XML::LibXML::Parser(3pUser Contributed Perl DocumentatXML::LibXML::Parser(3pm)NAMEXML::LibXML::Parser - Parsing XML Data with XML::LibXML
SYNOPSIS
use XML::LibXML '1.70';
# Parser constructor
$parser = XML::LibXML->new();
$parser = XML::LibXML->new(option=>value, ...);
$parser = XML::LibXML->new({option=>value, ...});
# Parsing XML
$dom = XML::LibXML->load_xml(
location => $file_or_url
# parser options ...
);
$dom = XML::LibXML->load_xml(
string => $xml_string
# parser options ...
);
$dom = XML::LibXML->load_xml(
string => (\$xml_string)
# parser options ...
);
$dom = XML::LibXML->load_xml({
IO => $perl_file_handle
# parser options ...
);
$dom = $parser->load_xml(...);
# Parsing HTML
$dom = XML::LibXML->load_html(...);
$dom = $parser->load_html(...);
# Parsing well-balanced XML chunks
$fragment = $parser->parse_balanced_chunk( $wbxmlstring, $encoding );
# Processing XInclude
$parser->process_xincludes( $doc );
$parser->processXIncludes( $doc );
# Old-style parser interfaces
$doc = $parser->parse_file( $xmlfilename );
$doc = $parser->parse_fh( $io_fh );
$doc = $parser->parse_string( $xmlstring);
$doc = $parser->parse_html_file( $htmlfile, \%opts );
$doc = $parser->parse_html_fh( $io_fh, \%opts );
$doc = $parser->parse_html_string( $htmlstring, \%opts );
# Push parser
$parser->parse_chunk($string, $terminate);
$parser->init_push();
$parser->push(@data);
$doc = $parser->finish_push( $recover );
# Set/query parser options
$parser->option_exists($name);
$parser->get_option($name);
$parser->set_option($name,$value);
$parser->set_options({$name=>$value,...});
# XML catalogs
$parser->load_catalog( $catalog_file );
PARSING
An 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();
$parser = XML::LibXML->new(option=>value, ...);
$parser = XML::LibXML->new({option=>value, ...});
Create a new XML and HTML parser instance. Each parser instance
holds default values for various parser options. Optionally, one
can pass a hash reference or a list of option => value pairs to set
a different default set of options. Unless specified otherwise,
the options "load_ext_dtd", and "expand_entities" are set to 1. See
"Parser Options" for a list of libxml2 parser's options.
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::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
load_xml
$dom = XML::LibXML->load_xml(
location => $file_or_url
# parser options ...
);
$dom = XML::LibXML->load_xml(
string => $xml_string
# parser options ...
);
$dom = XML::LibXML->load_xml(
string => (\$xml_string)
# parser options ...
);
$dom = XML::LibXML->load_xml({
IO => $perl_file_handle
# parser options ...
);
$dom = $parser->load_xml(...);
This function is available since XML::LibXML 1.70. It provides easy
to use interface to the XML parser that parses given file (or URL),
string, or input stream to a DOM tree. The arguments can be passed
in a HASH reference or as name => value pairs. The function can be
called as a class method or an object method. In both cases it
internally creates a new parser instance passing the specified
parser options; if called as an object method, it clones the
original parser (preserving its settings) and additionally applies
the specified options to the new parser. See the constructor "new"
and "Parser Options" for more information.
load_html
$dom = XML::LibXML->load_html(...);
$dom = $parser->load_html(...);
This function is available since XML::LibXML 1.70. It has the same
usage as "load_xml", providing interface to the HTML parser. See
"load_xml" for more information.
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. an 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, $encoding );
This function parses a well balanced XML string into a
XML::LibXML::DocumentFragment. The first arguments contains the
input string, the optional second argument can be used to specify
character encoding of the input (UTF-8 is assumed by default).
parse_xml_chunk
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 an XML
Document (see options section below). XML::LibXML allows one 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.
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 an XML
document that is available as a single string in memory, or
alternatively as a reference to a scalar containing a string.
Again, you can pass an optional base URI to the function.
my $doc = $parser->parse_string( $xmlstring, $baseuri );
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. See options labeled with HTML in
"Parser Options".
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. See options labeled with HTML in
"Parser Options".
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. See options labeled with HTML in
"Parser Options".
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:
init_push
$parser->init_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! This method
can be called repeatedly.
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.
Pull Parser (Reader)
XML::LibXML also provides a pull-parser interface similar to the
XmlReader interface in .NET. This interface is almost streaming, and is
usually faster and simpler to use than SAX. See XML::LibXML::Reader.
Direct SAX Parser
XML::LibXML provides a direct SAX parser in the XML::LibXML::SAX
module.
DOM based SAX Parser
XML::LibXML also 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::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 $XML::LibXML::setTagCompression = 1;
If skipXMLDeclaration is defined and not '0', the XML declaration is
omitted during serialization.
If skipDTD is defined and not '0', an existing DTD would not be
serialized with the document.
If setTagCompression is defined and not '0' empty tags are displayed as
open and closing tags rather than the shortcut. For example the empty
tag foo will be rendered as <foo></foo> rather than <foo/>.
PARSER OPTIONS
Handling of libxml2 parser options has been unified and improved in
XML::LibXML 1.70. You can now set default options for a particular
parser instance by passing them to the constructor as
"XML::LibXML->new({name=>value, ...})" or
"XML::LibXML->new(name=>value,...)". The options can be queried and
changed using the following methods (pre-1.70 interfaces such as
"$parser->load_ext_dtd(0)" also exist, see below):
option_exists
$parser->option_exists($name);
Returns 1 if the current XML::LibXML version supports the option
$name, otherwise returns 0 (note that this does not necessarily
mean that the option is supported by the underlying libxml2
library).
get_option
$parser->get_option($name);
Returns the current value of the parser option $name.
set_option
$parser->set_option($name,$value);
Sets option $name to value $value.
set_options
$parser->set_options({$name=>$value,...});
Sets multiple parsing options at once.
IMPORTANT NOTE: This documentation reflects the parser flags available
in libxml2 2.7.3. Some options have no effect if an older version of
libxml2 is used.
Each of the flags listed below is labeled
/parser/
if it can be used with a "XML::LibXML" parser object (i.e. passed
to "XML::LibXML->new", "XML::LibXML->set_option", etc.)
/html/
if it can be used passed to the "parse_html_*" methods
/reader/
if it can be used with the "XML::LibXML::Reader".
Unless specified otherwise, the default for boolean valued options is 0
(false).
The available options are:
URI /parser, html, reader/
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 base URI, that
is then used for the parsed document.
line_numbers
/parser, html, reader/
If this option is activated, libxml2 will store the line number of
each element node in the parsed document. The line number can be
obtained using the "line_number()" method of the
"XML::LibXML::Node" class (for non-element nodes this may report
the line number of the containing element). The line numbers are
also used for reporting positions of validation errors.
IMPORTANT: Due to limitations in the libxml2 library line numbers
greater than 65535 will be returned as 65535. Unfortunately, this
is a long and sad story, please see
<http://bugzilla.gnome.org/show_bug.cgi?id=325533> for more
details.
encoding
/html/
character encoding of the input
recover
/parser, html, reader/
recover from errors; possible values are 0, 1, and 2
A true value turns on recovery mode which allows one to parse
broken XML or HTML data. The recovery mode allows the parser to
return the successfully parsed portion of the input document. This
is useful for almost well-formed documents, where for example a
closing tag is missing somewhere. Still, XML::LibXML will only
parse until the first fatal (non-recoverable) error occurs,
reporting recoverable parsing errors as warnings. To suppress even
these warnings, use recover=>2.
Note that validation is switched off automatically in recovery
mode.
expand_entities
/parser, reader/
substitute entities; possible values are 0 and 1; default is 1
Note that although this flag disables entity substitution, it does
not prevent the parser from loading external entities; when
substitution of an external entity is disabled, the entity will be
represented in the document tree by an XML_ENTITY_REF_NODE node
whose subtree will be the content obtained by parsing the external
resource; Although this nesting is visible from the DOM it is
transparent to XPath data model, so it is possible to match nodes
in an unexpanded entity by the same XPath expression as if the
entity were expanded. See also ext_ent_handler.
ext_ent_handler
/parser/
Provide a custom external entity handler to be used when
expand_entities is set to 1. Possible value is a subroutine
reference.
This feature does not work properly in libxml2 < 2.6.27!
The subroutine provided is called whenever the parser needs to
retrieve the content of an external entity. It is called with two
arguments: the system ID (URI) and the public ID. The value
returned by the subroutine is parsed as the content of the entity.
This method can be used to completely disable entity loading, e.g.
to prevent exploits of the type described at
(<http://searchsecuritychannel.techtarget.com/generic/0,295582,sid97_gci1304703,00.html>),
where a service is tricked to expose its private data by letting it
parse a remote file (RSS feed) that contains an entity reference to
a local file (e.g. "/etc/fstab").
A more granular solution to this problem, however, is provided by
custom URL resolvers, as in
my $c = XML::LibXML::InputCallback->new();
sub match { # accept file:/ URIs except for XML catalogs in /etc/xml/
my ($uri) = @_;
return ($uri=~m{^file:/}
and $uri !~ m{^file:///etc/xml/})
? 1 : 0;
}
$c->register_callbacks([ \&match, sub{}, sub{}, sub{} ]);
$parser->input_callbacks($c);
load_ext_dtd
/parser, reader/
load the external DTD subset while parsing; possible values are 0
and 1. Unless specified, XML::LibXML sets this option to 1.
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.
complete_attributes
/parser, reader/
create default DTD attributes; possible values are 0 and 1
validation
/parser, reader/
validate with the DTD; possible values are 0 and 1
suppress_errors
/parser, html, reader/
suppress error reports; possible values are 0 and 1
suppress_warnings
/parser, html, reader/
suppress warning reports; possible values are 0 and 1
pedantic_parser
/parser, html, reader/
pedantic error reporting; possible values are 0 and 1
no_blanks
/parser, html, reader/
remove blank nodes; possible values are 0 and 1
no_defdtd
/html/
do not add a default DOCTYPE; possible values are 0 and 1
the default is (0) to add a DTD when the input html lacks one
expand_xinclude or xinclude
/parser, reader/
Implement XInclude substitution; possible values are 0 and 1
Expands XInclude tags immediately while parsing the document. Note
that the parser will use the URI resolvers installed via
"XML::LibXML::InputCallback" to parse the included document (if
any).
no_xinclude_nodes
/parser, reader/
do not generate XINCLUDE START/END nodes; possible values are 0 and
1
no_network
/parser, html, reader/
Forbid network access; possible values are 0 and 1
If set to true, 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 the flag "recover" for processing
documents requiring such resources while networking is off.
clean_namespaces
/parser, reader/
remove redundant namespaces declarations during parsing; possible
values are 0 and 1.
no_cdata
/parser, html, reader/
merge CDATA as text nodes; possible values are 0 and 1
no_basefix
/parser, reader/
not fixup XINCLUDE xml#base URIS; possible values are 0 and 1
huge
/parser, html, reader/
relax any hardcoded limit from the parser; possible values are 0
and 1. Unless specified, XML::LibXML sets this option to 0.
Note: the default value for this option was changed to protect
against denial of service through entity expansion attacks. Before
enabling the option ensure you have taken alternative measures to
protect your application against this type of attack.
gdome
/parser/
THIS OPTION IS EXPERIMENTAL!
Although quite powerful, XML::LibXML's DOM implementation is
incomplete with respect to the DOM level 2 or level 3
specifications. XML::GDOME is based on libxml2 as well, and
provides a rather complete DOM implementation by wrapping libgdome.
This flag 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!
Note: this feature was not seriously tested in recent XML::LibXML
releases.
For compatibility with XML::LibXML versions prior to 1.70, the
following methods are also supported for querying and setting the
corresponding parser options (if called without arguments, the methods
return the current value of the corresponding parser options; with an
argument sets the option to a given value):
$parser->validation();
$parser->recover();
$parser->pedantic_parser();
$parser->line_numbers();
$parser->load_ext_dtd();
$parser->complete_attributes();
$parser->expand_xinclude();
$parser->gdome_dom();
$parser->clean_namespaces();
$parser->no_network();
The following obsolete methods trigger parser options in some special
way:
recover_silently
$parser->recover_silently(1);
If called without an argument, returns true if the current value of
the "recover" parser option is 2 and returns false otherwise. With
a true argument sets the "recover" parser option to 2; with a false
argument sets the "recover" parser option to 0.
expand_entities
$parser->expand_entities(0);
Get/set the "expand_entities" option. If called with a true
argument, also turns the "load_ext_dtd" option to 1.
keep_blanks
$parser->keep_blanks(0);
This is actually the opposite of the "no_blanks" parser option. If
used without an argument retrieves negated value of "no_blanks". If
used with an argument sets "no_blanks" to the opposite value.
base_uri
$parser->base_uri( $your_base_uri );
Get/set the "URI" option.
XML CATALOGS
"libxml2" supports XML catalogs. Catalogs are used to map remote
resources to their local copies. Using catalogs can speed up parsing
processes if many external resources from remote addresses are loaded
into the parsed documents (such as DTDs or XIncludes).
Note that libxml2 has a global pool of loaded catalogs, so if you apply
the method "load_catalog" to one parser instance, all parser instances
will start using the catalog (in addition to other previously loaded
catalogs).
Note also that catalogs are not used when a custom external entity
handler is specified. At the current state it is not possible to make
use of both types of resolving systems at the same time.
load_catalog
$parser->load_catalog( $catalog_file );
Loads the XML catalog file $catalog_file.
# Global external entity loader (similar to ext_ent_handler option
# but this works really globally, also in XML::LibXSLT include etc..)
XML::LibXML::externalEntityLoader(\&my_loader);
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 is stored in $@. There are two
implementations: the old one throws $@ which is just a message string,
in the new one $@ is an object from the class XML::LibXML::Error; this
class overrides the operator "" so that when printed, the object
flattens to the usual error message.
XML::LibXML throws errors as they occur. 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 as arguments. 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.
AUTHORS
Matt Sergeant, Christian Glahn, Petr Pajas
VERSION
2.0108
COPYRIGHT
2001-2007, AxKit.com Ltd.
2002-2006, Christian Glahn.
2006-2009, Petr Pajas.
perl v5.18.1 2013-12-17 XML::LibXML::Parser(3pm)
