XPath(3) User Contributed Perl Documentation XPath(3)NAMEXML::XPath - a set of modules for parsing and evaluating XPath state-
ments
DESCRIPTION
This module aims to comply exactly to the XPath specification at
http://www.w3.org/TR/xpath and yet allow extensions to be added in the
form of functions. Modules such as XSLT and XPointer may need to do
this as they support functionality beyond XPath.
SYNOPSIS
use XML::XPath;
use XML::XPath::XMLParser;
my $xp = XML::XPath->new(filename => 'test.xhtml');
my $nodeset = $xp->find('/html/body/p'); # find all paragraphs
foreach my $node ($nodeset->get_nodelist) {
print "FOUND\n\n",
XML::XPath::XMLParser::as_string($node),
"\n\n";
}
DETAILS
There's an awful lot to all of this, so bear with it - if you stick it
out it should be worth it. Please get a good understanding of XPath by
reading the spec before asking me questions. All of the classes and
parts herein are named to be synonimous with the names in the specifi-
cation, so consult that if you don't understand why I'm doing something
in the code.
API
The API of XML::XPath itself is extremely simple to allow you to get
going almost immediately. The deeper API's are more complex, but you
shouldn't have to touch most of that.
new()
This constructor follows the often seen named parameter method call.
Parameters you can use are: filename, parser, xml, ioref and context.
The filename parameter specifies an XML file to parse. The xml parame-
ter specifies a string to parse, and the ioref parameter specifies an
ioref to parse. The context option allows you to specify a context
node. The context node has to be in the format of a node as specified
in XML::XPath::XMLParser. The 4 parameters filename, xml, ioref and
context are mutually exclusive - you should only specify one (if you
specify anything other than context, the context node is the root of
your document). The parser option allows you to pass in an already
prepared XML::Parser object, to save you having to create more than one
in your application (if, for example, you're doing more than just
XPath).
my $xp = XML::XPath->new( context => $node );
It is very much recommended that you use only 1 XPath object throughout
the life of your application. This is because the object (and it's
sub-objects) maintain certain bits of state information that will be
useful (such as XPath variables) to later calls to find(). It's also a
good idea because you'll use less memory this way.
nodeset = find($path, [$context])
The find function takes an XPath expression (a string) and returns
either an XML::XPath::NodeSet object containing the nodes it found (or
empty if no nodes matched the path), or one of XML::XPath::Literal (a
string), XML::XPath::Number, or XML::XPath::Boolean. It should always
return something - and you can use ->isa() to find out what it
returned. If you need to check how many nodes it found you should check
$nodeset->size. See XML::XPath::NodeSet. An optional second parameter
of a context node allows you to use this method repeatedly, for example
XSLT needs to do this.
findnodes($path, [$context])
Returns a list of nodes found by $path, optionally in context $context.
In scalar context returns an XML::XPath::NodeSet object.
findnodes_as_string($path, [$context])
Returns the nodes found reproduced as XML. The result is not guaranteed
to be valid XML though.
findvalue($path, [$context])
Returns either a "XML::XPath::Literal", a "XML::XPath::Boolean" or a
"XML::XPath::Number" object. If the path returns a NodeSet, $node-
set->to_literal is called automatically for you (and thus a
"XML::XPath::Literal" is returned). Note that for each of the objects
stringification is overloaded, so you can just print the value found,
or manipulate it in the ways you would a normal perl value (e.g. using
regular expressions).
exists($path, [$context])
Returns true if the given path exists.
matches($node, $path, [$context])
Returns true if the node matches the path (optionally in context $con-
text).
getNodeText($path)
Returns the text string for a particular XML node. Returns a string,
or undef if the node doesn't exist.
setNodeText($path, $text)
Sets the text string for a particular XML node. The node can be an
element or an attribute. If the node to be set is an attribute, and
the attribute node does not exist, it will be created automatically.
createNode($path)
Creates the node matching the path given. If part of the path given,
or all of the path do not exist, the necessary nodes will be created
automatically.
set_namespace($prefix, $uri)
Sets the namespace prefix mapping to the uri.
Normally in XML::XPath the prefixes in XPath node tests take their con-
text from the current node. This means that foo:bar will always match
an element <foo:bar> regardless of the namespace that the prefix foo is
mapped to (which might even change within the document, resulting in
unexpected results). In order to make prefixes in XPath node tests
actually map to a real URI, you need to enable that via a call to the
set_namespace method of your XML::XPath object.
clear_namespaces()
Clears all previously set namespace mappings.
$XML::XPath::Namespaces
Set this to 0 if you don't want namespace processing to occur. This
will make everything a little (tiny) bit faster, but you'll suffer for
it, probably.
Node Object Model
See XML::XPath::Node, XML::XPath::Node::Element,
XML::XPath::Node::Text, XML::XPath::Node::Comment,
XML::XPath::Node::Attribute, XML::XPath::Node::Namespace, and
XML::XPath::Node::PI.
On Garbage Collection
XPath nodes work in a special way that allows circular references, and
yet still lets Perl's reference counting garbage collector to clean up
the nodes after use. This should be totally transparent to the user,
with one caveat: If you free your tree before letting go of a sub-tree,
consider that playing with fire and you may get burned. What does this
mean to the average user? Not much. Provided you don't free (or let go
out of scope) either the tree you passed to XML::XPath->new, or if you
didn't pass a tree, and passed a filename or IO-ref, then provided you
don't let the XML::XPath object go out of scope before you let results
of find() and its friends go out of scope, then you'll be fine. Even if
you do let the tree go out of scope before results, you'll probably
still be fine. The only case where you may get stung is when the last
part of your path/query is either an ancestor or parent axis. In that
case the worst that will happen is you'll end up with a circular refer-
ence that won't get cleared until interpreter destruction time. You can
get around that by explicitly calling $node->DESTROY on each of your
result nodes, if you really need to do that.
Mail me direct if that's not clear. Note that it's not doom and gloom.
It's by no means perfect, but the worst that will happen is a long run-
ning process could leak memory. Most long running processes will there-
fore be able to explicitly be careful not to free the tree (or
XML::XPath object) before freeing results. AxKit, an application that
uses XML::XPath, does this and I didn't have to make any changes to the
code - it's already sensible programming.
If you really don't want all this to happen, then set the variable
$XML::XPath::SafeMode, and call $xp->cleanup() on the XML::XPath object
when you're finished, or $tree->dispose() if you have a tree instead.
Example
Please see the test files in t/ for examples on how to use XPath.
Support/Author
This module is copyright 2000 AxKit.com Ltd. This is free software, and
as such comes with NO WARRANTY. No dates are used in this module. You
may distribute this module under the terms of either the Gnu GPL, or
the Artistic License (the same terms as Perl itself).
For support, please subscribe to the Perl-XML mailing list at the URL
http://listserv.activestate.com/mailman/listinfo/perl-xml
Matt Sergeant, matt@sergeant.org
SEE ALSO
XML::XPath::Literal, XML::XPath::Boolean, XML::XPath::Number,
XML::XPath::XMLParser, XML::XPath::NodeSet, XML::XPath::PerlSAX,
XML::XPath::Builder.
perl v5.8.8 2001-06-12 XPath(3)
