xpath(3)

NAME

XML::XPath - a set of modules for parsing and evaluating
XPath statements

DESCRIPTION

This module aims to comply exactly to the XPath specifica
tion at http://www.w3.org/TR/xpath and yet allow exten
sions 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 "FOUND0,
        XML::XPath::XMLParser::as_string($node),
        "0;
}

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 specifica
tion, 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 parameter speci fies a string to parse, and the ioref parameter specifies an ioref to parse. The context option allows you to spec ify a context node. The context node has to be in the for mat 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 con text $context. In scalar context returns an XML::XPath::NodeSet object.
findnodes_as_string($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, $nodeset->to_literal is called automatically for you (and thus a "XML::XPath::Literal" is returned). Note that for each of the objects stringifica tion is overloaded, so you can just print the value found, or manipulate it in the ways you would a normal perl value (e.g. using regular expressions).
exists($path, [$context])
Returns true if the given path exists.
matches($node, $path, [$context])
Returns true if the node matches the path (optionally in context $context).
getNodeText($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 context 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 unex pected 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 pro
vided 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 par
ent axis. In that case the worst that will happen is
you'll end up with a circular reference 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 running process could leak mem
ory. Most long running processes will therefore 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->dis_
pose() 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/mail
man/listinfo/perl-xml

Matt Sergeant, matt@sergeant.org

docs.sk

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