WebScraperHelper: Simple Generation of SXPath Queries from SXML Examples

Neil Van Dyke

License: LGPL 3   Web: http://www.neilvandyke.org/webscraperhelper/

(require (planet neil/webscraperhelper:1:2))

1 Introduction

WebScraperHelper is intended as a programmer’s aid for crafting SXPath queries to extract information (e.g., news items, prices) from HTML Web pages that have been parsed by HtmlPrag. The current version of WebScraper accepts an example SXML (or SHTML) document and an example “goal” subtree of the document, and yields up to three different SXPath queries. A generated query can often be incorporated into a Web-scraping program as-is, for extracting information from documents with very similar formatting. Generated queries can also be used as starting points for hand-crafted queries.

For example, given the SXML document doc:

(define doc

'(*TOP* (html (head (title "My Title"))

(body (@ (bgcolor "white"))

(p "Summary: This is a document.")

(div (@ (id "ResultsSection"))

(h2 "Results")

(p "These are the results.")

(table (@ (id "ResultTable"))

(tr (td (b "Input:"))

(td "2 + 2"))

(tr (td (b "Output:"))

(td "Four")))

(p "Lookin' good!"))))))

evaluating the expression

  (webscraperhelper '(td "Four") doc)

will display generated queries like:

Absolute SXPath:           (html body div table (tr 2) (td 2))

Absolute SXPath with IDs:  (html body

(div (@ (equal? (id "ResultsSection"))))

(table (@ (equal? (id "ResultTable"))))

(tr 2) (td 2))

Relative SXPath with IDs:  (// (table (@ (equal? (id "ResultTable"))))

(tr 2) (td 2))

The queries can then be compiled with the sxpath procedure of the SXPath library:

(define query

(sxpath '(// (table (@ (equal? (id "ResultTable"))))

(tr 2) (td 2))))

(query doc) ==> ((td "Four"))

This version of WebScraperHelper requires R5RS, SRFI-11, and SRFI-16.

WebScraperHelper also comes with an advertising jingle (with apologies to greasy ground bovine additive Americana):

WebScraperHelper

helps a programmer

scrape the

Web a great deal!

2 Interactive Interface

In this version, the `interactive” interface is a procedure intended to be invoked manually from a REPL.

(webscraperhelper goal sxml ids) → any/c

goal : any/c

sxml : any/c

ids : any/c

Displays some XPath queries yielding SXML goal from document sxml.

goal is the desired SXML element node.

sxml is the document in SXML First Normal Form (1NF). Some nested nodelists emitted by SXML transformation tools, such as attributes nested in extra list levels, are not permitted.

The optional ids is a list of name symbols for element attributes that can be treate as unique identifiers. If ids is not given, then the default is ’(id). (Note: Since some Scheme implementations have case-insensitive readers, but SXML is case-sensitive, you may have to use (list (string->symbol "foo") (string->symbol "bar")) instead of ’(foo bar).)

3 Programmatic Interface

The programmatic interface, such as it is, will likely change substantially in a future version, as new ways of generating queries are implemented. The following procedures are therefore exposed only for tinkering, and are not really documented.

(find-wsh-path goal sxml) → any/c

goal : any/c

sxml : any/c

Yields a wsh-path to goal within sxml, or #f if no path could be found. The yielded path might share structure with sxml.

(wsh-path->sxpath-abs path) → any/c

path : any/c

(wsh-path->sxpath-absids+relids path ids) → any/c

path : any/c

ids : any/c

(wsh-path->sxpath-abs+absids+relids   path             ids)   →   any/c

path : any/c

ids : any/c

Translate a wsh-path to various SXPath queries. The yielded SXPath query lists should be considered immutable, as they might share structure with the original SXML from which path was generated, or multiple queries might share structure with each other.

4 History

• Version 0.5 – 2009-03-14 – PLaneT (1 2)

  Minor documentation change.

• Version 0.4 – 2009-02-24 – PLaneT (1 1)

  License now LGPL 3. Converted to author’s new Scheme administration system.

• Version 0.3 – 2005-07-04 – PLaneT (1 0)

  Documentation update, plus get it into PLaneT 299/3xx.

• Version 0.2 – 2004-08-16

  Corrected typographical error in attributions.

• Version 0.1 – 2004-07-31

  Initial version.

5 Legal

Copyright (c) 2004–2009 Neil Van Dyke. This program is Free Software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 3 of the License (LGPL 3), or (at your option) any later version. This program is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose. See http://www.gnu.org/licenses/ for details. For other licenses and consulting, please contact the author.