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XPath (XML Path Language) is an expression language designed to support the query or transformation of XML documents. It was defined by the World Wide Web Consortium (W3C) in 1999, [1] and can be used to compute values (e.g., strings, numbers, or Boolean values) from the content of an XML document.
XML documents must contain a root element (one that is the parent of all other elements). All elements in an XML document can contain sub elements, text and attributes. The tree represented by an XML document starts at the root element and branches to the lowest level of elements.
Nodes are of seven kinds, corresponding to different constructs in the syntax of XML: elements, attributes, text nodes, comments, processing instructions, namespace nodes, and document nodes. (The document node replaces the root node of XPath 1.0, because the XPath 2.0 model allows trees to be rooted at other kinds of node, notably elements.)
Compared to XPath 2.0, XPath 3.0 adds the following new features: . Inline function expressions Anonymous functions can be created in an expression context. For example, the expression function ($ a as xs:double, $ b as xs:double) as xs:double {$ a * $ b} creates a function that returns the product of its two arguments.
Originally, it was based on the XPath 1.0 data model which in turn is based on the XML Information Set. The XDM consists of flat sequences of zero or more items which can be typed or untyped, and are either atomic values or XML nodes (of seven kinds: document, element, attribute, text, namespace, processing instruction, and comment).
XQuery provides the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases [9] or office documents. XQuery contains a superset of XPath expression syntax to address specific parts of an XML document.
XPath (or XPath 1.0): an expression language for addressing portions of an XML document; XPath 2.0: a language for addressing portions of XML documents, successor of XPath 1.0; XPointer: a system for addressing components of XML based internet media; XProc : a W3C standard language to describe XML Pipeline
One or more Reference elements specify the resource being signed by URI reference and any transformations to be applied to the resource prior to signing. Transforms contains the transformations applied to the resource prior to signing. A transformation can be a XPath-expression that selects a defined subset of the document tree. [3]