NAME¶
XML::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", "expand_entities", and
"huge" 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 1.
- 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 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¶
2.0001
COPYRIGHT¶
2001-2007, AxKit.com Ltd.
2002-2006, Christian Glahn.
2006-2009, Petr Pajas.