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106 ” DOM Extension // Also returns a DOMNodeList with only the body node $list = $xpath->query(’//body’); ?> • In the first two examples, note that the root element (html) is referenced in the expression even though it is assumed to be the context node (since no other node is specified as the second parameter in either query call). • A single forward slash / indicates a parent-child relationship. /html/body addresses all body nodes that are children the document’s root html element (which in this case only amounts to a single result). • A double forward slash // indicates an ancestor-descendant relationship. //body addresses all body nodes that are descendants of the context node (which again only amounts to a single result). The single and double forward slash operators can be used multiple times and in combination with each other as shown below. i N a m e s p a c e s If you attempt to address nodes by their element name and receive no results when it appears you should, it’s possible that the document is namespacing nodes. The easiest way to get around this is to replace the element name with a condition. F o r example, if you are using the expression ul, an equivalent expression that disregards the namespace would be *[name()=“ul”] where * is a wildcard for all nodes and the name function compares the node name against a given value.
DOM Extension ” 107 R e l a t i v e Addressing The above examples use what is called absolute addressing, similar in concept to absolute filesystem paths. The next example covers use relative addressing. Where //body//ul/li selects li nodes, //body//ul/li/.. instead selects ul nodes by using relative addressing (specifically ..) to indicate that the parent node of the addressed node (li) should be returned rather than the addressed node itself. Compare this with the same sequence used to refer to the parent directory of the current directory on a UNIX filesystem. Addressing Attributes Examples thus far have dealt with element nodes in particular. XPath also supports the selection of attribute nodes. // Returns the id attribute nodes of all ul nodes $list = $xpath->query(’//ul/@id’); // Returns all ul nodes that have id attribute nodes $list = $xpath->query(’//ul/@id/..’); // Returns all id attribute nodes $list = $xpath->query(’//@id’); ?> • //ul/@li addresses all id attribute nodes associated with ul element nodes that are descendants of the context node. • N o t e how the use of relative addressing in the second example applies to attributes nodes just as it would apply to child element nodes.
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