XML and Scripting Languages

By Parand Tony Darugar
2005-05-18

Simple substitution

A simple method for transforming the XML source into HTML is to define pieces of HTML to be substituted for each XML tag. Using the popular XML::Parser module for Perl (see Resources), based on James Clark's Expat parser, we can parse the XML document and define callback routines for performing the substitutions.

Here is a simple invocation of the XML::Parser:



 use XML::Parser;

  

 my $parser = new XML::Parser(ErrorContext => 2);

 $parser->setHandlers(Start => \&start_handler,

                      End   => \&end_handler,

                      Char  => \&char_handler);

  

 $parser->parsefile($file);

This parses the given file, invoking the function start_handler each time a tag is started, and end_handler each time a tag is ended. The contents of the tag are processed by the char_handler function.

Given these callback functions, we can implement our simple substitution algorithm. First, we define a few substitutions:



  %startsub = (

 "stock_quote"        =>      "<hr><p>",

 "symbol"             =>      "<h2>",

 "price"              =>      "<br><b>Price:</b>"

 );

 

 %endsub = (

 "stock_quote"        =>      "",

 "symbol"             =>      "</h2>",

 "price"              =>      ""

 );

And now we write the handlers to perform the substitutions:



 sub start_handler

 {

     my $expat = shift; my $element = shift;

 

     # element is the name of the tag

     print $startsub{$element};

 }

 

 sub char_handler

 {

     my ($p, $data) = @_;

     print $data;

 

 }

The start_handler function simply prints the value to be substituted for the given tag. char_handler outputs the data it receives, which is the content of the tags. (The full program, with a few additions to handle attributes, is listed separately.) Running the program on our XML file, we get the following output:



 <hr><p>

  <h2>IBM</h2>

  

    <br><b>Date:</b><i>12/16/1999</i>

    <br><b>Time:</b><i>4:40PM</i>

  

  <br><b>Price:</b>type=ask value=109.1875 

  <br><b>Price:</b>type=open value=108 

  <br><b>Price:</b>type=dayhigh value=109.6875 

  <br><b>Price:</b>type=daylow value=105.75 

  <br><b>Change:</b>+2.1875

  <br><b>Volume:</b>7050200

The full output is available. Using this methodology, we can make simple XML to HTML transformations by defining substitutions.

Tutorial Pages:
» Converting XML to HTML
» Simple substitution
» Function-based substitution
» Tree-based processing
» Active XML documents
» Storing tag contents
» Retrieving the rules
» Acting on the rules
» Next steps
» Resources

First published by IBM DeveloperWorks

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