> into the <_http_method} attribute. You'll find the input fields in the <FORM> as <INPUT> elements. Supply values to these fields via the {'option'=>'value'} parameter of the next_result() method, and you're on your way.

See the EXAMPLES below for these two latter approaches.

BACK-END

The back-end of Scraper.pm receives the response from the search engine, handling the multiple pages which it may be composed of, parses the results, and returns to the caller an appropriate Perl representation of these results ("appropriate" means an array of hash tables of type WWW::Search::SearchResult). Scraper.pl (or some other Perl client) further processes this data, or presents in some human readable form.

There are a few common ways in which search engines return their results in the HTML response. These could be detected by Scraper.pm if it were intelligent enough, but unfortunately most search engines add so much administrative clutter, banner ads, "join" options, and so forth to the result that Scraper.pm usually needs some help in locating the real data.

The Scraper scripting language consists of both HTML parsing and string searching commands. While a strict HTML parse should produce the most reliable results, as a practical matter it is sometimes extremely difficult to grok just what the HTML structure of a response is (remember, these reponses are composed by increasingly complex application server programs.) Therefore, it is necessary to provide some hints as to where to start an interpretation by giving Scraper some kind of string searching command.

The string searching commands (BODY, COUNT, NEXT) will point Scraper to approximately the right place in the response page, while HTML parsing commands (TABLE, TR, TD, etc) will precisely extract the exact data. There are also ways to to callbacks into your sub-module to do exactly the type of parsing your engine requires.

Scraper performs its function by viewing the entire response page at once. Whenever a particular section of the page is recognized, it will process that section according to your instructions, then discard the recognized text from the page. It will repeat until no further sections are recognized.

We'll illustrate the exact syntax of this language in later examples, but the commands in this language include:

BODY

This command directs Scraper to a specific section of the result page. Two parameters on the BODY command give it a "start-string" and an "end-string". Scraper will search the result page for these two strings, throw away everything before the "start-string" (including the start-string), throw away everything after the "end-string" (including the end-string), then continue processing with the remainder.

This is a quick way to get rid of a lot of administrative clutter. Either of the parameters is optional, but one should be supplied or else it's a no-op.

Both start-string and end-string are treated as "regular expressions". If you don't know anything about regular expressions, that's ok. Just treat them as strings that you would search for in the result page; see the examples.

COUNT

This command provides a string by which Scraper will locate the "approximate count". It is a regular expression (see comments above). See the examples for some self-explanatory illustrations.

NEXT

This command provides a string by which Scraper will be able to locate the "NEXT" button. You supply a string that you expect to appear as text in the NEXT button anchor element (this is easier to find than you might expect. A simple search for "NEXT" often does the trick.)

It is a regular expression (see comments above). See the examples for some self-explanatory illustrations.

TABLE, TR, DL or FORM

These commands are the HTML parsing commands. They use strict HTML parsing to zero in on areas containing actual data.

TD, DT and DD

These commands declare the locations of the actual data elements. Some search engines use <TD> elements to present data, some use <DT> and <DD> elements. (Some use neither, which is why you'll need to use DATA or REGEX on these.) The first parameter on the TD, DT or DD command names the field in which the garnered data will be placed. A second parameter provides a reference to optional subroutine for further processing of the data.

DATA

DATA is like BODY in that it specifies a specific area of the document according to start and end strings. It's different in that it will take that text and store it as result data rather than further processing it.

A and AN

Often, data and a hyperlink (to a detail page, for instance) are presented simultaneously in an "anchor" element. Two parameters on the A command name the fields into which the garnered link and data will be placed.

There are two forms of the A command since some loosely coded HTML will supply the hyperlink without the quote marks. This creates some disturbing results sometimes, so if your data is in an anchor where the HREF is provided without quotes, then the AN operation will parse it more reliably.

HIT*

This command declares that the hits are about to be coming! The first (and only) parameter of the HIT* command specifies a Scraper script, as described here, for parsing all the data for one complete record. HIT* will process that script, stashing the record away, discarding the parsed text, and continuing until there is no more data to be found the corresponding text.

REGEX

Well, this would be self-explanatory if you knew anything about regular expressions, you dope! Oh, sorry . . .

The first parameter on the REGEX command is a regular expression. The rest of the parameters are a list naming which fields the matched variables of this regex ($1, $2, $3, etc) will be placed.

CALLBACK

CALLBACK is like DATA, in that it selects a section of the text based on start and end strings (which are treated as regular expressions). Then, rather than storing the data away, it will call a subroutine that you have provided in your sub-module. Your subroutine may decide to store some data into the hit; it can decide whether to discard the text after processing, or to keep it for further processing; it can decide whether the data justifies being a hit or not (unless some parsing step finds this justification, then the parsing will stop. TD, DATA, REGEX, et.al. will do that, but if you are using CALLBACK, you might not need the other data harvesting operations at all, so your callback subroutine must "declare" this justification when appropriate).

See the code for WWW::Search::Sherlock for an illustration of how this works. Sherlock uses this method for almost all its parsing. A sample Sherlock scraper frame is also listed below in the EXAMPLES.

RESIDUE

After each of the parsing commands is executed, the text within the scope of that command is discarded. In the end, there may be some residue left over. It may be convenient to put this residue in some field so you can see if you have missed any important data in your parsing.

BOGUS

Even after carefully designing a scraper frame, the HIT* section's parsing sometimes results in extra hits at the beginning or end of the page. A positive value for BOGUS clips that many responses from the beginning of the hit list; a negative value for BOGUS pops that many resonses off the end of the list.

TRACE

TRACE will print the entire text that is currently in context of the Scraper parser, to STDERR.

HTML

HTML is simply a command that you will place first in your script. Don't ask why, just do it.

SYNTAX

Scraper accepts its command script as a reference to a Perl array. You don't need to know how to build a Perl array; just follow these simple steps.

As noted above, every script begins with an HTML command

[ 'HTML' ]

You put the command is square brackets, and the name of the command in single quotes. HTML will have a single parameter, which is a reference to a Scraper script (in other words, another array).

[ 'HTML', [ ...Scraper script... ] ]

(You can see this is going to get messy with all these square brackets.)

Suppose we want to parse for just the NEXT button.

[ 'HTML', 
          [
                 [ 'NEXT', '<B>Next' ]
          ]
]

The basic syntax is, a set of square brackets, and a command name in single quotes, to designate a command. Following that command name may be one or two parameters, and following those parameters may be another list of commands. The list is within a set of square brackets, so often you will see two opening brackets together. At the end you will see a lot of closing brackets together (get used to counting brackets!).

EXAMPLES

CraigsList

CraigsList.com produces a relatively simple result page. Unfortunately, it does not use any of the standard methods for presenting data in a table, so we are required to use the REGEX method for locating data.

Most search engines will not require you to use REGEX. We've used CraigsList here not to illustrate REGEX, but to illustrate the structure of the Scraper scripting syntax more clearly. Just ignore the REGEX command in this script; realize that it parses a data string and puts the results in the fields named there.

[ 'HTML', 
   [
      [ 'BODY', '</FORM>', '' ,
         [
            [ 'COUNT', 'found (\d+) entries'] ,
              [ 'HIT*' ,
                [
                   [ 'REGEX', '(.*?)-.*?<a href=([^>]+)>(.*?)</a>(.*?)<.*?>(.*?)<', 
                               'date',  'url', 'title', 'location', 'description' ]
                ]
            ]
         ]
      ]
   ]
]

This tells Scraper to skip ahead, just past the first "</FORM>" string (it's only a coincidence that this string is also an HTML end-tag.) In the remainder of the result page, Scraper will find the appoximate COUNT in the string "found (\d+) entries" (the '\d+' means to find at least one digit), then the HITs will be found by applying the regular expression repeatedly to the rest.

JustTechJobs

JustTechJobs.com presents one of the prettiest results pages around. It is nice and very deeply structured (by Lotus-Domino, I think), which makes it very difficult to figure out manually. However, a few simple short-cuts produce a relatively simple Scraper script.

[ 'HTML', 
   [   
      [ 'COUNT', '\d+ - \d+ of (\d+) matches' ] ,
      [ 'NEXT', 1, '<b>Next ' ] ,
      [ 'HIT*' ,
         [
            [ 'BODY', '<input type="checkbox" name="check_', '',
               [  [ 'A', 'url', 'title' ] ,
                  [ 'TD' ],
                  [ 'TABLE', '#0',
                     [
                        [ 'TD' ] ,
                        [ 'TD', 'payrate' ],
                        [ 'TD' ] ,
                        [ 'TD', 'company' ],
                        [ 'TD' ] ,
                        [ 'TD', 'locations' ],
                        [ 'TD' ] ,
                        [ 'TD', 'description' ]
                     ]
                  ]
               ]
            ]
         ]
      ]
   ]
]

Note that the initial BODY command, that was used in CraigsLIst, is optional. We don't use it here since most of JustTechJobs' result page is data, with very little administrative clutter.

We pick up the COUNT right away, with a simple regular expression. Then the NEXT button is located and stashed. The rest of the result page is rich with content, so the actual data starts right away.

Because of the extreme complexity of this page (due to its automated generation) the simplest way to locate a data record is by scanning for a particular string. In this case, the string '<input type . . .check_' identifies a checkbox that starts each data record on the JustTechJobs page. We put this BODY command inside of a HIT* so that it is executed as many times as required to pick up all the data records on the page.

Within the area specified by the BODY command, you will find a table that contains the data. The first parameter of the TABLE command, '#0', means to skip zero tables and to just read the first one. The second parameter of the TABLE is a script telling Scraper how to interpret the data in the table. The primitive data in this table is contained in TD elements, as are labels for each of the data elements. We throw away those labels by specifying no destination field for the data.

The page, as composed by Lotus-Domino, literally consists of a form, containing several tables, one of which contains another table, which in turn contains data elements which are themselves two tables in which each of the job listings are presented in various forms (I think). Given such a complex page, this Scraper script is remarkably simple for interpreting it.

DICE

Ok, I know you all wanted to know what DICE.com looks like. Well, here it is:

[ 'HTML', 
   [  
      [ 'BODY', ' matching your query', '' ,
         [  
            [ 'NEXT', 1, '<img src="/images/rightarrow.gif" border=0>' ]
           ,[ 'COUNT', 'Jobs [-0-9]+ of (\d+) matching your query' ]
           ,[ 'HIT*' ,
               [  
                  [ 'DL',
                     [
                        [ 'DT', 'title', \&addURL ] 
                       ,[ 'DD', 'location', \&touchupLocation ]
                       ,[ 'RESIDUE', 'residue' ]
                     ]
                  ]
               ]
            ]
         ] 
      ]  
   ]
]

We'll leave this as an exercise for the reader (note that this is the "brief" form of the response page.)

Sherlock

Now that you're fairly well-versed in Scraper frame operations syntax, we'll use a fairly complex frame, automatically generated by WWW::Search::Sherlock, to illustrate how Sherlock uses the Scraper framework.

If you point Sherlock to the Yahoo plugin, it will generate the following Scraper frame to parse the result page.

[
  'HTML',
  [
    [
      'CALLBACK', \&resultList,
      'Inside Yahoo! Matches',
      'Yahoo! Category Matches',
      [
        [
          'HIT*',
          [
            [
              'CALLBACK', \&resultItem,
              '<b>',
              '<br>',
              [
                [
                  'CALLBACK',
                  \&resultData,
                  '<b>',
                  ':</b>',
                  'result_name'
                ]
              ],
              undef
            ]
          ],
          'result'
        ]
      ]
    ],
    [
      'CALLBACK',
      \&resultList,
      'Yahoo! Category Matches',
      'Yahoo! News Headline Matches',
      [
        [
          'HIT*',
          [
            [
              'CALLBACK', \&resultItem,
              '<dt><font face=arial size=-1>',
              '</a></li><p></dd>',
              [
                [
                  'CALLBACK', \&resultData,
                  '<li>',
                  '</a></li><p></dd>',
                  'result_name'
                ]
              ],
              undef
            ]
          ],
          'category'
        ]
      ]
    ]
  ]
]

You'll notice that there are three callback functions in here (six invocations). These are named after the parts-of-speech specified in the Sherlock technotes. These callback functions will process the data a little differently than the standard Scraper functions would.

In Sherlock, for instance, the start and end strings are considered part of the data, so throwing them away causes unfortunate results. Our callbacks handle the data more in the way that Sherlock's creators intended.

'resultList' corresponds to Scraper's BODY, 'resultItem' corresponds to Scraper's TABLE, and 'resultData' corresponds to Scraper's DATA. The next two parameters of each CALLBACK operation indicate the start and end strings for that callback function. (A fourth parameter allows you to pass more specific information from the Scraper frame to the callback function, as desired.) Of course, these callback functions then handle the data in the "Sherlock way", rather than the "Scraper way".

Note that the start string for the second resultList is the same as the end string of the first resultList. This is but one illustration of how Sherlock handles things differently than Scraper. But by using the CALLBACK operation, just about any type of special treatment can be created for Scraper.

We refer you to the code for WWW::Search::Sherlock for further education on how to compose your own CALLBACK functions.

AUTHOR

Glenn Wood, glenwood@alumni.caltech.edu.

COPYRIGHT

Copyright (C) 2001 Glenn Wood. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

cpanm WWW::Search::Scraper

perl -MCPAN -e shell
install WWW::Search::Scraper