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 firstname.lastname@example.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.
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 (
'). 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
children of the context node, or, in other words, the
para element grandchildren that have
An absolute location path consists of
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
|| RelativeLocationPath '/' Step|
A location step has three parts:
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 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
descendants of the context node:
that each node in the initial node-set must be a descendant of the
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].
|||Step||::=||AxisSpecifier NodeTest Predicate*|
The following axes are available:
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
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
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
selfaxes partition a document (ignoring attribute and namespace nodes): they do not overlap and together they contain all the nodes in the document.
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,
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
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
of the context node; if the context node has no
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
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
The node test
text() is true for any text node. For
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
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
|| NodeType '(' ')'|
|| 'processing-instruction' '(' Literal ')'|
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 is equivalent to
|||Predicate||::=||'[' PredicateExpr ']'|
Here are some examples of location paths using abbreviated syntax:
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
There is also an abbreviation for attributes:
attribute:: can be abbreviated to
example, a location path
para[@type="warning"] is short
child::para[attribute::type="warning"] and so selects
para children with a
type attribute with
value equal to
// is short for
/descendant-or-self::node()/. For example,
//para is short for
/descendant-or-self::node()/child::para and so will
para element in the document (even a
para element that is a document element will be selected
//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
NOTE: The location path
//paradoes not mean the same as the location path
/descendant::para. The latter selects the first descendant
paraelement; the former selects all descendant
paraelements that are the first
parachildren of their parents.
A location step of
. is short for
self::node(). This is particularly useful in
//. For example, the location path
.//para is short for
and so will select all
para descendant elements of the
Similarly, a location step of
.. is short for
parent::node(). For example,
parent::node()/child::title and so will
title children of the parent of the context
|||AbbreviatedRelativeLocationPath||::=||RelativeLocationPath '//' Step|
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.
Parentheses may be used for grouping.
|| '(' Expr ')'|
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.
|||FunctionCall||::=||FunctionName '(' ( Argument ( ',' Argument )* )? ')'|
A location path can be used as an expression. The expression returns the set of nodes selected by the path.
| 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::fooreturns the first
fooelement in reverse document order, because the axis that applies to the
predicate is the preceding axis; by contrast,
(preceding::foo)returns the first
fooelement in document order, because the axis that applies to the
predicate is the child axis.
// 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
used in a location path. As in location paths,
There are no types of objects that can be converted to node-sets.
|| UnionExpr '|' PathExpr|
|| FilterExpr '/' RelativeLocationPath|
|| FilterExpr '//' RelativeLocationPath|
|| FilterExpr Predicate|
An object of type boolean can have one of two values, true and false.
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.
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
comparisons that do not involve node-sets are defined for
!=. Third, comparisons that do not
involve node-sets are defined for
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
!=, 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
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.
$xis 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
$xhas the string-value
foo; the latter is true if and only if all nodes in
$xhave the string-value
When neither object to be compared is a node-set and the operator
>, then the objects are compared by converting both
objects to numbers and comparing the numbers according to IEEE 754.
< 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
>= 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
<=operators must be quoted according to XML 1.0 rules by using, for example,
<=. In the following example the value of the
testattribute is an XPath expression:<xsl:if test="@value < 10">...</xsl:if>
|| OrExpr 'or' AndExpr|
|| AndExpr 'and' EqualityExpr|
|| EqualityExpr '=' RelationalExpr|
|| EqualityExpr '!=' RelationalExpr|
|| RelationalExpr '<' AdditiveExpr|
|| RelationalExpr '>' AdditiveExpr|
|| RelationalExpr '<=' AdditiveExpr|
|| RelationalExpr '>=' AdditiveExpr|
NOTE: The effect of the above grammar is that the order of precedence is (lowest precedence first):
3 > 2 > 1is equivalent to
(3 > 2) > 1, which evaluates to false.
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.
+ operator performs addition.
- operator performs subtraction.
NOTE: Since XML allows
-in names, the
-operator typically needs to be preceded by whitespace. For example,
foo-barevaluates to a node-set containing the child elements named
foo - barevaluates to the difference of the result of converting the string-value of the first
foochild element to a number and the result of converting the string-value of the first
barchild to a number.
div operator performs floating-point division
according to IEEE 754.
mod operator returns the remainder from a
truncating division. For example,
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.
|| AdditiveExpr '+' MultiplicativeExpr|
|| AdditiveExpr '-' MultiplicativeExpr|
|| MultiplicativeExpr MultiplyOperator UnaryExpr|
|| MultiplicativeExpr 'div' UnaryExpr|
|| MultiplicativeExpr 'mod' UnaryExpr|
|| '-' UnaryExpr|
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].
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
, or an Operator, then a
* must be
recognized as a MultiplyOperator
and an NCName must be
recognized as an OperatorName.
|||ExprToken||::=||'(' | ')' | '[' | ']' | '.' | '..' | '@' | ',' | '::'|
|||Literal||::=||'"' [^"]* '"'|
|| "'" [^']* "'"|
|||Number||::=||Digits ('.' Digits?)?|
|| '.' Digits|
|| '/' | '//' | '|' | '+' | '-' | '=' | '!=' | '<' | '<=' | '>' | '>='|
|||OperatorName||::=||'and' | 'or' | 'mod' | 'div'|
|||FunctionName||::=||QName - NodeType|
|| NCName ':' '*'|
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.
Function: number last()
The last function returns a number equal to the context size from the expression evaluation context.
Function: number position()
The position function returns a number equal to the context position from the expression evaluation context.
Function: number count(node-set)
The count function returns the number of nodes in the argument node-set.
Function: node-set id(object)
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.
Function: string local-name(node-set?)
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.
Function: string namespace-uri(node-set?)
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.
Function: string name(node-set?)
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.
Function: string string(object?)
The string function converts an object to a string as follows:
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
If the argument is omitted, it defaults to a node-set with the context node as its only member.
stringfunction is not intended for converting numbers into strings for presentation to users. The
xsl:numberelement in [XSLT] provide this functionality.
Function: string concat(string, string, string*)
The concat function returns the concatenation of its arguments.
Function: boolean starts-with(string, string)
The starts-with function returns true if the first argument string starts with the second argument string, and otherwise returns false.
Function: boolean contains(string, string)
The contains function returns true if the first argument string contains the second argument string, and otherwise returns false.
Function: string substring-before(string, string)
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,
Function: string substring-after(string, string)
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,
Function: string substring(string, number, number?)
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,
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,
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.substringmethod 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:
Function: number string-length(string?)
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.
Function: string normalize-space(string?)
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.
Function: string translate(string, string, string)
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,
"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.
Function: boolean boolean(object)
The boolean function converts its argument to a boolean as follows:
Function: boolean not(boolean)
The not function returns true if its argument is false, and false otherwise.
Function: boolean true()
The true function returns true.
Function: boolean false()
The false function returns false.
Function: boolean lang(string)
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
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:
<para xml:lang="en"/> <div xml:lang="en"><para/></div> <para xml:lang="EN"/> <para xml:lang="en-us"/>
Function: number number(object?)
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
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.
Function: number sum(node-set)
The sum function returns the sum, for each node in the argument node-set, of the result of converting the string-values of the node to a number.
Function: number floor(number)
The floor function returns the largest (closest to positive infinity) number that is not greater than the argument and that is an integer.
Function: number ceiling(number)
The ceiling function returns the smallest (closest to negative infinity) number that is not less than the argument and that is an integer.
Function: number round(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.
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:
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
nodeValuemethod (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.
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.
The root node does not have an expanded-name.
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
typeattribute of the
ExpETypeelement; the namespace URI of the expanded-name corresponds to the
nsattribute of the
ExpETypeelement, and is null if the
nsattribute of the
ExpETypeelement 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.
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.
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.
=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: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
nameattribute of the
ExpANameelement; the namespace URI of the expanded-name corresponds to the
nsattribute of the
ExpANameelement, and is null if the
nsattribute of the
ExpANameelement 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]).
Each element has an associated set of namespace nodes, one for each
distinct namespace prefix that is in scope for the element (including
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:
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.
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.
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
A comment node does not have an expanded-name.
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.
<![CDATA[<]]> in the source document will
treated the same as
<. Both will result in a
< character in a text node in the tree. Thus, a
CDATA section is treated as if the
]]> were removed and every occurrence of
& were replaced by
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].
A text node does not have an expanded-name.
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.
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).
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
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.