XML::LibXML::Parser

Section: User Contributed Perl Documentation (3)
Updated: 2003-11-07
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NAME

XML::LibXML::Parser - Parsing XML Data with XML::LibXML  

SYNOPSIS

  $parser = XML::LibXML->new();
  $doc = $parser->parse_file( $xmlfilename );
  $doc = $parser->parse_fh( $io_fh );
  $doc = $parser->parse_string( $xmlstring);
  $doc = $parser->parse_html_file( $htmlfile );
  $doc = $parser->parse_html_fh( $io_fh );
  $doc = $parser->parse_html_string( $htmlstring );
  $doc = $parser->parse_sgml_file( $sgmlfile );
  $doc = $parser->parse_sgml_fh( $io_fh );
  $doc = $parser->parse_sgml_string( $sgmlstring );
  $fragment = $parser->parse_balanced_chunk( $wbxmlstring );
  $fragment = $parser->parse_xml_chunk( $wbxmlstring );
  $parser->process_xincludes( $doc );
  $parser->processXIncludes( $doc );
  $parser->parse_chunk($string, $terminate);
  $parser->start_push();
  $parser->push(@data);
  $doc = $parser->finish_push( $recover );
  $parser->validation(1);
  $parser->recover(1);
  $parser->expand_entities(0);
  $parser->keep_blanks(0);
  $parser->pedantic_parser(1)
  $parser->line_numbers(1)
  $parser->load_ext_dtd(1);
  $parser->complete_attributes(1);
  $parser->expand_xinclude(1);
  $parser->load_catalog( $catalog_file );
  $parser->base_uri( $your_base_uri );
  $parser->gdome_dom(1);
  $parser->clean_namespaces( 1 );
  $parser->match_callback($subref);
  $parser->open_callback($subref);
  $parser->read_callback($subref);
  $parser->close_callback($subref);

 

DESCRIPTION

 

SYNOPSIS

  use XML::LibXML;
  my $parser = XML::LibXML->new();

  my $doc = $parser->parse_string(<<'EOT');
  <some-xml/>
  EOT
  my $fdoc = $parser->parse_file( $xmlfile );

  my $fhdoc = $parser->parse_fh( $xmlstream );

  my $fragment = $parser->parse_xml_chunk( $xml_wb_chunk );

 

PARSING

A XML document is read into a datastructure such as a DOM tree by a piece of software, called a parser. XML::LibXML currently provides four diffrent parser interfaces:
* A DOM Pull-Parser
* A DOM Push-Parser
* A SAX Parser
* A DOM based SAX Parser.
 

Creating a Parser Instance

XML::LibXML provides an OO interface to the libxml2 parser functions. Thus you have to create a parser instance before you can parse any XML data.
new
  $parser = XML::LibXML->new();

There is nothing much to say about the constructor. It simply creates a new parser instance.

Although libxml2 uses mainly global flags to alter the behaviour of the parser, each XML::LibXML parser instance has its own flags or callbacks and does not interfere with other instances.

 

DOM Parser

One of the common parser interfaces of XML::LibXML is the DOM parser. This parser reads XML data into a DOM like datastructure, so each tag can get accessed and transformed.

XML::LibXML's DOM parser is not only capable to parse XML data, but also (strict) HTML and SGML files. There are three ways to parse documents - as a string, as a Perl filehandle, or as a filename. The return value from each is a XML::LibXML::Document object, which is a DOM object.

All of the functions listed below will throw an exception if the document is invalid. To prevent this causing your program exiting, wrap the call in an eval{} block

parse_file
  $doc = $parser->parse_file( $xmlfilename );

This function reads an absolute filename into the memory. It causes XML::LibXML to use libxml2's file parser instead of letting perl reading the file such as with parse_fh(). If you need to parse files directly, this function would be the faster choice, since this function is about 6-8 times faster then parse_fh().

parse_fh
  $doc = $parser->parse_fh( $io_fh );

parse_fh() parses a IOREF or a subclass of IO::Handle.

Because the data comes from an open handle, libxml2's parser does not know about the base URI of the document. To set the base URI one should use parse_fh() as follows:

  my $doc = $parser->parse_fh( $io_fh, $baseuri );

parse_string
  $doc = $parser->parse_string( $xmlstring);

This function is similar to parse_fh(), but it parses a XML document that is available as a single string in memory. Again, you can pass an optional base URI to the function.

  my $doc = $parser->parse_stirng( $xmlstring, $baseuri );

parse_html_file
  $doc = $parser->parse_html_file( $htmlfile );

Similar to parse_file() but parses HTML (strict) documents.

parse_html_fh
  $doc = $parser->parse_html_fh( $io_fh );

Similar to parse_fh() but parses HTML (strict) streams.

parse_html_string
  $doc = $parser->parse_html_string( $htmlstring );

Similar to parse_file() but parses HTML (strict) strings.

parse_sgml_file
  $doc = $parser->parse_sgml_file( $sgmlfile );

Similar to parse_file() but parses SGML documents.

parse_sgml_fh
  $doc = $parser->parse_sgml_fh( $io_fh );

Similar to parse_file() but parses SGML streams.

parse_sgml_string
  $doc = $parser->parse_sgml_string( $sgmlstring );

Similar to parse_file() but parses SGML strings.

Parsing HTML may cause problems, especially if the ampersand ('&') is used. This is a common problem if HTML code is parsed that contains links to CGI-scripts. Such links cause the parser to throw errors. In such cases libxml2 still parses the entire document as there was no error, but the error causes XML::LibXML to stop the parsing process. However, the document is not lost. Such HTML documents should be parsed using the recover flag. By default recovering is deactivated.

The functions described above are implemented to parse well formed documents. In some cases a program gets well balanced XML instead of well formed documents (e.g. a XML fragment from a Database). With XML::LibXML it is not required to wrap such fragments in the code, because XML::LibXML is capable even to parse well balanced XML fragments.

parse_balanced_chunk
  $fragment = $parser->parse_balanced_chunk( $wbxmlstring );

This function parses a well balanced XML string into a XML::LibXML::DocumentFragment.

parse_xml_chunk
  $fragment = $parser->parse_xml_chunk( $wbxmlstring );

This is the old name of parse_balanced_chunk(). Because it may causes confusion with the push parser interface, this function should be used anymore.

By default XML::LibXML does not process XInclude tags within a XML Document (see options section below). XML::LibXML allows to post process a document to expand XInclude tags.

process_xincludes
  $parser->process_xincludes( $doc );

After a document is parsed into a DOM structure, you may want to expand the documents XInclude tags. This function processes the given document structure and expands all XInclude tags (or throws an error) by using the flags and callbacks of the given parser instance.

Note that the resulting Tree contains some extra nodes (of type XML_XINCLUDE_START and XML_XINCLUDE_END) after successfully processing the document. These nodes indicate where data was included into the original tree. if the document is serialized, these extra nodes will not show up.

Remember: A Document with processed XIncludes differs from the original document after serialization, because the original XInclude tags will not get restored!

If the parser flag ``expand_xincludes'' is set to 1, you need not to post process the parsed document.

processXIncludes
  $parser->processXIncludes( $doc );

This is an alias to process_xincludes, but through a JAVA like function name.

 

Push Parser

XML::LibXML provides a push parser interface. Rather than pulling the data from a given source the push parser waits for the data to be pushed into it.

This allows one to parse large documents without waiting for the parser to finish. The interface is especially useful if a program needs to preprocess the incoming pieces of XML (e.g. to detect document boundaries).

While XML::LibXML parse_*() functions force the data to be a wellformed XML, the push parser will take any arbitrary string that contains some XML data. The only requirement is that all the pushed strings are together a well formed document. With the push parser interface a programm can interrupt the parsing process as required, where the parse_*() functions give not enough flexibility.

Different to the pull parser implemented in parse_fh() or parse_file(), the push parser is not able to find out about the documents end itself. Thus the calling program needs to indicate explicitly when the parsing is done.

In XML::LibXML this is done by a single function:

parse_chunk
  $parser->parse_chunk($string, $terminate);

parse_chunk() tries to parse a given chunk of data, which isn't nessecarily well balanced data. The function takes two parameters: The chunk of data as a string and optional a termination flag. If the termination flag is set to a true value (e.g. 1), the parsing will be stopped and the resulting document will be returned as the following exable describes:

  my $parser = XML::LibXML->new;
  for my $string ( "<", "foo", ' bar="hello worls"', "/>") {
       $parser->parse_chunk( $string );
  }
  my $doc = $parser->parse_chunk("", 1); # terminate the parsing

Internally XML::LibXML provides three functions that control the push parser process:

start_push
  $parser->start_push();

Initializes the push parser.

push
  $parser->push(@data);

This function pushes the data stored inside the array to libxml2's parser. Each entry in @data must be a normal scalar!

finish_push
  $doc = $parser->finish_push( $recover );

This function returns the result of the parsing process. If this function is called without a parameter it will complain about non wellformed documents. If $restore is 1, the push parser can be used to restore broken or non well formed (XML) documents as the following example shows:

  eval {
      $parser->push( "<foo>", "bar" );
      $doc = $parser->finish_push();    # will report broken XML
  };
  if ( $@ ) {
     # ...
  }

This can be annoying if the closing tag is missed by accident. The following code will restore the document:

  eval {
      $parser->push( "<foo>", "bar" );
      $doc = $parser->finish_push(1);   # will return the data parsed
                                        # unless an error happened
  };

  print $doc->toString(); # returns "<foo>bar</foo>"

Of course finish_push() will return nothing if there was no data pushed to the parser before.

 

DOM based SAX Parser

XML::LibXML provides a DOM based SAX parser. The SAX parser is defined in XML::LibXML::SAX::Parser. As it is not a stream based parser, it parses documents into a DOM and traverses the DOM tree instead.

The API of this parser is exactly the same as any other Perl SAX2 parser. See XML::SAX::Intro for details.

Aside from the regular parsing methods, you can access the DOM tree traverser directly, using the generate() method:

  my $doc = build_yourself_a_document();
  my $saxparser = $XML::LibXML::SAX::Parser->new( ... );
  $parser->generate( $doc );

This is useful for serializing DOM trees, for example that you might have done prior processing on, or that you have as a result of XSLT processing.

WARNING

This is NOT a streaming SAX parser. As I said above, this parser reads the entire document into a DOM and serialises it. Some people couldn't read that in the paragraph above so I've added this warning.

If you want a streaming SAX parser look at the XML::LibXML::SAX man page  

SERIALIZATION

XML::LibXML provides some functions to serialize nodes and documents. The serialization functions are described on the XML::LibXML::Node manpage or the XML::LibXML::Document manpage. XML::LibXML checks three global flags that alter the serialization process:
* skipXMLDeclaration
* skipDTD
* setTagCompression

of that three functions only setTagCompression is available for all serialization functions.

Because XML::LibXML does these flags not itself, one has to define them locally as the following example shows:

  local $XML::LibXML::skipXMLDeclaration = 1;
  local $XML::LibXML::skipDTD = 1;
  local $XML::LibXML::setTagCompression = 1;

If skipXMLDeclaration is defined and not '0', the XML declaration is omitted during serialization.

If skipDTD is defined and not '0', an existing DTD would not be serialized with the document.

If setTagCompression is defined and not '0' empty tags are displayed as open and closing tags ranther than the shortcut. For example the empty tag foo will be rendered as <foo></foo> rather than <foo/>.  

PARSER OPTIONS

LibXML options are global (unfortunately this is a limitation of the underlying implementation, not this interface). They can either be set using $parser->option(...), or XML::LibXML->option(...), both are treated in the same manner. Note that even two parser processes will share some of the same options, so be careful out there!

Every option returns the previous value, and can be called without parameters to get the current value.

validation
  $parser->validation(1);

Turn validation on (or off). Defaults to off.

recover
  $parser->recover(1);

Turn the parsers recover mode on (or off). Defaults to off.

This allows one to parse broken XML data into memory. This switch will only work with XML data rather than HTML data. Also the validation will be switched off automaticly.

The recover mode helps to recover documents that are almost wellformed very efficiently. That is for example a document that forgets to close the document tag (or any other tag inside the document). The recover mode of XML::LibXML has problems restoring documents that are more like well ballanced chunks.

XML::LibXML will only parse until the first fatal error occours.

expand_entities
  $parser->expand_entities(0);

Turn entity expansion on or off, enabled by default. If entity expansion is off, any external parsed entities in the document are left as entities. Probably not very useful for most purposes.

keep_blanks
  $parser->keep_blanks(0);

Allows you to turn off XML::LibXML's default behaviour of maintaining whitespace in the document.

pedantic_parser
  $parser->pedantic_parser(1)

You can make XML::LibXML more pedantic if you want to.

line_numbers
  $parser->line_numbers(1)

If this option is activated XML::LibXML will store the line number of a node. This gives more information where a validation error occoured. It could be also used to find out about the position of a node after parsing (see also XML::LibXML::Node::line_number())

By default line numbering is switched off (0).

load_ext_dtd
  $parser->load_ext_dtd(1);

Load external DTD subsets while parsing.

complete_attributes
  $parser->complete_attributes(1);

Complete the elements attributes lists with the ones defaulted from the DTDs. By default, this option is enabled.

expand_xinclude
  $parser->expand_xinclude(1);

Expands XIinclude tags immediately while parsing the document. This flag assures that the parser callbacks are used while parsing the included document.

load_catalog
  $parser->load_catalog( $catalog_file );

Will use $catalog_file as a catalog during all parsing processes. Using a catalog will significantly speed up parsing processes if many external resources are loaded into the parsed documents (such as DTDs or XIncludes).

Note that catalogs will not be available if an external entity handler was specified. At the current state it is not possible to make use of both types of resolving systems at the same time.

base_uri
  $parser->base_uri( $your_base_uri );

In case of parsing strings or file handles, XML::LibXML doesn't know about the base uri of the document. To make relative references such as XIncludes work, one has to set a separate base URI, that is then used for the parsed documents.

gdome_dom
  $parser->gdome_dom(1);

THIS FLAG IS EXPERIMENTAL!

Although quite powerful XML:LibXML's DOM implementation is limited if one needs or wants full DOM level 2 or level 3 support. XML::GDOME is based on libxml2 as well but provides a rather complete DOM implementation by wrapping libgdome. This allows you to make use of XML::LibXML's full parser options and XML::GDOME's DOM implementation at the same time.

To make use of this function, one has to install libgdome and configure XML::LibXML to use this library. For this you need to rebuild XML::LibXML!

clean_namespaces
  $parser->clean_namespaces( 1 );

libxml2 2.6.0 and later allows to strip redundant namespace declarations from the DOM tree. To do this, one has to set clean_namespaces() to 1 (TRUE). By default no namespace cleanup is done.

 

Input Callbacks

If libxml2 has to load external documents during parsing, this may cause strange results, if the location is not a HTTP, FTP or relative location. To get around this limitation, one may add its own input handler, to open, read and close particular locations or URI classes.

The input callbacks are used whenever LibXML has to get something other than external parsed entities from somewhere. The input callbacks in LibXML are stacked on top of the original input callbacks within the libxml library. This means that if you decide not to use your own callbacks (see match()), then you can revert to the default way of handling input. This allows, for example, to only handle certain URI schemes.

Callbacks are only used on files, but not on strings or filehandles. This is because LibXML requires the match event to find out about which callback set is shall be used for the current input stream. LibXML can decide this only before the stream is open. For LibXML strings and filehandles are already opened streams.

The following callbacks are defined:

match_callback
  $parser->match_callback($subref);

If you want to handle the URI, simply return a true value from this callback.

open_callback
  $parser->open_callback($subref);

Open something and return it to handle that resource.

read_callback
  $parser->read_callback($subref);

Read a certain number of bytes from the resource. This callback is called even if the entire Document has already read. This callback has to return a string which will be parsed by the libxml2 parser.

close_callback
  $parser->close_callback($subref);

Close the handle associated with the resource.

It is important that one must not create a new parser instance and parse some XML data from within any callback. This is forbidden, because the new parser will override the existing callbacks and will leave the calling parser in an undefined state. Most likely memory violations will follow and break the running parsing process without returning control to the perl layer.

The following example explains the concept a bit. It is a purely fictitious example that uses a MyScheme::Handler object that responds to methods similar to an IO::Handle.

  $parser->match_callback(\&match_uri);

    $parser->open_callback(\&open_uri);

    $parser->read_callback(\&read_uri);

    $parser->close_callback(\&close_uri);

    sub match_uri {
      my $uri = shift;
      return $uri =~ /^myscheme:/;
    }

    sub open_uri {
      my $uri = shift;
      return MyScheme::Handler->new($uri);
    }

    sub read_uri {
      my $handler = shift;
      my $length = shift;
      my $buffer;
      read($handler, $buffer, $length);
      return $buffer;
    }

    sub close_uri {
      my $handler = shift;
      close($handler);
    }

A more realistic example can be found in the ``example'' directory.

Since the parser requires all callbacks defined it is also possible to set all callbacks with a single call of callbacks(). This would implify the example code to:

  $parser->callbacks( \&match_uri, \&open_uri, \&read_uri, \&close_uri);

All functions that are used to set the callbacks, can also be used to retrieve the callbacks from the parser.

Optionaly it is possible to apply global callback on the XML::LibXML class level. This allows multiple parses to share the same callbacks. To set these global callbacks one can use the callback access functions directly on the class.

  XML::LibXML->callbacks( \&match_uri, \&open_uri, \&read_uri, \&close_uri);

The previous code snippet will set the callbacks from the first example as global callbacks.  

ERROR REPORTING

XML::LibXML throws exceptions during parsing, validation or XPath processing (and some other occations). These errors can be caught by using eval blocks. The error then will be stored in $@. Alternatively one can use the get_last_error() function of XML::LibXML. It will return the same string that is stored in $@. Using get_last_error() makes it still nessecary to eval the statement, since these function groups will die() on errors.

Note, that the use of get_last_error() still requires eval blocks. XML::LibXML throws errors as they occour and does not wait if a user test for them. This is a very common misunderstanding in the use of XML::LibXML. If the eval is ommited, XML::LibXML will allways halt your script by ``croaking'' (see Carp man page for details).

Also note that an increasing number throws errors if bad data is passed. If you cannot asure valid data passed to XML::LibXML you should eval these functions.

get_last_error() can be called either by the class itself or by a parser instance:

  $errstring = XML::LibXML->get_last_error();
  $errstring = $parser->get_last_error();

However, XML::LibXML exceptions are global. That means if get_last_error() is called on an parser instance, the last global error will be returned. This is not necessarily the error caused by the parser instance itself.  

AUTHORS

Matt Sergeant, Christian Glahn, =head1 VERSION

1.58  

COPYRIGHT

2001-2004, AxKit.com Ltd; 2002-2004 Christian Glahn, All rights reserved.


 

Index

NAME
SYNOPSIS
DESCRIPTION
SYNOPSIS
PARSING
Creating a Parser Instance
DOM Parser
Push Parser
DOM based SAX Parser
SERIALIZATION
PARSER OPTIONS
Input Callbacks
ERROR REPORTING
AUTHORS
COPYRIGHT

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