ch05_01.htm

Perl & XML. by Erik T. Ray and Jason McIntosh ISBN 0-596-00205-X First Edition, published April

replica of the original document with a few modifications.
Specifically, it makes these changes to a document:
</p>

<ul><li>
<p>Turns every XML comment into a <tt class="literal">&lt;comment&gt;</tt>
element
</p>
</li><li>
<p>Deletes processing instructions </p>
</li><li>
<p>Removes tags, but leaves the content, for
<tt class="literal">&lt;literal&gt;</tt> elements that occur within
<tt class="literal">&lt;programlisting&gt;</tt> elements at any level
</p>
</li></ul>
<p>The code for this program is listed in <a href="ch05_01.htm#perlxml-CHP-5-EX-1">Example 5-1</a>.
Like the last program, we initialize the parser with a set of
handlers, except this time they are bundled together in a convenient
package: an object called <tt class="literal">MyHandler</tt>. Notice that
we've implemented a few more handlers, since we want
to be able to deal with comments, processing instructions, and the
document prolog.
</p>

<a name="perlxml-CHP-5-EX-1" /><div class="example">
<h4 class="objtitle">Example 5-1. Filter program </h4>
<blockquote><pre class="code"># initialize the parser
#
use XML::Parser::PerlSAX;
my $parser = XML::Parser::PerlSAX-&gt;new( Handler =&gt; MyHandler-&gt;new( ) );

if( my $file = shift @ARGV ) {
    $parser-&gt;parse( Source =&gt; {SystemId =&gt; $file} );
} else {
    my $input = "";
    while( &lt;STDIN&gt; ) { $input .= $_; }
    $parser-&gt;parse( Source =&gt; {String =&gt; $input} );
}
exit;

#
# global variables
#
my @element_stack;                # remembers element names
my $in_intset;                    # flag: are we in the internal subset?

###
### Document Handler Package
###
package MyHandler;

#
# initialize the handler package
#
sub new {
    my $type = shift;
    return bless {}, $type;
}

#
# handle a start-of-element event: output start tag and attributes
#
sub start_element {
    my( $self, $properties ) = @_;
    # note: the hash %{$properties} will lose attribute order

    # close internal subset if still open
    output( "]&gt;\n" ) if( $in_intset );
    $in_intset = 0;

    # remember the name by pushing onto the stack
    push( @element_stack, $properties-&gt;{'Name'} );

    # output the tag and attributes UNLESS it's a &lt;literal&gt;
    # inside a &lt;programlisting&gt;
    unless( stack_top( 'literal' ) and
            stack_contains( 'programlisting' )) {
        output( "&lt;" . $properties-&gt;{'Name'} );
        my %attributes = %{$properties-&gt;{'Attributes'}};
        foreach( keys( %attributes )) {
            output( " $_=\"" . $attributes{$_} . "\"" );
        }
        output( "&gt;" );
    }
} 

#
# handle an end-of-element event: output end tag UNLESS it's from a
# &lt;literal&gt; inside a &lt;programlisting&gt;
#
sub end_element {
    my( $self, $properties ) = @_;
    output( "&lt;/" . $properties-&gt;{'Name'} . "&gt;" )
         unless( stack_top( 'literal' ) and
                stack_contains( 'programlisting' ));
    pop( @element_stack );
}

#
# handle a character data event
#
sub characters {
    my( $self, $properties ) = @_;
    # parser unfortunately resolves some character entities for us,
    # so we need to replace them with entity references again
    my $data = $properties-&gt;{'Data'};
    $data =~ s/\&amp;/\&amp;/;
    $data =~ s/&lt;/\&amp;lt;/;
    $data =~ s/&gt;/\&amp;gt;/;
    output( $data );
}

#
# handle a comment event: turn into a &lt;comment&gt; element
#
sub comment {
    my( $self, $properties ) = @_;
    output( "&lt;comment&gt;" . $properties-&gt;{'Data'} . "&lt;/comment&gt;" );
}

#
# handle a PI event: delete it
#
sub processing_instruction {
  # do nothing!
}

#
# handle internal entity reference (we don't want them resolved)
#
sub entity_reference {
    my( $self, $properties ) = @_;
    output( "&amp;" . $properties-&gt;{'Name'} . ";" );
}

sub stack_top {
    my $guess = shift;
    return $element_stack[ $#element_stack ] eq $guess;
}

sub stack_contains {
    my $guess = shift;
    foreach( @element_stack ) {
        return 1 if( $_ eq $guess );
    }
    return 0;
}

sub output {
    my $string = shift;
    print $string;
}</pre></blockquote>
</div>

<p>Looking closely at the handlers, we see that one argument is passed,
in addition to the obligatory object reference
<tt class="literal">$self</tt>. This argument is a reference to a hash of
properties about the event. This technique has one disadvantage: in
the element start handler, the attributes are stored in a hash, which
has no memory of the original attribute order. Semantically, this is
not a big deal, since XML is supposed to be ignorant of attribute
order. However, there may be cases when you want to replicate that
order.<a href="#FOOTNOTE-25">[25]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-25" /><p>[25]In the case of our filter, we might want to
compare the versions from before and after processing using a utility
such as the Unix program <em class="emphasis">diff</em>. Such a comparison
would yield many false differences where the order of attributes
changed. Instead of using <em class="emphasis">diff</em>, you should
consider using the module <tt class="literal">XML::SemanticDiff</tt> by Kip
Hampton. This module would ignore syntactic differences and compare
only the semantics of two documents.</p> </blockquote>

<p>As a filter, this program preserves everything about the original
document, except for the few details that have to be changed. The
program preserves the document prolog, processing instructions, and
comments. Even entity references should be preserved as they are
instead of being resolved (as the parser may want to do). Therefore,
the program has a few more handlers than in the last example, from
which we were interested only in extracting very specific
information.
</p>

<p>Let's test this program now. Our input datafile is
listed in <a href="ch05_01.htm#perlxml-CHP-5-EX-2">Example 5-2</a>.
</p>

<a name="perlxml-CHP-5-EX-2" /><div class="example">
<h4 class="objtitle">Example 5-2. Data for the filter </h4>
<blockquote><pre class="code">&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE book
  SYSTEM "/usr/local/prod/sgml/db.dtd"
[
  &lt;!ENTITY thingy "hoo hah blah blah"&gt;
]&gt;

&lt;book id="mybook"&gt;
&lt;?print newpage?&gt;
  &lt;title&gt;GRXL in a Nutshell&lt;/title&gt;
  &lt;chapter id="intro"&gt;
    &lt;title&gt;What is GRXL?&lt;/title&gt;
&lt;!-- need a better title --&gt;
    &lt;para&gt;
Yet another acronym.  That was our attitude at first, but then we saw 
the amazing uses of this new technology called
&lt;literal&gt;GRXL&lt;/literal&gt;.  Consider the following program:
    &lt;/para&gt;
&lt;?print newpage?&gt;
    &lt;programlisting&gt;AH aof -- %%%%
{{{{{{ let x = 0 }}}}}}
  print!  &lt;lineannotation&gt;&lt;literal&gt;wow&lt;/literal&gt;&lt;/lineannotation&gt;
or not!&lt;/programlisting&gt;
&lt;!-- what font should we use? --&gt;
    &lt;para&gt;
What does it do?  Who cares?  It's just lovely to look at.  In fact,
I'd have to say, "&amp;thingy;".
    &lt;/para&gt;
&lt;?print newpage?&gt;
  &lt;/chapter&gt;
&lt;/book&gt;</pre></blockquote>
</div>

<p>The result, after running the program on the data, is shown in <a href="ch05_01.htm#perlxml-CHP-5-EX-3">Example 5-3</a>. 
</p>

<a name="perlxml-CHP-5-EX-3" /><div class="example">
<h4 class="objtitle">Example 5-3. Output from the filter </h4>
<blockquote><pre class="code">&lt;book id="mybook"&gt;
  &lt;title&gt;GRXL in a Nutshell&lt;/title&gt;
  &lt;chapter id="intro"&gt;
    &lt;title&gt;What is GRXL?&lt;/title&gt;
&lt;comment&gt; need a better title &lt;/comment&gt;
    &lt;para&gt;
Yet another acronym.  That was our attitude at first, but then we saw 
the amazing uses of this new technology called
&lt;literal&gt;GRXL&lt;/literal&gt;.  Consider the following program:
    &lt;/para&gt;

    &lt;programlisting&gt;AH aof -- %%%%
{{{{{{ let x = 0 }}}}}}
  print!  &lt;lineannotation&gt;wow&lt;/lineannotation&gt;
or not!&lt;/programlisting&gt;
&lt;comment&gt; what font should we use? &lt;/comment&gt;
    &lt;para&gt;
What does it do?  Who cares?  It's just lovely to look at.  In fact,
I'd have to say, "&amp;thingy;".
    &lt;/para&gt;

  &lt;/chapter&gt;
&lt;/book&gt;</pre></blockquote>
</div>

<p>Here's what the filter did right. It turned an XML
comment into a <tt class="literal">&lt;comment&gt;</tt> element and deleted
the processing instruction. The <tt class="literal">&lt;literal&gt;</tt>
element in the <tt class="literal">&lt;programlisting&gt;</tt> was removed,
with its contents left intact, while other
<tt class="literal">&lt;literal&gt;</tt> elements were preserved. Entity
references were left unresolved, as we wanted. So far, so good. But
something's missing. The XML declaration, document
type declaration, and internal subset are gone. Without the
declaration for the entity <tt class="literal">thingy</tt>, this document
is not valid. It looks like the handlers<a name="INDEX-378" /> <a name="INDEX-379" /> we had available to us were not
sufficient.
</p>

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