Status Update (October 2016): Although XPath 1.0 remains widely used, and is referenced normatively from other W3C specifications, readers are advised that later versions exist, and that no further maintenance (including correction of reported errors) is planned for this document. Readers interested in the most recent version of the XPath specification are encouraged to refer to https://www.w3.org/TR/xpath-3/.
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.
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.
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:
node-set (an unordered collection of nodes without duplicates)
boolean (true or false)
number (a floating-point number)
string (a sequence of UCS characters)
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:
a node (the
context node)
a pair of non-zero positive integers (the context
position and the context size)
a set of variable bindings
a function library
the set of namespace declarations in scope for the
expression
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].
2 Location Paths
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:
child::para selects the
para element children of the context node
child::* selects all element
children of the context node
child::text() selects all text
node children of the context node
child::node() selects all the
children of the context node, whatever their node type
attribute::name selects the
name attribute of the context node
attribute::* selects all the
attributes of the context node
descendant::para selects the
para element descendants of the context node
ancestor::div selects all div
ancestors of the context node
ancestor-or-self::div selects the
div ancestors of the context node and, if the context node is a
div element, the context node as well
descendant-or-self::para selects the
para element descendants of the context node and, if the context node is
a para element, the context node as well
self::para selects the context node if it is a
para element, and otherwise selects nothing
child::chapter/descendant::para
selects the para element descendants of the
chapter element children of the context node
child::*/child::para selects
all para grandchildren of the context node
/ selects the document root (which is
always the parent of the document element)
/descendant::para selects all the
para elements in the same document as the context node
/descendant::olist/child::item selects all the
item elements that have an olist parent and
that are in the same document as the context node
child::para[position()=1] selects the first
para child of the context node
child::para[position()=last()] selects the last
para child of the context node
child::para[position()=last()-1] selects
the last but one para child of the context node
child::para[position()>1] selects all
the para children of the context node other than the
first para child of the context node
following-sibling::chapter[position()=1]
selects the next chapter sibling of the context node
preceding-sibling::chapter[position()=1]
selects the previous chapter sibling of the context
node
/descendant::figure[position()=42] selects
the forty-second figure element in the
document
/child::doc/child::chapter[position()=5]/child::section[position()=2]
selects the second section of the fifth
chapter of the doc document
element
child::para[attribute::type="warning"]
selects all para children of the context node that have a
type attribute with value warning
child::para[attribute::type='warning'][position()=5]
selects the fifth para child of the context node that has
a type attribute with value
warning
child::para[position()=5][attribute::type="warning"]
selects the fifth para child of the context node if that
child has a type attribute with value
warning
child::chapter[child::title='Introduction']
selects the chapter children of the context node that
have one or more title children with string-value equal to
Introduction
child::chapter[child::title] selects the
chapter children of the context node that have one or
more title children
child::*[self::chapter or self::appendix]
selects the chapter and appendix children of
the context node
child::*[self::chapter or
self::appendix][position()=last()] selects the last
chapter or appendix child of the context
node
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 / by itself
selects the root node of the document containing the context node. If
it is followed by a relative location path, then the location path
selects the set of nodes that would be selected by the relative
location path relative to the root node of the document containing the
context node.
an axis, which specifies the tree relationship between the
nodes selected by the location step and the context node,
a node test, which specifies the node type and expanded-name of the nodes selected
by the location step, and
zero or more predicates, which use arbitrary expressions to
further refine the set of nodes selected by the location
step.
The syntax for a location step is the axis name and node test
separated by a double colon, followed by zero or more expressions each
in square brackets. For example, in
child::para[position()=1], child is the name
of the axis, para is the node test and
[position()=1] is a predicate.
The node-set selected by the location step is the node-set that
results from generating an initial node-set from the axis and
node-test, and then filtering that node-set by each of the predicates
in turn.
The initial node-set consists of the nodes having the relationship
to the context node specified by the axis, and having the node type
and expanded-name specified
by the node test. For example, a location step
descendant::para selects the para element
descendants of the context node: descendant specifies
that each node in the initial node-set must be a descendant of the
context; para specifies that each node in the initial
node-set must be an element named para. The available
axes are described in [2.2 Axes]. The available node tests
are described in [2.3 Node Tests]. The meaning of some
node tests is dependent on the axis.
The initial node-set is filtered by the first predicate to generate
a new node-set; this new node-set is then filtered using the second
predicate, and so on. The final node-set is the node-set selected by
the location step. The axis affects how the expression in each
predicate is evaluated and so the semantics of a predicate is defined
with respect to an axis. See [2.4 Predicates].
the child axis contains the children of the
context node
the descendant axis contains the descendants of
the context node; a descendant is a child or a child of a child and so
on; thus the descendant axis never contains attribute or namespace
nodes
the parent axis contains the parent of the context node, if there is
one
the ancestor axis contains the ancestors of the
context node; the ancestors of the context node consist of the
parent of context node and the
parent's parent and so on; thus, the ancestor axis will always include
the root node, unless the context node is the root node
the following-sibling axis contains all the
following siblings of the context node; if the
context node is an attribute node or namespace node, the
following-sibling axis is empty
the preceding-sibling axis contains all the
preceding siblings of the context node; if the context node is an
attribute node or namespace node, the preceding-sibling
axis is empty
the following axis contains all nodes in the
same document as the context node that are after the context node in
document order, excluding any descendants and excluding attribute
nodes and namespace nodes
the preceding axis contains all nodes in the
same document as the context node that are before the context node in
document order, excluding any ancestors and excluding attribute nodes
and namespace nodes
the attribute axis contains the attributes of
the context node; the axis will be empty unless the context node is an
element
the namespace axis contains the namespace nodes
of the context node; the axis will be empty unless the context node
is an element
the self axis contains just the context node
itself
the descendant-or-self axis contains the context
node and the descendants of the context node
the ancestor-or-self axis contains the context
node and the ancestors of the context node; thus, the ancestor axis
will always include the root node
NOTE: The ancestor, descendant,
following, preceding and self
axes partition a document (ignoring attribute and namespace nodes):
they do not overlap and together they contain all the nodes in the
document.
Axes
[6]
AxisName
::=
'ancestor'
| 'ancestor-or-self'
| 'attribute'
| 'child'
| 'descendant'
| 'descendant-or-self'
| 'following'
| 'following-sibling'
| 'namespace'
| 'parent'
| 'preceding'
| 'preceding-sibling'
| 'self'
2.3 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.
A node test that is a QName
is true if and only if the type of the node (see [5 Data Model])
is the principal node type and has
an expanded-name equal to
the expanded-name specified
by the QName. For example,
child::para selects the para element
children of the context node; if the context node has no
para children, it will select an empty set of nodes.
attribute::href selects the href attribute
of the context node; if the context node has no href
attribute, it will select an empty set of nodes.
A QName in the node test is
expanded into an expanded-name using the namespace
declarations from the expression context. This is the same way
expansion is done for element type names in start and end-tags except
that the default namespace declared with xmlns is not
used: if the QName does not have
a prefix, then the namespace URI is null (this is the same way
attribute names are expanded). It is an error if the QName has a prefix for which there is
no namespace declaration in the expression context.
A node test * is true for any node of the principal
node type. For example, child::* will select all element
children of the context node, and attribute::* will
select all attributes of the context node.
A node test can have the form NCName:*. In this
case, the prefix is expanded in the same way as with a QName, using the context namespace
declarations. It is an error if there is no namespace declaration for
the prefix in the expression context. The node test will be true for
any node of the principal type whose expanded-name has the namespace URI
to which the prefix expands, regardless of the local part of the
name.
The node test text() is true for any text node. For
example, child::text() will select the text node
children of the context node. Similarly, the node test
comment() is true for any comment node, and the node test
processing-instruction() is true for any processing
instruction. The processing-instruction() test may have
an argument that is Literal; in this case, it
is true for any processing instruction that has a name equal to the
value of the Literal.
A node test node() is true for any node of any type
whatsoever.
An axis is either a forward axis or a reverse axis. An axis that
only ever contains the context node or nodes that are after the
context node in document
order is a forward axis. An axis that only ever contains
the context node or nodes that are before the context node in document order is a reverse axis.
Thus, the ancestor, ancestor-or-self, preceding, and preceding-sibling
axes are reverse axes; all other axes are forward axes. Since the self
axis always contains at most one node, it makes no difference whether
it is a forward or reverse axis. The proximity position of a
member of a node-set with respect to an axis is defined to be the
position of the node in the node-set ordered in document order if the
axis is a forward axis and ordered in reverse document order if the
axis is a reverse axis. The first position is 1.
A predicate filters a node-set with respect to an axis to produce a
new node-set. For each node in the node-set to be filtered, the PredicateExpr is evaluated with that node
as the context node, with the number of nodes in the node-set as the
context size, and with the proximity position of the node
in the node-set with respect to the axis as the context position; if
PredicateExpr evaluates to true for
that node, the node is included in the new node-set; otherwise, it is
not included.
A PredicateExpr is evaluated by
evaluating the Expr and converting the result
to a boolean. If the result is a number, the result will be converted
to true if the number is equal to the context position and will be
converted to false otherwise; if the result is not a number, then the
result will be converted as if by a call to the
boolean function. Thus a location path
para[3] is equivalent to
para[position()=3].
Here are some examples of location paths using abbreviated
syntax:
para selects the para element children of
the context node
* selects all element children of the
context node
text() selects all text node children of the
context node
@name selects the name attribute of
the context node
@* selects all the attributes of the
context node
para[1] selects the first para child of
the context node
para[last()] selects the last para child
of the context node
*/para selects all para grandchildren of
the context node
/doc/chapter[5]/section[2] selects the second
section of the fifth chapter of the
doc
chapter//para selects the para element
descendants of the chapter element children of the
context node
//para selects all the para descendants of
the document root and thus selects all para elements in the
same document as the context node
//olist/item selects all the item
elements in the same document as the context node that have an
olist parent
. selects the context node
.//para selects the para element
descendants of the context node
.. selects the parent of the context node
../@lang selects the lang attribute
of the parent of the context node
para[@type="warning"] selects all para
children of the context node that have a type attribute with
value warning
para[@type="warning"][5] selects the fifth
para child of the context node that has a type
attribute with value warning
para[5][@type="warning"] selects the fifth
para child of the context node if that child has a
type attribute with value warning
chapter[title="Introduction"] selects the
chapter children of the context node that have one or
more title children with string-value equal to
Introduction
chapter[title] selects the chapter
children of the context node that have one or more title
children
employee[@secretary and @assistant] selects all
the employee children of the context node that have both a
secretary attribute and an assistant
attribute
The most important abbreviation is that child:: can be
omitted from a location step. In effect, child is the
default axis. For example, a location path div/para is
short for child::div/child::para.
There is also an abbreviation for attributes:
attribute:: can be abbreviated to @. For
example, a location path para[@type="warning"] is short
for child::para[attribute::type="warning"] and so selects
para children with a type attribute with
value equal to warning.
// 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 (even a
para element that is a document element will be selected
by //para since the document element node is a child of
the root node); div//para is short for
div/descendant-or-self::node()/child::para and so
will select all para descendants of div
children.
NOTE: The location path //para[1] does
not mean the same as the location path
/descendant::para[1]. The latter selects the first
descendant para element; the former selects all descendant
para elements that are the first para
children of their parents.
A location step of . is short for
self::node(). This is particularly useful in
conjunction with //. For example, the location path
.//para is short for
and so will select all para descendant elements of the
context node.
Similarly, a location step of .. is short for
parent::node(). For example, ../title is
short for parent::node()/child::title and so will
select the title children of the parent of the context
node.
A VariableReference evaluates
to the value to which the variable name is bound in the set of
variable bindings in the context. It is an error if the variable name
is not bound to any value in the set of variable bindings in the
expression context.
A FunctionCall expression is
evaluated by using the FunctionName to
identify a function in the expression evaluation context function
library, evaluating each of the Arguments,
converting each argument to the type required by the function, and
finally calling the function, passing it the converted arguments. It
is an error if the number of arguments is wrong or if an argument
cannot be converted to the required type. The result of the FunctionCall expression is the result
returned by the function.
An argument is converted to type string as if by calling the
string function. An argument is converted to
type number as if by calling the number function.
An argument is converted to type boolean as if by calling the
boolean function. An argument that is not of
type node-set cannot be converted to a node-set.
A location path can be used as an expression. The expression
returns the set of nodes selected by the path.
The | operator computes the union of its operands,
which must be node-sets.
Predicates are used to filter
expressions in the same way that they are used in location paths. It
is an error if the expression to be filtered does not evaluate to a
node-set. The Predicate filters the
node-set with respect to the child axis.
NOTE: The meaning of a Predicate
depends crucially on which axis applies. For example,
preceding::foo[1] returns the first foo
element in reverse document order, because the axis that
applies to the [1] predicate is the preceding axis; by
contrast, (preceding::foo)[1] returns the first
foo element in document order, because the
axis that applies to the [1] predicate is the child
axis.
The / and // operators compose an
expression and a relative location path. It is an error if the
expression does not evaluate to a node-set. The /
operator does composition in the same way as when / is
used in a location path. As in location paths, // is
short for /descendant-or-self::node()/.
There are no types of objects that can be converted to node-sets.
An object of type boolean can have one of two values, true and
false.
An or expression is evaluated by evaluating each
operand and converting its value to a boolean as if by a call to the
boolean function. The result is true if either
value is true and false otherwise. The right operand is not evaluated
if the left operand evaluates to true.
An and expression is evaluated by evaluating each
operand and converting its value to a boolean as if by a call to the
boolean function. The result is true if both
values are true and false otherwise. The right operand is not
evaluated if the left operand evaluates to false.
An EqualityExpr (that is not just
a RelationalExpr) or a RelationalExpr (that is not just an AdditiveExpr) is evaluated by comparing the
objects that result from evaluating the two operands. Comparison of
the resulting objects is defined in the following three paragraphs.
First, comparisons that involve node-sets are defined in terms of
comparisons that do not involve node-sets; this is defined uniformly
for =, !=, <=,
<, >= and >. Second,
comparisons that do not involve node-sets are defined for
= and !=. Third, comparisons that do not
involve node-sets are defined for <=,
<, >= and >.
If both objects to be compared are node-sets, then the comparison
will be true if and only if there is a node in the first node-set and
a node in the second node-set such that the result of performing the
comparison on the string-values of the two nodes is
true. If one object to be compared is a node-set and the other is a
number, then the comparison will be true if and only if there is a
node in the node-set such that the result of performing the comparison
on the number to be compared and on the result of converting the
string-value of that node to
a number using the number function is true. If
one object to be compared is a node-set and the other is a string,
then the comparison will be true if and only if there is a node in the
node-set such that the result of performing the comparison on the
string-value of the node and
the other string is true. If one object to be compared is a node-set
and the other is a boolean, then the comparison will be true if and
only if the result of performing the comparison on the boolean and on
the result of converting the node-set to a boolean using the
boolean function is true.
When neither object to be compared is a node-set and the operator
is = or !=, then the objects are compared by
converting them to a common type as follows and then comparing them.
If at least one object to be compared is a boolean, then each object
to be compared is converted to a boolean as if by applying the
boolean function. Otherwise, if at least one
object to be compared is a number, then each object to be compared is
converted to a number as if by applying the
number function. Otherwise, both objects to be
compared are converted to strings as if by applying the
string function. The = comparison
will be true if and only if the objects are equal; the !=
comparison will be true if and only if the objects are not equal.
Numbers are compared for equality according to IEEE 754 [IEEE 754]. Two booleans are equal if either both are true or
both are false. Two strings are equal if and only if they consist of
the same sequence of UCS characters.
NOTE: If $x is bound to a node-set, then
$x="foo" does not mean the same as
not($x!="foo"): the former is true if and only if
some node in $x has the string-value
foo; the latter is true if and only if all
nodes in $x have the string-value
foo.
When neither object to be compared is a node-set and the operator
is <=, <, >= or
>, then the objects are compared by converting both
objects to numbers and comparing the numbers according to IEEE 754.
The < comparison will be true if and only if the first
number is less than the second number. The <=
comparison will be true if and only if the first number is less than
or equal to the second number. The > comparison will
be true if and only if the first number is greater than the second
number. The >= comparison will be true if and only if
the first number is greater than or equal to the second number.
NOTE:
When an XPath expression occurs in an XML document, any
< and <= operators must be quoted
according to XML 1.0 rules by using, for example,
< and <=. In the following
example the value of the test attribute is an XPath
expression:
NOTE: The effect of the above grammar is that the order of
precedence is (lowest precedence first):
or
and
=, !=
<=, <, >=,
>
and the operators are all left associative.
For example, 3 > 2 > 1 is equivalent to (3
> 2) > 1, which evaluates to false.
3.5 Numbers
A number represents a floating-point number. A number can have any
double-precision 64-bit format IEEE 754 value [IEEE 754].
These include a special "Not-a-Number" (NaN) value,
positive and negative infinity, and positive and negative zero. See
Section 4.2.3 of [JLS] for a summary of the key
rules of the IEEE 754 standard.
The numeric operators convert their operands to numbers as if by
calling the number function.
The + operator performs addition.
The - operator performs subtraction.
NOTE: Since XML allows - in names, the -
operator typically needs to be preceded by whitespace. For example,
foo-bar evaluates to a node-set containing the child
elements named foo-bar; foo - bar evaluates
to the difference of the result of converting the string-value of the first
foo child element to a number and the result of
converting the string-value
of the first bar child to a number.
The div operator performs floating-point division
according to IEEE 754.
The mod operator returns the remainder from a
truncating division. For example,
5 mod 2 returns 1
5 mod -2 returns 1
-5 mod 2 returns -1
-5 mod -2 returns -1
NOTE: This is the same as the % operator in Java and
ECMAScript.
NOTE: This is not the same as the IEEE 754 remainder operation, which
returns the remainder from a rounding division.
Strings consist of a sequence of zero or more characters, where a
character is defined as in the XML Recommendation [XML].
A single character in XPath thus corresponds to a single Unicode
abstract character with a single corresponding Unicode scalar value
(see [Unicode]); this is not the same thing as a 16-bit
Unicode code value: the Unicode coded character representation for an
abstract character with Unicode scalar value greater that U+FFFF is a
pair of 16-bit Unicode code values (a surrogate pair). In many
programming languages, a string is represented by a sequence of 16-bit
Unicode code values; implementations of XPath in such languages must
take care to ensure that a surrogate pair is correctly treated as a
single XPath character.
NOTE: It is possible in Unicode for there to be two strings that
should be treated as identical even though they consist of the
distinct sequences of Unicode abstract characters. For example, some
accented characters may be represented in either a precomposed or
decomposed form. Therefore, XPath expressions may return unexpected
results unless both the characters in the XPath expression and in the
XML document have been normalized into a canonical form. See [Character Model].
3.7 Lexical Structure
When tokenizing, the longest possible token is always returned.
For readability, whitespace may be used in expressions even though not
explicitly allowed by the grammar: ExprWhitespace may be freely added within
patterns before or after any ExprToken.
The following special tokenization rules must be applied in the
order specified to disambiguate the ExprToken grammar:
If there is a preceding token and the preceding token is not
one of @, ::, (,
[, , or an Operator, then a * must be
recognized as a MultiplyOperator
and an NCName must be
recognized as an OperatorName.
This section describes functions that XPath implementations must
always include in the function library that is used to evaluate
expressions.
Each function in the function library is specified using a function
prototype, which gives the return type, the name of the function, and
the type of the arguments. If an argument type is followed by a
question mark, then the argument is optional; otherwise, the argument
is required.
The id function selects elements by their
unique ID (see [5.2.1 Unique IDs]). When the argument to
id is of type node-set, then the result is the
union of the result of applying id to the
string-value of each of the
nodes in the argument node-set. When the argument to
id is of any other type, the argument is
converted to a string as if by a call to the
string function; the string is split into a
whitespace-separated list of tokens (whitespace is any sequence of
characters matching the production S);
the result is a node-set containing the elements in the same document
as the context node that have a unique ID equal to any of the tokens
in the list.
id("foo") selects the element with unique ID
foo
id("foo")/child::para[position()=5] selects
the fifth para child of the element with unique ID
foo
The local-name function returns the local part
of the expanded-name of the
node in the argument node-set that is first in document order. If the argument
node-set is empty or the first node has no expanded-name, an empty string is
returned. If the argument is omitted, it defaults to a node-set with
the context node as its only member.
The namespace-uri function returns the
namespace URI of the expanded-name of the node in the
argument node-set that is first in document order. If the argument
node-set is empty, the first node has no expanded-name, or the namespace URI
of the expanded-name is
null, an empty string is returned. If the argument is omitted, it
defaults to a node-set with the context node as its only member.
NOTE: The string returned by the
namespace-uri function will be empty except for
element nodes and attribute nodes.
The name function returns a string containing
a QName representing the
expanded-name of the node in
the argument node-set that is first in document order. The QName must represent the expanded-name with respect to the
namespace declarations in effect on the node whose expanded-name is being represented.
Typically, this will be the QName that occurred in the XML
source. This need not be the case if there are namespace declarations
in effect on the node that associate multiple prefixes with the same
namespace. However, an implementation may include information about
the original prefix in its representation of nodes; in this case, an
implementation can ensure that the returned string is always the same
as the QName used in the XML
source. If the argument node-set is empty or the first node has no
expanded-name, an empty
string is returned. If the argument it omitted, it defaults to a
node-set with the context node as its only member.
NOTE: The string returned by the name function
will be the same as the string returned by the
local-name function except for element nodes and
attribute nodes.
The string function converts an object to a string
as follows:
A node-set is converted to a string by returning the string-value of the node in the
node-set that is first in document
order. If the node-set is empty, an empty string is
returned.
A number is converted to a string as follows
NaN is converted to the string NaN
positive zero is converted to the string
0
negative zero is converted to the string
0
positive infinity is converted to the string
Infinity
negative infinity is converted to the string
-Infinity
if the number is an integer, the number is represented in
decimal form as a Number with no decimal
point and no leading zeros, preceded by a minus sign (-)
if the number is negative
otherwise, the number is represented in decimal form as a Number including a decimal point with at least
one digit before the decimal point and at least one digit after the
decimal point, preceded by a minus sign (-) if the number
is negative; there must be no leading zeros before the decimal point
apart possibly from the one required digit immediately before the
decimal point; beyond the one required digit after the decimal point
there must be as many, but only as many, more digits as are needed to
uniquely distinguish the number from all other IEEE 754 numeric
values.
The boolean false value is converted to the string
false. The boolean true value is converted to the
string true.
An object of a type other than the four basic types is
converted to a string in a way that is dependent on that
type.
If the argument is omitted, it defaults to a node-set with the
context node as its only member.
NOTE: The string function is not intended for
converting numbers into strings for presentation to users. The
format-number function and xsl:number
element in [XSLT] provide this
functionality.
The substring-before function returns the substring
of the first argument string that precedes the first occurrence of the
second argument string in the first argument string, or the empty
string if the first argument string does not contain the second
argument string. For example,
substring-before("1999/04/01","/") returns
1999.
The substring-after function returns the
substring of the first argument string that follows the first
occurrence of the second argument string in the first argument string,
or the empty string if the first argument string does not contain the
second argument string. For example,
substring-after("1999/04/01","/") returns
04/01, and
substring-after("1999/04/01","19") returns
99/04/01.
The substring function returns the substring of the
first argument starting at the position specified in the second
argument with length specified in the third argument. For example,
substring("12345",2,3) returns "234".
If the third argument is not specified, it returns
the substring starting at the position specified in the second
argument and continuing to the end of the string. For example,
substring("12345",2) returns "2345".
More precisely, each character in the string (see [3.6 Strings]) is considered to have a numeric position: the
position of the first character is 1, the position of the second
character is 2 and so on.
NOTE: This differs from Java and ECMAScript, in which the
String.substring method treats the position of the first
character as 0.
The returned substring contains those
characters for which the position of the character is greater than or
equal to the rounded value of the second argument and, if the third
argument is specified, less than the sum of the rounded value of the
second argument and the rounded value of the third argument; the
comparisons and addition used for the above follow the standard IEEE
754 rules; rounding is done as if by a call to the
round function. The following examples illustrate
various unusual cases:
The string-length returns the number of
characters in the string (see [3.6 Strings]). If the
argument is omitted, it defaults to the context node converted to a
string, in other words the string-value of the context node.
The normalize-space function returns the argument
string with whitespace normalized by stripping leading and trailing
whitespace and replacing sequences of whitespace characters by a
single space. Whitespace characters are the same as those allowed by the S production in XML. If the argument is
omitted, it defaults to the context node converted to a string, in
other words the string-value
of the context node.
The translate function returns the first
argument string with occurrences of characters in the second argument
string replaced by the character at the corresponding position in the
third argument string. For example,
translate("bar","abc","ABC") returns the string
BAr. If there is a character in the second argument
string with no character at a corresponding position in the third
argument string (because the second argument string is longer than the
third argument string), then occurrences of that character in the
first argument string are removed. For example,
translate("--aaa--","abc-","ABC") returns
"AAA". If a character occurs more than once in the second
argument string, then the first occurrence determines the replacement
character. If the third argument string is longer than the second
argument string, then excess characters are ignored.
NOTE: The translate function is not a sufficient
solution for case conversion in all languages. A future version of
XPath may provide additional functions for case conversion.
The lang function returns true or false depending on
whether the language of the context node as specified by
xml:lang attributes is the same as or is a sublanguage of
the language specified by the argument string. The language of the
context node is determined by the value of the xml:lang
attribute on the context node, or, if the context node has no
xml:lang attribute, by the value of the
xml:lang attribute on the nearest ancestor of the context
node that has an xml:lang attribute. If there is no such
attribute, then lang returns false. If there is such an
attribute, then lang returns true if the attribute
value is equal to the argument ignoring case, or if there is some
suffix starting with - such that the attribute value is
equal to the argument ignoring that suffix of the attribute value and
ignoring case. For example, lang("en") would return true
if the context node is any of these five elements:
The number function converts its argument to a
number as follows:
a string that consists of optional whitespace followed by an
optional minus sign followed by a Number
followed by whitespace is converted to the IEEE 754 number that is
nearest (according to the IEEE 754 round-to-nearest rule)
to the mathematical value represented by the string; any other
string is converted to NaN
boolean true is converted to 1; boolean false is converted to
0
a node-set is first converted to a string as if by a call to the
string function and then converted in the same way as a
string argument
an object of a type other than the four basic types is
converted to a number in a way that is dependent on that
type
If the argument is omitted, it defaults to a node-set with the
context node as its only member.
NOTE: The number function should not be used
for conversion of numeric data occurring in an element in an XML
document unless the element is of a type that represents numeric data
in a language-neutral format (which would typically be transformed
into a language-specific format for presentation to a user). In
addition, the number function cannot be used
unless the language-neutral format used by the element is consistent
with the XPath syntax for a Number.
The round function returns the number that is
closest to the argument and that is an integer. If there are two such
numbers, then the one that is closest to positive infinity is
returned. If the argument is NaN, then NaN is returned. If the
argument is positive infinity, then positive infinity is returned. If
the argument is negative infinity, then negative infinity is
returned. If the argument is positive zero, then positive zero is
returned. If the argument is negative zero, then negative zero is
returned. If the argument is less than zero, but greater than or
equal to -0.5, then negative zero is returned.
NOTE: For these last two cases, the result of calling the
round function is not the same as the result of
adding 0.5 and then calling the floor
function.
5 Data Model
XPath operates on an XML document as a tree. This section describes
how XPath models an XML document as a tree. This model is conceptual
only and does not mandate any particular implementation. The
relationship of this model to the XML Information Set [XML Infoset] is described in [B XML Information Set Mapping].
XML documents operated on by XPath must conform to the XML
Namespaces Recommendation [XML Names].
The tree contains nodes. There are seven types of node:
root nodes
element nodes
text nodes
attribute nodes
namespace nodes
processing instruction nodes
comment nodes
For every type of
node, there is a way of determining a string-value for a
node of that type. For some types of node, the string-value is part
of the node; for other types of node, the string-value is computed
from the string-value of descendant nodes.
NOTE: For element nodes and root nodes, the string-value of a node
is not the same as the string returned by the DOM
nodeValue method (see [DOM]).
Some types of
node also have an expanded-name, which is a pair
consisting of a local part and a namespace URI. The local part is a
string. The namespace URI is either null or a string. The namespace
URI specified in the XML document can be a URI reference as defined in
[RFC2396]; this means it can have a fragment identifier
and can be relative. A relative URI should be resolved into an
absolute URI during namespace processing: the namespace URIs of
expanded-names of nodes in
the data model should be absolute. Two expanded-names are equal if they have
the same local part, and either both have a null namespace URI or both
have non-null namespace URIs that are equal.
There is an
ordering, document order, defined on all the nodes in the
document corresponding to the order in which the first character of
the XML representation of each node occurs in the XML representation
of the document after expansion of general entities. Thus, the root
node will be the first node. Element nodes occur before their
children. Thus, document order orders element nodes in order of the
occurrence of their start-tag in the XML (after expansion of
entities). The attribute nodes and namespace nodes of an element occur
before the children of the element. The namespace nodes are defined
to occur before the attribute nodes. The relative order of namespace
nodes is implementation-dependent. The relative order of attribute
nodes is implementation-dependent. Reverse document order is the reverse of document order.
Root nodes and element nodes have an ordered list of child nodes.
Nodes never share children: if one node is not the same node as
another node, then none of the children of the one node will be the
same node as any of the children of another node. Every node other than the root node has
exactly one parent, which is either an element node or
the root node. A root node or an element node is the parent
of each of its child nodes. The descendants of a node are the
children of the node and the descendants of the children of the
node.
5.1 Root Node
The root node is the root of the tree. A root node does not occur
except as the root of the tree. The element node for the document
element is a child of the root node. The root node also has as
children processing instruction and comment nodes for processing
instructions and comments that occur in the prolog and after the end
of the document element.
The string-value of the
root node is the concatenation of the string-values of all text node
descendants of the root
node in document order.
There is an element node for every element in the document. An
element node has an expanded-name computed by expanding
the QName of the element
specified in the tag in accordance with the XML Namespaces
Recommendation [XML Names]. The namespace URI of the
element's expanded-name will
be null if the QName has no
prefix and there is no applicable default namespace.
NOTE: In the notation of Appendix A.3 of [XML Names],
the local part of the expanded-name corresponds to the
type attribute of the ExpEType element; the
namespace URI of the expanded-name corresponds to the ns
attribute of the ExpEType element, and is null if the
ns attribute of the ExpEType element is
omitted.
The children of an element node are the element nodes, comment
nodes, processing instruction nodes and text nodes for its content.
Entity references to both internal and external entities are expanded.
Character references are resolved.
The string-value of an
element node is the concatenation of the string-values of all text node
descendants of the element
node in document order.
5.2.1 Unique IDs
An element node may have a unique identifier (ID). This is the
value of the attribute that is declared in the DTD as type
ID. No two elements in a document may have the same
unique ID. If an XML processor reports two elements in a document as
having the same unique ID (which is possible only if the document is
invalid) then the second element in document order must be treated as
not having a unique ID.
NOTE: If a document does not have a DTD, then no element in the
document will have a unique ID.
5.3 Attribute Nodes
Each element node has an associated set of attribute nodes; the
element is the parent of each of
these attribute nodes; however, an attribute node is not a child of
its parent element.
NOTE: This is different from the DOM, which does not treat the
element bearing an attribute as the parent of the attribute (see
[DOM]).
Elements never share attribute nodes: if one element node is not
the same node as another element node, then none of the attribute
nodes of the one element node will be the same node as the attribute
nodes of another element node.
NOTE: The = operator tests whether two nodes have the
same value, not whether they are the same node. Thus
attributes of two different elements may compare as equal using
=, even though they are not the same node.
A defaulted attribute is treated the same as a specified attribute.
If an attribute was declared for the element type in the DTD, but the
default was declared as #IMPLIED, and the attribute was
not specified on the element, then the element's attribute set does
not contain a node for the attribute.
Some attributes, such as xml:lang and
xml:space, have the semantics that they apply to all
elements that are descendants of the element bearing the attribute,
unless overridden with an instance of the same attribute on another
descendant element. However, this does not affect where attribute
nodes appear in the tree: an element has attribute nodes only for
attributes that were explicitly specified in the start-tag or
empty-element tag of that element or that were explicitly declared in
the DTD with a default value.
An attribute node has an expanded-name and a string-value. The expanded-name is computed by
expanding the QName specified in
the tag in the XML document in accordance with the XML Namespaces
Recommendation [XML Names]. The namespace URI of the
attribute's name will be null if the QName of the attribute does not have
a prefix.
NOTE: In the notation of Appendix A.3 of [XML Names],
the local part of the expanded-name corresponds to the
name attribute of the ExpAName element; the
namespace URI of the expanded-name corresponds to the ns
attribute of the ExpAName element, and is null if the
ns attribute of the ExpAName element is
omitted.
An attribute node has a string-value. The string-value is the normalized value
as specified by the XML Recommendation [XML]. An
attribute whose normalized value is a zero-length string is not
treated specially: it results in an attribute node whose string-value is a zero-length
string.
NOTE: It is possible for default attributes to be declared in an
external DTD or an external parameter entity. The XML Recommendation
does not require an XML processor to read an external DTD or an
external parameter unless it is validating. A stylesheet or other facility that assumes
that the XPath tree contains default attribute values declared in an
external DTD or parameter entity may not work with some non-validating
XML processors.
There are no attribute nodes corresponding to attributes that
declare namespaces (see [XML Names]).
5.4 Namespace Nodes
Each element has an associated set of namespace nodes, one for each
distinct namespace prefix that is in scope for the element (including
the xml prefix, which is implicitly declared by the XML
Namespaces Recommendation [XML Names]) and one for
the default namespace if one is in scope for the element. The element
is the parent of each of these
namespace nodes; however, a namespace node is not a child of
its parent element. Elements never share namespace nodes: if one element
node is not the same node as another element node, then none of the
namespace nodes of the one element node will be the same node as the
namespace nodes of another element node. This means that an element
will have a namespace node:
for every attribute on the element whose name starts with
xmlns:;
for every attribute on an ancestor element whose name starts
xmlns: unless the element itself or a nearer ancestor
redeclares the prefix;
for an xmlns attribute, if the element or some
ancestor has an xmlns attribute, and the value of the
xmlns attribute for the nearest such element is
non-empty
NOTE: An attribute xmlns="" "undeclares"
the default namespace (see [XML Names]).
A namespace node has an expanded-name: the local part is
the namespace prefix (this is empty if the namespace node is for the
default namespace); the namespace URI is always null.
The string-value of a
namespace node is the namespace URI that is being bound to the
namespace prefix; if it is relative, it must be resolved just like a
namespace URI in an expanded-name.
5.5 Processing Instruction Nodes
There is a processing instruction node for every processing
instruction, except for any processing instruction that occurs within
the document type declaration.
A processing instruction has an expanded-name: the local part is
the processing instruction's target; the namespace URI is null. The
string-value of a processing
instruction node is the part of the processing instruction following
the target and any whitespace. It does not include the terminating
?>.
NOTE: The XML declaration is not a processing instruction.
Therefore, there is no processing instruction node corresponding to the
XML declaration.
5.6 Comment Nodes
There is a comment node for every comment, except for any comment that
occurs within the document type declaration.
The string-value of
comment is the content of the comment not including the opening
<!-- or the closing -->.
Character data is grouped into text nodes. As much character data
as possible is grouped into each text node: a text node never has an
immediately following or preceding sibling that is a text node. The
string-value of a text node
is the character data. A text node always has at least one character
of data.
Each character within a CDATA section is treated as character data.
Thus, <![CDATA[<]]> in the source document will
treated the same as <. Both will result in a
single < character in a text node in the tree. Thus, a
CDATA section is treated as if the <![CDATA[ and
]]> were removed and every occurrence of
< and & were replaced by
< and & respectively.
NOTE: When a text node that contains a < character
is written out as XML, the < character must be escaped
by, for example, using <, or including it in a
CDATA section.
Characters inside comments, processing instructions and attribute
values do not produce text nodes. Line-endings in external entities
are normalized to #xA as specified in the XML Recommendation [XML].
XPath is intended primarily as a component that can be used by
other specifications. Therefore, XPath relies on specifications that
use XPath (such as [XPointer] and [XSLT]) to
specify criteria for conformance of implementations of XPath and does
not define any conformance criteria for independent implementations of
XPath.
T. Berners-Lee, R. Fielding, and
L. Masinter. Uniform Resource Identifiers (URI): Generic
Syntax. IETF RFC 2396. See http://www.ietf.org/rfc/rfc2396.txt.
ISO (International
Organization for Standardization). ISO/IEC 10646-1:1993,
Information technology -- Universal Multiple-Octet Coded Character Set
(UCS) -- Part 1: Architecture and Basic Multilingual Plane.
International Standard. See http://www.iso.ch/cate/d18741.html.
World Wide Web Consortium. XSL
Transformations (XSLT). W3C Recommendation. See http://www.w3.org/TR/xslt
B XML Information Set Mapping (Non-Normative)
The nodes in the XPath data model can be derived from the
information items provided by the XML Information Set [XML Infoset] as follows:
NOTE: A new version of the XML Information Set Working Draft, which
will replace the May 17 version, was close to completion at the time
when the preparation of this version of XPath was completed and was
expected to be released at the same time or shortly after the release
of this version of XPath. The mapping is given for this new version
of the XML Information Set Working Draft. If the new version of the
XML Information Set Working has not yet been released, W3C members may
consult the internal Working Group version
http://www.w3.org/XML/Group/1999/09/WD-xml-infoset-19990915.html
(members
only).
The root node comes from the document information item. The
children of the root node come from the children and children - comments
properties.
An element node comes from an element information item. The
children of an element node come from the children and children - comments properties. The
attributes of an element node come from the attributes property. The namespaces
of an element node come from the in-scope namespaces property. The
local part of the expanded-name of the element node
comes from the local name
property. The namespace URI of the expanded-name of the element node
comes from the namespace URI
property. The unique ID of the element node comes from the children property of the attribute
information item in the attributes property that has an attribute type property equal to
ID.
An attribute node comes from an attribute information item.
The local part of the expanded-name of the attribute node
comes from the local name
property. The namespace URI of the expanded-name of the attribute node
comes from the namespace URI
property. The string-value of
the node comes from concatenating the character code property of each member
of the children
property.
A text node comes from a sequence of one or more consecutive
character information items. The string-value of the node comes from
concatenating the character code
property of each of the character information items.
A processing instruction node comes from a processing
instruction information item. The local part of the expanded-name of the node comes from
the target property. (The
namespace URI part of the expanded-name of the node is null.)
The string-value of the node
comes from the content
property. There are no processing instruction nodes for processing
instruction items that are children of document type declaration
information item.
A comment node comes from a comment information item. The
string-value of the node
comes from the content property.
There are no comment nodes for comment information items that are
children of document type declaration information item.
A namespace node comes from a namespace declaration
information item. The local part of the expanded-name of the node comes from
the prefix property. (The
namespace URI part of the expanded-name of the node is null.)
The string-value of the node
comes from the namespace URI
property.