XPath expression for selecting all nodes with a common attribute

a book I'm reading on XML says that to select all nodes in an XML file that have a specific attribute, use the syntax:

//*/@_attribute_

What I don't understand is why the asterisk is needed. As I understand it, the expression // selects all descendants of the root node. So, wouldn't //@lang, for example, select all descendants of the root node that have an attribute called "lang"? I can't even interpret what the asterisk even means in the above expression (I know the asterisk in general 开发者_StackOverflowmeans "all"). If someone could break it down for me I'd really appreciate it.

Thanks

Hi, a book I'm reading on XML says that to select all nodes in an XML file that have a specific attribute, use the syntax:

//*/@attribute

That's wrong. It will be expanded to:

/descendant-or-self::node()/child::*/attribute::attribute

Meaning: All attribute attributes of any element child of a node being the root document itself or one of its descendats

You need:

/descendant::*[attribute::attribute]

Or the abbreviated form

//*[@attribute]

About the *: formaly is a name test not a node type test. In XPath 1.0 there is no element type test. In XPath 2.0 you have element(). So, why select only elements? Well, it doesn't. The axis have a principal node type, from http://www.w3.org/TR/xpath/#node-tests :

Every axis has a principal node type. If an axis can contain elements, then the principal node type is element; otherwise, it is the type of the nodes that the axis can contain. Thus,

• For the attribute axis, the principal node type is attribute.
• For the namespace axis, the principal node type is namespace.
• For other axes, the principal node type is element.

That's why *,child::*,self::*,descendant::*, etc. selects elements, but @* or attribute::* or namespace::* selects attributes or in scope namespaces.

About predicate (the [@attribute] part): this expression is evaluate with each of the nodes selects by last step. It expects a boolean value for filtering. The boolean value for a node set (this is the result for attribute::attribute) is false for an empty node set, and true otherwise.

The title of this question is:

XPath expression for selecting all nodes with a common attribute

However nowhere does the text of the question discuss how tho find all nodes that have a common attribute -- so the title may be incorrect.

To find all nodes that have a common attribute named x (BTW, only element-nodes can have attributes), use:

//*[@x]

Use:

//@x

to select all attributes named x in the XML document. This is probably the shortest expression to do so.

There is nothing wrong with:

//*/@x

except that it is slightly longer.

It is a shorthand for:

/descendant-or-self::node()/child::*/attribute::x

and also selects all x attributes in the XML document.

Someone may think that this expression doesn't select the x attribute of the top element in the document. This is a wrong conclusion, because the first location step:

/descendant-or-self::node()

selects every node in the document, including the root (/) itself.

This means that:

/descendant-or-self::node()/child::*

selects every element, including the top element (which is the only child of the root node in a well-formed XML document).

So, when the last location step /@x is finally added, this will select all the x attributes of all nodes selected so far by the first two location steps -- that is all x attributes of all element-nodes in the XML document.