XPath is a pattern expression used to select a set of XML nodes.
You can select:
- from node name
- from attributes
- from text
This is a pipeline syntax that should read from left to right with the command being separated by a / or //
should be read:
- start from the root node (always)
- then //h1 - select all descendant nodes (ie //) the first , h1 element
- then from the previous selected node (known also the context node) (ie the h1 node):
- select the next sibling following-sibling
- no matter its tag name (*)
Get all node by tag name
- // means go down the whole tree (the whole document)
- a is the tag name
The same as full qualified expression working for all namespace would be:
- // means the whole tree (recursive)
- * means select the node
- local-name()='tagName' means select the node with the tag name equal to tagName
Select by attribute
The below expression will select:
Select by namespace
- // means the whole tree (recursive)
- * means select any node
- namespace-uri()='http://mynamespaceUri' means select the node with the namespace URI name equal to http://mynamespaceUri
There is two syntax:
- an abbreviated one 1)
- and a fully qualified
Example: select all para in the tree
- abbreviated syntax: //para
- is a short for /descendant-or-self::node()/child::para
Node Navigation and Content
An XML document is a tree-structured (hierarchical) collection of nodes. As with a hierarchical directory structure, it is useful to specify a path that points to a particular node in the hierarchy (hence the name of the specification: XPath).
In fact, much of the notation of directory paths is carried over intact:
|/||The forward slash||Path separator|| An absolute path from the root of the document starts with a /.
A relative path from a given location starts with anything else.
|..||A double period||The parent of the current node||And its content for the functions|
|.||A single period||The current node||And its content for the functions|
For example, in an Extensible HTML (XHTML) document, the path /h1/h2/ would indicate an h2 element under an h1. (Recall that in XML, element names are case-sensitive, so this kind of specification works much better in XHTML than it would in plain HTML, because HTML is case-insensitive).
A name specified in an XPath expression refers to an element. For example, h1 in /h1/h2 refers to an h1 element.
In a pattern-matching specification such as XPath, the specification /h1/h2 selects all h2 elements that lie under an h1 element.
The square-bracket notation () is normally associated with indexing.
To select a specific h2 element, you use square brackets  for indexing. The path /h1/h2 would therefore select the fifth h2 element under the fourth h1 element.
The expression @type=“unordered” specifies an attribute named type whose value is unordered. An expression such as LIST/@type specifies the type attribute of a LIST element.
The expression LIST[@type=“unordered”] selects all LIST elements whose type value is unordered.
Examples that use the extended square-bracket notation:
- /PROJECT[.=“MyProject”]: Selects a PROJECT named “MyProject”.
- /PROJECT[STATUS]: Selects all projects that have a STATUS child element.
- /PROJECT[STATUS=“Critical”]: Selects all projects that have a STATUS child element with the string-value Critical.
Combining Index Addresses
The XPath specification defines quite a few addressing mechanisms, and they can be combined in many different ways in order to get interesting combinations:
- LIST[@type=“ordered”]: Selects all LIST elements of with the type attribute and a value of ordered, and returns the third.
- LIST[@type=“ordered”]: Selects the third LIST element, but only if it is of the type ordered.
By definition, an unqualified XPath expression selects a set of XML nodes that matches that specified pattern.
For example, /HEAD matches all top-level HEAD entries, whereas /HEAD matches only the first.
|*||Matches any element node|
|node()||Matches any node of any kind: element node, text node, attribute node, processing instruction node, namespace node, or comment node.|
|text()||selects all text node|
|@*||Matches any attribute node.|
In the project database example, /*/PERSON[.=“Fred”] matches any PROJECT or ACTIVITY element that names Fred.
- preceding sibling of the first h2 node
- next/following sibling of the second h2 node
is short for
double forward slash (tree traversal)
So far, all the patterns you have seen have specified an exact number of levels in the hierarchy.
For example, /HEAD specifies any HEAD element at the first level in the hierarchy, whereas /*/* specifies any element at the second level in the hierarchy.
To specify an indeterminate level in the hierarchy, use a double forward slash (//).
For example, the XPath expression //PARA selects all paragraph elements in a document, wherever they may be found.
The // pattern can also be used within a path. So the expression
indicates all paragraph elements in a subtree that begins from /HEAD/LIST.
// is short for /descendant-or-self::node()/.
For example, //para is short for /descendant-or-self::node()/child::para and so will select any para element in the document.
XPath expressions yield either a set of nodes, a string, a Boolean (a true/false value), or a number.
||||Alternative. For example, PARA|LIST selects all PARA and LIST elements.|
|or, and||Returns the or/and of two Boolean values.|
|=, !=||Equal or not equal, for Booleans, strings, and numbers.|
|<, >, ⇐, >=||Less than, greater than, less than or equal to, greater than or equal to, for numbers.|
|+, -, *, div, mod||Add, subtract, multiply, floating-point divide, and modulus (remainder) operations (e.g., 6 mod 4 = 2).|
Expressions can be grouped in parentheses, so you do not have to worry about operator precedence.
Note - Operator precedence is a term that answers the question, “If you specify a + b * c, does that mean (a+b) * c or a + (b*c)?” (The operator precedence is roughly the same as that shown in the table).
String-Value of an Element
The string-value of an element is the concatenation of all descendent text nodes, no matter how deep. Consider this mixed-content XML data:
<PARA>This paragraph contains a <b>bold</b> word</PARA>
The string-value of theelement is “This paragraph contains a bold word”. In particular, note that is a child of and that the text bold is a child of .
The point is that all the text in all children of a node joins in the concatenation to form the string-value.
Also, it is worth understanding that the text in the abstract data model defined by XPath is fully normalized. So whether the XML structure contains the entity reference < or < in a CDATA section, the element's string-value will contain the < character. Therefore, when generating HTML or XML with an XSLT stylesheet, you must convert occurrences of < to < or enclose them in a CDATA section. Similarly, occurrences of & must be converted to &.
The XPath specification 4)is the foundation for a variety of specifications:
- including XSLT. Xpath is used to query nodes from the source document and apply styling templates to them to create a result document.
- and linking/addressing specifications such as XPointer.