22.5.1. Problem

22.5.2. Solution

use XML::LibXML;
my $parser = XML::LibXML->new;
$parser->validation(1);
$parser->parse_file($FILENAME);

use XML::Xerces;

my $parser = XML::Xerces::DOMParser->new;
$parser->setValidationScheme($XML::Xerces::DOMParser::Val_Always);

my $error_handler = XML::Xerces::PerlErrorHandler->new( );
$parser->setErrorHandler($error_handler);

$parser->parse($FILENAME);

22.5.3. Discussion

To make XML::LibXML parse the DTD, add this line to the books.xml file:

<!DOCTYPE books
  SYSTEM "books.dtd">

When the document validates, the program produces no output—XML::LibXML successfully parses the document into a DOM structure that is quietly destroyed when the program ends. Edit the books.xml file, however, and you see the errors the XML::LibXML emits when it discovers broken XML.

For example, changing the id attribute to unique_id causes this error message:

'books.xml:0: validity error: No declaration for attribute unique_id
of element book
    <book unique_id="1">
                       ^
books.xml:0: validity error: Element book does not carry attribute id
    </book>
          ^
' at /usr/local/perl5-8/Library/Perl/5.8.0/darwin/XML/LibXML.pm line
405.
 at checker-1 line 7

XML::LibXML does a good job of reporting unknown attributes and tags. However, it's not so good at reporting out-of-order elements. If you return books.xml to its correct state, and then swap the order of a title and an edition element, you get this message:

'books.xml:0: validity error: Element book content does not follow the
DTD
    </book>
          ^
' at /usr/local/perl5-8/Library/Perl/5.8.0/darwin/XML/LibXML.pm line
405.
 at checker-1 line 7

In this case, XML::LibXML says that something in the book element didn't follow the DTD, but it couldn't tell us precisely what it violated in the DTD or how.

At the time of this writing, you must use XML::Xerces to validate while using SAX, or to validate against W3C Schema. Both of these features (and RelaxNG validation) are planned for XML::LibXML, but weren't available at the time of printing.

use XML::Xerces;

# create a new parser that always validates
my $p = XML::Xerces::DOMParser->new( );
$p->setValidationScheme($XML::Xerces::DOMParser::Val_Always);

# make it die when things fail to parse
my $error_handler = XML::Xerces::PerlErrorHandler->new( );
$p->setErrorHandler($error_handler);

$p->parse($FILENAME);

To validate against a schema, you must tell XML::Xerces where the schema is and that it should be used:

$p->setFeature("http://xml.org/sax/features/validation", 1);
$p->setFeature("http://apache.org/xml/features/validation/dynamic", 0);
$p->setFeature("http://apache.org/xml/features/validation/schema", $SCHEMAFILE);

You can pass three possible values to setValidationScheme:

$XML::Xerces::DOMParser::Val_Always
$XML::Xerces::DOMParser::Val_Never
$XML::Xerces::DOMParser::Val_Auto

The default is to never validate. Always validating raises an error if the file does not have a DTD or Schema. Auto raises an error only if the file has a DTD or Schema, but it fails to validate against that DTD or Schema.

22.5.4. See Also