Parsing XML Data with XML::LibXML

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 data structure such as a DOM tree by a piece of software, called a parser. XML::LibXML currently provides four different 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 data structure, 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 files. There are three ways to parse documents - as a string, as a Perl filehandle, or as a filename/URL. The return value from each is a XML::LibXML DOM Document Class 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 parses an XML document from a file or network; $xmlfilename can be either a filename or an URL. Note that for parsing files, this function is the fastest choice, 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_string( $xmlstring, $baseuri );

parse_html_file 	

$doc = $parser->parse_html_file( $htmlfile, \%opts );

Similar to parse_file() but parses HTML (strict) documents; $htmlfile can be filename or URL.
An optional second argument can be used to pass some options to the HTML parser as a HASH reference. Possible options are: Possible options are: encoding and URI for libxml2 < 2.6.27, and for later versions of libxml2 additionally: recover, suppress_errors, suppress_warnings, pedantic_parser, no_blanks, and no_network.
parse_html_fh 	

$doc = $parser->parse_html_fh( $io_fh, \%opts );

Similar to parse_fh() but parses HTML (strict) streams.
An optional second argument can be used to pass some options to the HTML parser as a HASH reference. Possible options are: encoding and URI for libxml2 < 2.6.27, and for later versions of libxml2 additionally: recover, suppress_errors, suppress_warnings, pedantic_parser, no_blanks, and no_network. Note: encoding option may not work correctly with this function in libxml2 < 2.6.27 if the HTML file declares charset using a META tag.
parse_html_string 	

$doc = $parser->parse_html_string( $htmlstring, \%opts );

Similar to parse_string() but parses HTML (strict) strings.
An optional second argument can be used to pass some options to the HTML parser as a HASH reference. Possible options are: encoding and URI for libxml2 < 2.6.27, and for later versions of libxml2 additionally: recover, suppress_errors, suppress_warnings, pedantic_parser, no_blanks, and no_network.
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's DOM L2 Document Fragment Implementation.
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 not 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 pre-process the incoming pieces of XML (e.g. to detect document boundaries).
While XML::LibXML parse_*() functions force the data to be a well-formed 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 program 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 necessarily 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 example describes:

my $parser = XML::LibXML->new;
for my $string ( "<", "foo", ' bar="hello world"', "/>") {
     $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 well-formed 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 the module 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 direct SAX parser man page

Serialization
XML::LibXML provides some functions to serialize nodes and documents. The serialization functions are described on the Abstract Base Class of XML::LibXML Nodes manpage or the XML::LibXML DOM Document Class 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 rather 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 automatically.
The recover mode helps to recover documents that are almost well-formed 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 balanced chunks.
XML::LibXML will only parse until the first fatal error occurs, reporting recoverable parsing errors as warnings. To suppress these warnings use $parser->recover_silently(1); or, equivalently, $parser->recover(2).
recover_silently 	

$parser->recover_silently(1);

Turns the parser warnings off (or on). Defaults to on.
This allows to switch off warnings printed to STDERR when parsing documents with recover(1).
Please note that calling recover_silently(0) also turns the parser recover mode off and calling recover_silently(1) automatically activates the parser recover mode.
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 white-space 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 occurred. It could be also used to find out about the position of a node after parsing (see also XML::LibXML::Node::line_number()).
IMPORTANT: Due to limitations in the libxml2 library line numbers greater than 65535 will be returned as 65535. Please see http://bugzilla.gnome.org/show_bug.cgi?id=325533 for more details.
By default line numbering is switched off (0).
load_ext_dtd 	

$parser->load_ext_dtd(1);

Load external DTD subsets while parsing.
This flag is also required for DTD Validation, to provide complete attribute, and to expand entities, regardless if the document has an internal subset. Thus switching off external DTD loading, will disable entity expansion, validation, and complete attributes on internal subsets as well.
If you leave this parser flag untouched, everything will work, because the default is 1 (activated)
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.
no_network 	

$parser->no_network(1);

Turn networking support on or off, enabled by default. If networking is off, all attempts to fetch non-local resources (such as DTD or external entities) will fail (unless custom callbacks are defined). It may be necessary to use $parser->recover(1) for processing documents requiring such resources while networking is off.

Error Reporting
XML::LibXML throws exceptions during parsing, validation or XPath processing (and some other occasions). These errors can be caught by using eval blocks. The error then will be stored in $@.
XML::LibXML throws errors as they occurs 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 omitted, XML::LibXML will always halt your script by "croaking" (see Carp man page for details).
Also note that an increasing number of functions throw errors if bad data is passed. If you cannot assure valid data passed to XML::LibXML you should eval these functions.
Note: since version 1.59, get_last_error() is no longer available in XML::LibXML for thread-safety reasons.