XPath Evaluation
The XML::LibXML::XPathContext class provides an almost complete interface to libxml2's XPath implementation. With XML::LibXML::XPathContext is is possible to evaluate XPath expressions in the context of arbitrary node, context size, and context position, with a user-defined namespace-prefix mapping, custom XPath functions written in Perl, and even a custom XPath variable resolver.

Examples

Namespaces
This example demonstrates registerNs() method. It finds all paragraph nodes in an XHTML document.

my $xc = XML::LibXML::XPathContext->new($xhtml_doc);
$xc->registerNs('xhtml', 'http://www.w3.org/1999/xhtml');
my @nodes = $xc->findnodes('//xhtml:p');

Custom XPath functions
This example demonstrates registerFunction() method by defining a function filtering nodes based on a Perl regular expression:

sub grep_nodes { 
  my ($nodelist,$regexp) =  @_;
  my $result = XML::LibXML::NodeList->new;
  for my $node ($nodelist->get_nodelist()) {
    $result->push($node) if $node->textContent =~ $regexp;
  }
  return $result;
};

my $xc = XML::LibXML::XPathContext->new($node);
$xc->registerFunction('grep_nodes', \&grep_nodes);
my @nodes = $xc->findnodes('//section[grep_nodes(para,"\bsearch(ing|es)?\b")]');

Variables
This example demonstrates registerVarLookup() method. We use XPath variables to recycle results of previous evaluations:

sub var_lookup {
  my ($varname,$ns,$data)=@_;
  return $data->{$varname};
}
        
my $areas = XML::LibXML->new->parse_file('areas.xml');
my $empl = XML::LibXML->new->parse_file('employees.xml');
        
my $xc = XML::LibXML::XPathContext->new($empl);
        
my %variables = ( 
  A => $xc->find('/employees/employee[@salary>10000]'),
  B => $areas->find('/areas/area[district='Brooklyn']/street'), 
);
        
# get names of employees from $A working in an area listed in $B
$xc->registerVarLookupFunc(\&var_lookup, \%variables);
my @nodes = $xc->findnodes('$A[work_area/street = $B]/name');

Methods
new 	

my $xpc = XML::LibXML::XPathContext->new();

Creates a new XML::LibXML::XPathContext object without a context node.

my $xpc = XML::LibXML::XPathContext->new($node);

Creates a new XML::LibXML::XPathContext object with the context node set to $node.
registerNs 	

$xpc->registerNs($prefix, $namespace_uri)

Registers namespace $prefix to $namespace_uri.
unregisterNs 	

$xpc->unregisterNs($prefix)

Unregisters namespace $prefix.
lookupNs 	

$uri = $xpc->lookupNs($prefix)

Returns namespace URI registered with $prefix. If $prefix is not registered to any namespace URI returns undef.
registerVarLookupFunc 	

$xpc->registerVarLookupFunc($callback, $data)

Registers variable lookup function $prefix. The registered function is executed by the XPath engine each time an XPath variable is evaluated. It takes three arguments: $data, variable name, and variable ns-URI and must return one value: a number or string or any XML::LibXML:: object that can be a result of findnodes: Boolean, Literal, Number, Node (e.g. Document, Element, etc.), or NodeList. For convenience, simple (non-blessed) array references containing only Abstract Base Class of XML::LibXML Nodes objects can be used instead of a XML::LibXML::NodeList.
getVarLookupData 	

$data = $xpc->getVarLookupData();

Returns the data that have been associated with a variable lookup function during a previous call to registerVarLookupFunc.
getVarLookupFunc 	

$callback = $xpc->getVarLookupFunc();

Returns the variable lookup function previously registered with registerVarLookupFunc.
unregisterVarLookupFunc 	

$xpc->unregisterVarLookupFunc($name);

Unregisters variable lookup function and the associated lookup data.
registerFunctionNS 	

$xpc->registerFunctionNS($name, $uri, $callback)

Registers an extension function $name in $uri namespace. $callback must be a CODE reference. The arguments of the callback function are either simple scalars or XML::LibXML::* objects depending on the XPath argument types. The function is responsible for checking the argument number and types. Result of the callback code must be a single value of the following types: a simple scalar (number, string) or an arbitrary XML::LibXML::* object that can be a result of findnodes: Boolean, Literal, Number, Node (e.g. Document, Element, etc.), or NodeList. For convenience, simple (non-blessed) array references containing only Abstract Base Class of XML::LibXML Nodes objects can be used instead of a XML::LibXML::NodeList.
unregisterFunctionNS 	

$xpc->unregisterFunctionNS($name, $uri)

Unregisters extension function $name in $uri namespace. Has the same effect as passing undef as $callback to registerFunctionNS.
registerFunction 	

$xpc->registerFunction($name, $callback)

Same as registerFunctionNS but without a namespace.
unregisterFunction 	

$xpc->unregisterFunction($name)

Same as unregisterFunctionNS but without a namespace.
findnodes 	

@nodes = $xpc->findnodes($xpath)

@nodes = $xpc->findnodes($xpath, $context_node )

$nodelist = $xpc->findnodes($xpath, $context_node )

Performs the xpath statement on the current node and returns the result as an array. In scalar context returns a XML::LibXML::NodeList object. Optionally, a node may be passed as a second argument to set the context node for the query.
The xpath expression can be passed either as a string or or as a XML::LibXML::XPathExpression object.
find 	

$object = $xpc->find($xpath )

$object = $xpc->find($xpath, $context_node )

Performs the xpath expression using the current node as the context of the expression, and returns the result depending on what type of result the XPath expression had. For example, the XPath 1 * 3 + 52 results in a XML::LibXML::Number object being returned. Other expressions might return a XML::LibXML::Boolean object, or a XML::LibXML::Literal object (a string). Each of those objects uses Perl's overload feature to ``do the right thing'' in different contexts. Optionally, a node may be passed as a second argument to set the context node for the query.
The xpath expression can be passed either as a string or or as a XML::LibXML::XPathExpression object.
findvalue 	

$value = $xpc->findvalue($xpath )

$value = $xpc->findvalue($xpath, $context_node )

Is exactly equivalent to:

$xpc->find( $xpath, $context_node )->to_literal;

That is, it returns the literal value of the results. This enables you to ensure that you get a string back from your search, allowing certain shortcuts. This could be used as the equivalent of <xsl:value-of select=``some_xpath''/>. Optionally, a node may be passed in the second argument to set the context node for the query.
The xpath expression can be passed either as a string or or as a XML::LibXML::XPathExpression object.
exists 	

$bool = $xpc->exists( $xpath_expression, $context_node );

This method behaves like findnodes, except that it only returns a boolean value (1 if the expression matches a node, 0 otherwise) and may be faster than findnodes, because the XPath evaluation may stop early on the first match (this is true for libxml2 >= 2.6.27).
For XPath expressions that do not return node-set, the method returns true if the returned value is a non-zero number or a non-empty string.
setContextNode 	

$xpc->setContextNode($node)

Set the current context node.
getContextNode 	

my $node = $xpc->getContextNode;

Get the current context node.
setContextPosition 	

$xpc->setContextPosition($position)

Set the current context position. By default, this value is -1 (and evaluating XPath function position() in the initial context raises an XPath error), but can be set to any value up to context size. This usually only serves to cheat the XPath engine to return given position when position() XPath function is called. Setting this value to -1 restores the default behavior.
getContextPosition 	

my $position = $xpc->getContextPosition;

Get the current context position.
setContextSize 	

$xpc->setContextSize($size)

Set the current context size. By default, this value is -1 (and evaluating XPath function last() in the initial context raises an XPath error), but can be set to any non-negative value. This usually only serves to cheat the XPath engine to return the given value when last() XPath function is called. If context size is set to 0, position is automatically also set to 0. If context size is positive, position is automatically set to 1. Setting context size to -1 restores the default behavior.
getContextSize 	

my $size = $xpc->getContextSize;

Get the current context size.
setContextNode 	

$xpc->setContextNode($node)

Set the current context node.

Bugs And Caveats
XML::LibXML::XPathContext objects are reentrant, meaning that you can call methods of an XML::LibXML::XPathContext even from XPath extension functions registered with the same object or from a variable lookup function. On the other hand, you should rather avoid registering new extension functions, namespaces and a variable lookup function from within extension functions and a variable lookup function, unless you want to experience untested behavior.

Authors
Ilya Martynov and Petr Pajas, based on XML::LibXML and XML::LibXSLT code by Matt Sergeant and Christian Glahn.

Historical remark
Prior to XML::LibXML 1.61 this module was distributed separately for maintenance reasons.