XPath is a language for addressing parts of an XML
document, designed to be used by both XSLT and
XPointer.
This document has been reviewed by W3C Members and other interested
parties and has been endorsed by the Director as a W3C Recommendation. It
is a stable document and may be used as reference material or cited as
a normative reference from other documents. W3C's role in making the
Recommendation is to draw attention to the specification and to
promote its widespread deployment. This enhances the functionality and
interoperability of the Web.
The list of known errors in this specification is available at
http://www.w3.org/1999/11/REC-xpath-19991116-errata.
Comments on this specification may be sent to www-xpath-comments@w3.org; archives
of the comments are available.
The English version of this specification is the only normative
version. However, for translations of this document, see http://www.w3.org/Style/XSL/translations.html.
A list of current W3C Recommendations and other technical documents
can be found at http://www.w3.org/TR.
This specification is joint work of the XSL Working Group and the
XML Linking Working Group and so is part of the W3C Style activity and
of the W3C XML
activity.
1 Introduction
2 Location Paths
2.1 Location Steps
2.2 Axes
2.3 Node Tests
2.4 Predicates
2.5 Abbreviated Syntax
3 Expressions
3.1 Basics
3.2 Function Calls
3.3 Node-sets
3.4 Booleans
3.5 Numbers
3.6 Strings
3.7 Lexical Structure
4 Core Function Library
4.1 Node Set Functions
4.2 String Functions
4.3 Boolean Functions
4.4 Number Functions
5 Data Model
5.1 Root Node
5.2 Element Nodes
5.2.1 Unique IDs
5.3 Attribute Nodes
5.4 Namespace Nodes
5.5 Processing Instruction Nodes
5.6 Comment Nodes
5.7 Text Nodes
6 Conformance
Appendices
A References
A.1 Normative References
A.2 Other References
B XML Information Set Mapping (Non-Normative)
XPath is the result of an effort to provide a common syntax and
semantics for functionality shared between XSL Transformations [XSLT] and XPointer [XPointer]. The primary purpose
of XPath is to address parts of an XML [XML] document.
In support of this primary purpose, it also provides basic facilities
for manipulation of strings, numbers and booleans. XPath uses a
compact, non-XML syntax to facilitate use of XPath within URIs and XML
attribute values. XPath operates on the abstract, logical structure
of an XML document, rather than its surface syntax. XPath gets its
name from its use of a path notation as in URLs for navigating through
the hierarchical structure of an XML document.
In addition to its use for addressing, XPath is also designed so
that it has a natural subset that can be used for matching (testing
whether or not a node matches a pattern); this use of XPath is
described in XSLT.
XPath models an XML document as a tree of nodes. There are
different types of nodes, including element nodes, attribute nodes and
text nodes. XPath defines a way to compute a string-value for each type of node.
Some types of nodes also have names. XPath fully supports XML
Namespaces [XML Names]. Thus, the name of a node is
modeled as a pair consisting of a local part and a possibly null
namespace URI; this is called an expanded-name. The data model is
described in detail in [5 Data Model].
The primary syntactic construct in XPath is the expression. An
expression matches the production Expr. An
expression is evaluated to yield an object, which has one of the
following four basic types:
Expression evaluation occurs with respect to a context. XSLT and
XPointer specify how the context is determined for XPath expressions
used in XSLT and XPointer respectively. The context consists of:
The context position is always less than or equal to the
context size.
The variable bindings consist of a mapping from variable names to
variable values. The value of a variable is an object, which can be of
any of the types that are possible for the value of an expression,
and may also be of additional types not specified here.
The function library consists of a mapping from function names to
functions. Each function takes zero or more arguments and returns a
single result. This document defines a core function library that all
XPath implementations must support (see [4 Core Function Library]).
For a function in the core function library, arguments and result are
of the four basic types. Both XSLT and XPointer extend XPath by
defining additional functions; some of these functions operate on the
four basic types; others operate on additional data types defined by
XSLT and XPointer.
The namespace declarations consist of a mapping from prefixes to
namespace URIs.
The variable bindings, function library and namespace declarations
used to evaluate a subexpression are always the same as those used to
evaluate the containing expression. The context node, context
position, and context size used to evaluate a subexpression are
sometimes different from those used to evaluate the containing
expression. Several kinds of expressions change the context node; only
predicates change the context position and context size (see [2.4 Predicates]). When the evaluation of a kind of expression is
described, it will always be explicitly stated if the context node,
context position, and context size change for the evaluation of
subexpressions; if nothing is said about the context node, context
position, and context size, they remain unchanged for the
evaluation of subexpressions of that kind of expression.
XPath expressions often occur in XML attributes. The grammar
specified in this section applies to the attribute value after XML 1.0
normalization. So, for example, if the grammar uses the character
<, this must not appear in the XML source as
< but must be quoted according to XML 1.0 rules by,
for example, entering it as <. Within expressions,
literal strings are delimited by single or double quotation marks,
which are also used to delimit XML attributes. To avoid a quotation
mark in an expression being interpreted by the XML processor as
terminating the attribute value the quotation mark can be entered as a
character reference (" or
'). Alternatively, the expression can use single
quotation marks if the XML attribute is delimited with double
quotation marks or vice-versa.
One important kind of expression is a location path. A location
path selects a set of nodes relative to the context node. The result
of evaluating an expression that is a location path is the node-set
containing the nodes selected by the location path. Location paths
can recursively contain expressions that are used to filter sets of
nodes. A location path matches the production LocationPath.
In the following grammar, the non-terminals QName and NCName are defined in [XML Names], and S is defined in
[XML]. The grammar uses the same EBNF notation as
[XML] (except that grammar symbols always have initial
capital letters).
Expressions are parsed by first dividing the character string to be
parsed into tokens and then parsing the resulting sequence of tokens.
Whitespace can be freely used between tokens. The tokenization
process is described in [3.7 Lexical Structure].
Although location paths are not the most general grammatical
construct in the language (a LocationPath is a special case of an Expr), they are the most important construct and
will therefore be described first.
Every location path can be expressed using a straightforward but
rather verbose syntax. There are also a number of syntactic
abbreviations that allow common cases to be expressed concisely. This
section will explain the semantics of location paths using the
unabbreviated syntax. The abbreviated syntax will then be explained
by showing how it expands into the unabbreviated syntax (see [2.5 Abbreviated Syntax]).
Here are some examples of location paths using the unabbreviated
syntax:
There are two kinds of location path: relative location paths
and absolute location paths.
A relative location path consists of a sequence of one or more
location steps separated by /. The steps in a relative
location path are composed together from left to right. Each step in
turn selects a set of nodes relative to a context node. An initial
sequence of steps is composed together with a following step as
follows. The initial sequence of steps selects a set of nodes
relative to a context node. Each node in that set is used as a
context node for the following step. The sets of nodes identified by
that step are unioned together. The set of nodes identified by
the composition of the steps is this union. For example,
child::div/child::para selects the
para element children of the div element
children of the context node, or, in other words, the
para element grandchildren that have div
parents.
An absolute location path consists of / optionally
followed by a relative location path. A /<