This is one document in a set of eight documents that are being progressed to Edited Recommendation together (XPath 2.0, XQuery 1.0, XQueryX 1.0, XSLT 2.0, Data Model (XDM), Functions and Operators, Formal Semantics, Serialization).
This document, published on 14 December 2010, is an Edited
This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. 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.
This specification is designed to be referenced normatively from
other specifications defining a host language for it; it is not
intended to be implemented outside a host language. The
implementability of this specification has been tested in the context
of its normative inclusion in host languages defined by the
Please report errors in and submit comments on this document using W3C's
This document was produced by
XPath 2.0 is an expression
language that allows the processing of values conforming to the data
model defined in
The primary purpose of XPath is to address the
nodes of
XPath is designed to be embedded in a
XQuery Version 1.0 is an extension of XPath Version 2.0. Any expression that is syntactically valid and executes successfully in both XPath 2.0 and XQuery 1.0 will return the same result in both languages. Since these languages are so closely related, their grammars and language descriptions are generated from a common source to ensure consistency, and the editors of these specifications work together closely.
XPath also depends on and is closely related to the following specifications:
The type system of XPath is based on
The built-in function library and the operators supported by
XPath are defined in
This document specifies a grammar for XPath, using the
same basic EBNF notation used in
In the grammar productions in this document, named symbols are underlined and literal text is enclosed in double quotes. For example, the following production describes the syntax of a function call:
The production should be read as follows: A function call consists of a QName followed by an open-parenthesis. The open-parenthesis is followed by an optional argument list. The argument list (if present) consists of one or more expressions, separated by commas. The optional argument list is followed by a close-parenthesis.
Certain aspects of language
processing are described in this specification as
A language aspect described in this specification as
This document normatively defines the dynamic semantics of
XPath. The static semantics of XPath are normatively defined
in
The basic building block of XPath is the
This specification contains no
assumptions or requirements regarding the character set encoding of strings
of
Like XML, XPath is a case-sensitive language. Keywords in
XPath use lower-case characters and are not reserved—that is, names in XPath expressions are allowed to be the same as language keywords, except for certain unprefixed function-names listed in
xs:string
. The xs:QName
.
Names in XPath are called xs:anyURI
type in
xs = http://www.w3.org/2001/XMLSchema
fn = http://www.w3.org/2005/xpath-functions
err = http://www.w3.org/2005/xqt-errors
(see
Element nodes have a property called
In
The individual components of the
true
if rules for backward compatibility with XPath Version 1.0 are in effect; otherwise it is false
.
xs:anyURI
type in
xs:anyURI
type in
xs:anyURI
type in
An expression that binds a variable (such as a
for
,
some
, or every
expression) extends the
The
xs:string
and xs:anyURI
(and types derived from them) when no
explicit collation is
specified.
fn:resolve-uri
function.)xs:anyURI
type in
fn:doc
function. The type is the fn:doc
with the given URI as its
literal argument. fn:doc
is a
string literal that is not present in fn:doc
is document-node()?
.
The purpose of the fn:doc
.
fn:collection
function. The type is the type of the
sequence of nodes that would result from calling the
fn:collection
function with this URI as its
argument.fn:collection
is a string literal that is not present in
fn:collection
is node()*
.
The purpose of the fn:collection
.
fn:collection
function with no arguments.node()*
.
The individual
components of the
The
Certain language constructs, notably the E1/E2
and the E1[E2]
, create a new focus
for the evaluation of a sub-expression. In these constructs, E2
is evaluated once for each item in the
sequence that results from evaluating E1
. Each time E2
is evaluated, it is evaluated with a
different focus. The focus for evaluating E2
is referred to below as the E1
is referred to as the E2
is being evaluated. When this evaluation
is complete, evaluation of the containing expression continues with
its original focus unchanged.
.
). When an expression E1/E2
or E1[E2]
is evaluated, each item in the
sequence obtained by evaluating E1
becomes the context item in the inner focus for an evaluation of E2
.
fn:position()
. When an expression E1/E2
or E1[E2]
is evaluated, the context position in
the inner focus for an evaluation of E2
is the position of the context item in the sequence obtained by
evaluating E1
. The position of the
first item in a sequence is always 1 (one). The context position is
always less than or equal to the context size.
fn:last()
. When an expression
E1/E2
or E1[E2]
is evaluated, the context size in the
inner focus for an evaluation of E2
is
the number of items in the sequence obtained by evaluating E1
.
fn:current-dateTime
function. If invoked multiple times during the execution of
xs:dayTimeDuration
. See
fn:doc
function when applied to that URI.
If there are one or more URIs in D
, then the document-uri property of D
must either be absent, or must
be one of these URIs.
This means that given a document node $N
, the result of
fn:doc(fn:document-uri($N)) is $N
will always be True, unless
fn:document-uri($N)
is an empty sequence.
fn:collection
function when that URI is supplied as the
argument.
For every document node D
that is in the target of a mapping in D
must either be absent, or must be a
URI U
such that U
to D
."
This means that for any document node $N
retrieved using the
fn:collection
function, either directly or by navigating to the root of a
node that was returned, the result of fn:doc(fn:document-uri($N)) is $N
will always be True, unless fn:document-uri($N)
is an empty sequence. This
implies a requirement for the fn:doc
and fn:collection
functions to be
consistent in their effect. If the implementation uses catalogs or
user-supplied URI resolvers to dereference URIs supplied to the fn:doc
function, the implementation of the fn:collection
function must take these
mechanisms into account. For example, an implementation might achieve this
by mapping the collection URI to a set of document URIs, which are then
resolved using the same catalog or URI resolver that is used by the fn:doc
function.
fn:collection
function with no arguments.
XPath is defined in terms
of the
Figure 1: Processing Model Overview
Figure 1 provides a schematic overview of the processing steps that
are discussed in detail below. Some of these steps are completely
outside the domain of XPath; in Figure 1, these are depicted
outside the line that represents the boundaries of the language, an
area labeled
Before
A document may be parsed using an XML parser that
generates an
The Information Set or PSVI may be
transformed into an
The above steps provide an example of how an
type-name
property.) The type annotation of a node is a
The value of an attribute is represented directly within the
attribute node. An attribute node whose type is unknown (such as might
occur in a schemaless document) is given the xs:untypedAtomic
.
The value of an element is represented by the children of the
element node, which may include text nodes and other element
nodes. The xs:untyped
. An element that has been validated and found to be partially valid is annotated with the schema type xs:anyType
. If an element node is annotated as xs:untyped
, all its descendant element nodes are also annotated as xs:untyped
. However, if an element node is annotated as xs:anyType
, some of its descendant element nodes may have a more specific
The
XPath defines two phases of processing called
the
Within each phase, an implementation is free to use any strategy or algorithm whose result conforms to the specifications in this document.
During the static analysis phase, the
The
Each expression is then assigned a
During the
The purpose of the
The dynamic evaluation phase can occur only if no errors were detected during the
The dynamic evaluation phase depends on the
xs:integer*
, denoting a sequence of zero or more integers, but at evaluation time its value may have the dynamic type xs:integer
, denoting exactly one integer.)
If an operand of an expression is found
to have a
Even though static typing can catch many xs:untypedAtomic
. This is not a
When the
The host language may provide a serialization option.
In order for XPath to
be well defined, the input
Some of the consistency constraints use the term
For every node that has a type annotation, if that type annotation is found in the
For every element name
Every element name, attribute name, or schema type name referenced in
Any reference to a global element, attribute, or type name in
the
For each mapping of a string to a
document node in
For each mapping of a string to a sequence of nodes in
The sequence of nodes in the
The value of the
For each (variable, type) pair in
In the xml
must not be bound to any namespace URI other than http://www.w3.org/XML/1998/namespace
, and no prefix other than xml
may be bound to this namespace URI.
As described in
The outcome of the
If more than one error is present, or if an error condition comes within the scope of more than one error defined in this specification, then any non-empty subset of these errors may be reported.
During the ()
or data(())
is empty-sequence()
, a
Independently of whether the fn:error()
function must not be evaluated during the
In addition to the errors defined in this
specification, an implementation may raise a
The errors defined in this specification are identified by QNames that have the form err:XPYYnnnn
, where:
err
denotes the namespace for XPath and XQuery errors, http://www.w3.org/2005/xqt-errors
. This binding of the namespace prefix err
is used for convenience in this document, and is not normative.
XP
identifies the error as an XPath error.
YY
denotes the error category, using the following encoding:
ST
denotes a static error.
DY
denotes a dynamic error.
TY
denotes a type error.
nnnn
is a unique numeric code.
The namespace URI for XPath and XQuery errors is not expected to change from one version of XPath to another. However, the contents of this namespace may be extended to include additional error definitions.
The method by which an XPath processor reports error information to the external environment is
An error can be represented by a URI reference that is derived from the error QName as follows: an error with namespace URI NS
LP
NS
#
LP
err:XPST0017
could be represented as http://www.w3.org/2005/xqt-errors#XPST0017
.
Along with a code identifying an error, implementations may wish to return additional information, such
as the location of the error or the processing phase in which it was detected. If an implementation chooses to do so, then the mechanism that
it uses to return this information is
Except as noted in this document, if any operand of an expression
raises a expr1 and expr2
may return the value false
if either operand returns false
,
or may raise a dynamic error if either operand raises a dynamic
error.
If more than one operand of an expression raises an error, the implementation may choose which error is raised by the expression. For example, in this expression:
both the sub-expressions ($x div $y)
and xs:decimal($z)
may
raise an error. The
implementation may choose which error is raised by the "+
"
expression. Once one operand raises an error, the implementation is
not required, but is permitted, to evaluate any other operands.
A dynamic error may be raised by a div
operator raises an error if its operands are xs:decimal
values and its second operand
is equal to zero. Errors raised by built-in functions and operators are defined in
A dynamic error can also be raised explicitly by calling the
fn:error
function, which only raises an error and never
returns a value. This function is defined in app
is bound to a namespace containing application-defined error codes):
Because different implementations may
choose to evaluate or optimize an expression in different ways,
certain aspects of the detection and reporting of
An implementation is always free to evaluate the operands of an operator in any order.
In some cases, a processor can determine the result of an expression without accessing all the data that would be implied by the formal expression semantics. For example, the formal description of $s[1]
should be evaluated by examining all the items in sequence $s
, and selecting all those that satisfy the predicate position()=1
. In practice, many implementations will recognize that they can evaluate this expression by taking the first item in the sequence and then exiting. If $s
is defined by an expression such as //book[author eq 'Berners-Lee']
, then this strategy may avoid a complete scan of a large document and may therefore greatly improve performance. However, a consequence of this strategy is that a dynamic error or type error that would be detected if the expression semantics were followed literally might not be detected at all if the evaluation exits early. In this example, such an error might occur if there is a book
element in the input data with more than one author
subelement.
The extent to which a processor may optimize its access to data, at the cost of not detecting errors, is defined by the following rules.
Consider an expression
There is an exception to this rule: If a processor evaluates an operand $e eq 0
results in a type error if the value of $e
contains two or more items. A processor is not allowed to decide, after evaluating the first item in the value of $e
and finding it equal to zero, that the only possible outcomes are the value true
or a type error caused by the cardinality violation. It must establish that the value of $e
contains no more than one item.
These rules apply to all the operands of an expression considered in combination: thus if an expression has two operands
The rules cascade: if
The effect of these rules is that the processor is free to stop examining further items in a sequence as soon as it can establish that further items would not affect the result except possibly by causing an error. For example, the processor may return true
as the result of the expression S1 = S2
as soon as it finds a pair of equal values from the two sequences.
Another consequence of these rules is that where none of the items in a sequence contributes to the result of an expression, the processor is not obliged to evaluate any part of the sequence. Again, however, the processor cannot dispense with a required cardinality check: if an empty sequence is not permitted in the relevant context, then the processor must ensure that the operand is not an empty sequence.
Examples:
If an implementation can find (for example, by using an index) that at
least one item returned by $expr1
in the following example has the value 47
, it is allowed to
return true
as the result of the some
expression, without searching for
another item returned by $expr1
that would raise an error if it were evaluated.
In the following example, if an implementation can find (for example, by using an index) the
product
element-nodes that have an id
child with the value 47
, it is allowed to return these nodes as the
result of the product
node that
would raise an error because it has an id
child whose value is not an integer.
For a variety of reasons, including optimization, implementations may rewrite expressions into a different form. There are a number of rules that limit the extent of this freedom:
Other than the raising or not raising of errors, the result of evaluating a rewritten expression must conform to the semantics defined in this specification for the original expression.
This allows an implementation to return a result in cases where the original expression would have raised an error, or to raise an error in cases where the original expression would have returned a result. The main cases where this is likely to arise in practice are (a) where a rewrite changes the order of evaluation, such that a subexpression causing an error is evaluated when the expression is written one way and is not evaluated when the expression is written a different way, and (b) where intermediate results of the evaluation cause overflow or other out-of-range conditions.
This rule does not mean that the result of the expression will always
be the same in non-error cases as if it had not been rewritten, because there
are many cases where the result of an expression is to some degree
Conditional and typeswitch expressions
must not raise a dynamic error in
respect of subexpressions occurring in a branch that is not selected,
and must not
return the value delivered by a branch unless that branch is selected.
Thus, the following example must not raise a
dynamic error if the document abc.xml
does not exist:
As stated earlier, an expression
must not be rewritten to dispense with a
required cardinality check: for example, string-length(//title)
must raise an
error if the document contains more than one title element.
Expressions must not be rewritten in such a way as to create or remove static errors. For example, there is a rule that in casting a string to a QName the operand must be a string literal. This rule applies to the original expression and not to any rewritten form of the expression.
Expression rewrite is illustrated by the following examples.
Consider the expression Parts that have exactly one color that is Red are returned. If some part has color Red together with some other color, an error is
raised. The existence of some part that has no color Red but has multiple non-Red
colors does not trigger an error.//part[color eq "Red"]
. An implementation might
choose to rewrite this expression as //part[color = "Red"][color eq
"Red"]
. The implementation might then process the expression as follows:
First process the "=
" predicate by probing an index on parts by color to
quickly find all the parts that have a Red color; then process the "eq
"
predicate by checking each of these parts to make sure it has only a
single color. The result would be as follows:
The expression in the following example cannot raise a casting error if it is evaluated exactly as written (i.e., left to right). Since neither predicate depends on the context position, an implementation might choose to reorder the predicates to achieve better performance (for example, by taking advantage of an index). This reordering could cause the expression to raise an error.
To avoid unexpected errors caused by expression rewrite, tests that are designed to prevent dynamic errors should be expressed using conditional expressions. For example, the above expression can be written as follows:
This section explains some concepts that are important to the processing of XPath expressions.
An ordering called
Document order is a total ordering, although the relative order of some nodes is
Within a tree, document order satisfies the following constraints:
The root node is the first node.
Every node occurs before all of its children and descendants.
Namespace nodes immediately follow the element node with
which they are associated. The relative order of namespace nodes is
stable but
Attribute nodes immediately follow the
The relative order of siblings is the order in which they occur
in the children
property of their parent node.
Children and descendants occur before following siblings.
The relative order of nodes in distinct trees is stable but
The semantics of some
XPath operators depend on a process called fn:data
function
on the sequence, as defined in
The semantics of
fn:data
are repeated here for convenience. The result of
fn:data
is the sequence of atomic values produced by
applying the following rules to each item in the input
sequence:
If the item is an atomic value, it is returned.
If the item is a node,
its
Atomization is used in processing the following types of expressions:
Arithmetic expressions
Comparison expressions
Function calls and returns
Cast expressions
Under certain circumstances (listed below), it is necessary to find
the fn:boolean
function to the value, as
defined in
The dynamic semantics of fn:boolean
are repeated here for convenience:
If its operand is an empty sequence, fn:boolean
returns false
.
If its operand is a sequence whose first item is a node, fn:boolean
returns true
.
If its operand is a xs:boolean
or derived from xs:boolean
, fn:boolean
returns the value of its operand unchanged.
If its operand is a xs:string
, xs:anyURI
, xs:untypedAtomic
, or a type derived from one of these, fn:boolean
returns false
if the operand value has zero length; otherwise it returns true
.
If its operand is a fn:boolean
returns false
if the operand value is NaN
or is numerically equal to zero; otherwise it returns true
.
In all other cases, fn:boolean
raises a type error [err:FORG0006].
The static semantics of fn:boolean
are defined in
The
Logical expressions (and
, or
)
The fn:not
function
Certain types of a[b]
Conditional expressions (if
)
Quantified expressions (some
, every
)
General comparisons, in
The definition of xs:boolean
, for example in a cast
expression or when passing a value to a function whose expected parameter is of type xs:boolean
.
XPath has a set of functions that provide access to
input data. These functions are of particular importance because they provide a way in which an expression can reference a document or a collection of documents. The input functions are described informally here; they are defined in
An expression can access input data either by calling one
of the input functions or by referencing some part of the
The input functions supported by XPath are as follows:
The fn:doc
function takes a string containing a URI. If that URI is associated with a document in fn:doc
returns a document node whose content is the
The fn:collection
function with one argument takes a string containing a URI. If that URI is associated with a collection in fn:collection
returns the data model representation of that collection; otherwise it raises a fn:collection("http://example.org")//customer
identifies all the customer
elements that are
descendants of nodes found in the collection whose URI is
http://example.org
.
The fn:collection
function with zero arguments returns the
The type system of XPath is based on
xs:NOTATION
or xs:anyAtomicType
, in which case its derived
types can be so used). Every schema type is either a
Atomic types represent the intersection between the categories of xs:integer
or my:hatsize
, is both a
The http://www.w3.org/2001/XMLSchema
,
represented in this document by the namespace prefix
xs
. The schema types in this namespace are defined in
xs:untyped
is used as the skip
mode.xs:untyped
.
xs:untypedAtomic
is an atomic type that is used to denote untyped atomic data, such as text that has not been assigned a more specific type.skip
mode is represented in the xs:untypedAtomic
. No predefined schema types are derived from xs:untypedAtomic
.
xs:dayTimeDuration
is derived by restriction from xs:duration
. The lexical representation of xs:dayTimeDuration
is restricted to contain only day, hour, minute, and second
components.
xs:yearMonthDuration
is derived by restriction from xs:duration
. The lexical representation of xs:yearMonthDuration
is
restricted to contain only year and month
components.
xs:anyAtomicType
is an atomic type that includes all atomic values (and no values that
are not atomic). Its base type is
xs:anySimpleType
from which all simple types, including atomic,
list, and union types, are derived. All primitive atomic types, such as
xs:decimal
and xs:string
, have xs:anyAtomicType
as their base type.
xs:anyAtomicType
will not appear as the type of an actual value in an
The relationships among the schema types in the xs
namespace are illustrated in Figure 2. A more complete description of the XPath type hierarchy can be found in
Figure 2: Hierarchy of Schema Types used in XPath
Every node
has a fn:data
function to
the node.fn:string
function to the node.fn:data
and fn:string
can be found in
An implementation may store both the xs:integer
value 30
, its string value might be "30
" or "0030
".
The
As a convenience to the reader, the relationship between
For text and document nodes, the typed value of the node is the same as its
string value, as an instance of the type xs:untypedAtomic
. The
string value of a document node is formed by concatenating the string
values of all its descendant text nodes, in
The typed value of a commentxs:string
.
The typed value of an attribute node with
the xs:anySimpleType
or xs:untypedAtomic
is the same as its
string value, as an instance of xs:untypedAtomic
. The
typed value of an attribute node with any other type annotation is
derived from its string value and type annotation using the lexical-to-value-space mapping defined in
Example: A1 is an attribute
having string value "3.14E-2"
and type annotation
xs:double
. The typed value of A1 is the
xs:double
value whose lexical representation is
3.14E-2
.
Example: A2 is an attribute with type
annotation xs:IDREFS
, which is a list datatype whose item type is the atomic datatype xs:IDREF
. Its string value is
"bar baz faz
". The typed value of A2 is a sequence of
three atomic values ("bar
", "baz
",
"faz
"), each of type xs:IDREF
. The typed
value of a node is never treated as an instance of a named list
type. Instead, if the type annotation of a node is a list type (such
as xs:IDREFS
), its typed value is treated as a sequence
of the atomic type from which it is derived (such as
xs:IDREF
).
For an element node, the
relationship between typed value and string value depends on the
node's
If the type annotation is xs:untyped
or xs:anySimpleType
or
denotes a complex type with mixed content (including xs:anyType
), then the typed value of the
node is equal to its string value, as an instance of
xs:untypedAtomic
. However, if the nilled
property of the node is true
, then its typed value is the empty sequence.
Example: E1 is an element node
having type annotation xs:untyped
and string value
"1999-05-31
". The typed value of E1 is
"1999-05-31
", as an instance of
xs:untypedAtomic
.
Example: E2 is an element node
with the type annotation formula
, which is a complex type
with mixed content. The content of E2 consists of the character
"H
", a child element named subscript
with
string value "2
", and the character "O
". The
typed value of E2 is "H2O
" as an instance of
xs:untypedAtomic
.
If the type
annotation denotes a simple type or a complex type with simple
content, then the typed value of the node is derived from its string
value and its type annotation in a way that is consistent with schema
validation. However, if the nilled
property of the node is true
, then its typed value is the empty sequence.
Example: E3 is an element node with the type
annotation cost
, which is a complex type that has several
attributes and a simple content type of xs:decimal
. The
string value of E3 is "74.95
". The typed value of E3 is
74.95
, as an instance of
xs:decimal
.
Example: E4 is an element node with the
type annotation hatsizelist
, which is a simple type
derived from the atomic type hatsize
, which in turn is
derived from xs:integer
. The string value of E4 is
"7 8 9
". The typed value of E4 is a sequence of three
values (7
, 8
, 9
), each of type
hatsize
.
Example: E5 is an element node with the type annotation my:integer-or-string
which is a union type with member types xs:integer
and xs:string
. The string value of E5 is "47
". The typed value of E5 is 47
as an xs:integer
, since xs:integer
is the member type that validated the content of E5. In general, when the type annotation of a node is a union type, the typed value of the node will be an instance of one of the member types of the union.
If an implementation stores only the string value of a node, and the type annotation of the node is a union type, the implementation must be able to deliver the typed value of the node as an instance of the appropriate member type.
If the type annotation denotes a complex type with empty content, then the typed value of the node is the empty sequence and its string value is the zero-length string.
If the type annotation
denotes a complex type with element-only content, then the typed value
of the node is fn:data
function raises a
Example: E6 is an
element node with the type annotation weather
, which is a
complex type whose content type specifies
element-only
. E6 has two child elements named
temperature
and precipitation
. The typed
value of E6 is fn:data
function
applied to E6 raises an error.
Whenever it is necessary to refer to a type in an XPath expression, the
With the
exception of the special type empty-sequence()
, a item()
, which
permits any kind of item, item types divide into element()
) and xs:integer
).
Item types representing element and attribute nodes may specify the
required element(*, us:address)
denotes any element node whose type
annotation is (or is derived from) the schema type named us:address
.
Here are some examples of
xs:date
refers to the built-in atomic schema type named xs:date
attribute()?
refers to an optional attribute node
element()
refers to any element node
element(po:shipto, po:address)
refers to an element node that has the name po:shipto
and has the type annotation po:address
(or a schema type derived from po:address
)
element(*, po:address)
refers to an element node of any name that has the type annotation po:address
(or a type derived from po:address
)
element(customer)
refers to an element node named customer
with any type annotation
schema-element(customer)
refers to an element node whose name is customer
(or is in the substitution group headed by customer
) and whose type annotation matches the schema type declared for a customer
element in the
node()*
refers to a sequence of zero or more nodes of any kind
item()+
refers to a sequence of one or more nodes or atomic values
instance of
expression returns true
if the false
if
it does not.
QNames appearing in a eq
operator.
The rules for
Some of the rules for
xs:integer
value is used where an xs:decimal
value is expected, the value retains its type as xs:integer
.
The definition of derives-from(
)
, which takes
an actual simple or complex schema type derives-from
is
defined below and is defined formally in
derives-from(
)
returns true
if
There exists some schema type derives-from(
)
and derives-from(
)
are true.
derives-from(
)
returns false
if
No schema type derives-from(
)
and derives-from(
)
are true.
derives-from(
)
raises a
The rules for
The empty-sequence()
matches a value that is the empty sequence.
An
An
An
?
matches zero or one items
*
matches zero or more items
+
matches one or more items
As a consequence of these rules, any *
or ?
matches a
value that is an empty sequence.
An derives-from(
)
is true
. If a QName that is used as an
Example: The
xs:decimal
matches the value
12.34
(a decimal literal). xs:decimal
also
matches a value whose type is shoesize
, if
shoesize
is an atomic type derived by restriction from
xs:decimal
.
The names of non-atomic
types such as xs:IDREFS
are not accepted in this context,
but can often be replaced by an atomic type with an occurrence
indicator, such as
xs:IDREF+
.
item()
matches
any single item.
Example: item()
matches the atomic
value 1
or the element
<a/>
.
node()
matches any node.
text()
matches any text node.
processing-instruction()
matches any processing-instruction node.
processing-instruction(
)
matches any processing-instruction node whose PITarget is equal to fn:normalize-space(N)
. If fn:normalize-space(N)
is not in the lexical space of NCName, a type error is raised
Example:
processing-instruction(xml-stylesheet)
matches any
processing instruction whose PITarget is
xml-stylesheet
.
For backward compatibility with
XPath 1.0, the PITarget of a
processing instruction may also be expressed as a
string literal, as in this example:
processing-instruction("xml-stylesheet")
.
comment()
matches any comment node.
document-node()
matches any document
node.
document-node(
)
matches any document node that contains exactly one element node, optionally accompanied by one or more comment and processing instruction nodes, if
Example:
document-node(element(book))
matches a document node
containing
exactly one element node that is matched by the ElementTest
element(book)
.
An
An
element()
and
element(*)
match any
single element node, regardless of its name or
type annotation.
element(
)
matches any element node whose name is nilled
property.
Example: element(person)
matches any element node whose name is person
.
element(
,
)
matches an element node whose name is derives-from(
)
is true
, where nilled
property of the node is false
.
Example: element(person, surgeon)
matches a
non-nilled element node whose name is person
and whose
type annotation is surgeon
(or is derived from surgeon
).
element(
?)
matches an element node whose name is derives-from(
)
is true
, where nilled
property of the node may be either true
or false
.
Example: element(person, surgeon?)
matches a nilled or non-nilled element node whose name is person
and whose type
annotation is surgeon
(or is derived from surgeon
).
element(*,
)
matches an element
node regardless of its name, if
derives-from(
)
is
true
, where nilled
property of the node is false
.
Example: element(*, surgeon)
matches any non-nilled element node whose type annotation is
surgeon
(or is derived from surgeon
), regardless of its name.
element(*,
?)
matches an element
node regardless of its name, if
derives-from(
)
is
true
, where nilled
property of the node may be either true
or false
.
Example: element(*, surgeon?)
matches any nilled or non-nilled element node whose type annotation is
surgeon
(or is derived from surgeon
), regardless of its name.
A
schema-element(
)
If the
A
The name of the candidate node matches the specified
derives-from(
)
is true
, where
If the element declaration for
nillable
, then the
nilled
property of the candidate node is false
.
Example: The schema-element(customer)
matches a candidate element node if customer
is a top-level element declaration in the customer
or is in a customer
, the type annotation of the candidate node is the same as or derived from the schema type declared for the customer
element, and either the candidate node is not nilled
or customer
is declared to be nillable
.
An
attribute()
and attribute(*)
match any single attribute node,
regardless of its name or type annotation.
attribute(
)
matches any attribute node whose name is
Example: attribute(price)
matches any attribute node whose name is price
.
attribute(
)
matches an attribute node whose name is derives-from(
)
is true
, where
Example: attribute(price, currency)
matches an
attribute node whose name is price
and whose type
annotation is
currency
(or is derived from currency
).
attribute(*,
)
matches an attribute
node regardless of its name, if
derives-from(
)
is
true
, where
Example:
attribute(*, currency)
matches any attribute node whose
type annotation is currency
(or is derived from currency
), regardless of its
name.
A
schema-attribute(
)
If the
A
The name of the candidate node matches the specified
derives-from(
)
is true
, where
Example: The schema-attribute(color)
matches a candidate attribute node if color
is a top-level attribute declaration in the color
, and the type annotation of the candidate node is the same as or derived from the schema type declared for the color
attribute.
Comments may be used to provide informative annotation for
Comments are strings, delimited by the symbols (:
and :)
. Comments may be nested.
A comment may be used anywhere
The following is an example of a comment:
This section discusses each of the basic kinds of expression. Each kind of expression has a name such as PathExpr
, which is introduced on the left side of the grammar production that defines the expression. Since XPath is a composable language, each kind of expression is defined in terms of other expressions whose operators have a higher precedence. In this way, the precedence of operators is represented explicitly in the grammar.
The order in which expressions are discussed in this document does not reflect the order of operator precedence. In general, this document introduces the simplest kinds of expressions first, followed by more complex expressions. For the complete grammar, see Appendix [
The XPath operator that has lowest precedence is the
The symbol
After the comma, the expressions that have next lowest precedence are
The value of a .
" and no e
or E
character is an atomic value of type xs:integer
. The value of a numeric literal containing ".
" but no e
or E
character is an atomic value of type xs:decimal
. The value of a numeric literal containing an e
or E
character is an atomic value of type xs:double
. The value of the numeric literal is determined by casting it to the
appropriate type according to the rules for casting from xs:untypedAtomic
to a numeric type as specified in
The value of a xs:string
and whose value is the string denoted by the characters between the
delimiting apostrophes or quotation marks. If the literal is delimited by apostrophes, two adjacent apostrophes within the literal are interpreted as a single apostrophe. Similarly, if the literal is delimited by quotation marks, two adjacent quotation marks within the literal are interpreted as one quotation mark.
Here are some examples of literal expressions:
"12.5"
denotes the string containing the characters '1', '2', '.', and
'5'.
12
denotes the xs:integer
value twelve.
12.5
denotes the xs:decimal
value twelve and one half.
125E2
denotes the xs:double
value twelve thousand, five hundred.
"He said, ""I don't like it."""
denotes a string containing two quotation marks and one apostrophe.
When XPath expressions are embedded in contexts where quotation marks have special significance, such as inside XML attributes, additional escaping may be needed.
The xs:boolean
values true
and false
can be represented by calls to the fn:true()
and fn:false()
, respectively.
Values of other atomic types can be constructed by
calling the
xs:integer("12")
returns the integer value twelve.
xs:date("2001-08-25")
returns an item whose type is xs:date
and whose value represents the date 25th August 2001.
xs:dayTimeDuration("PT5H")
returns an item whose type is xs:dayTimeDuration
and whose value represents a duration of five hours.
Constructor functions can also be used to create special values that have no literal representation, as in the following examples:xs:float("NaN")
returns the special floating-point value, "Not a Number."xs:double("INF")
returns the special double-precision value, "positive infinity."
It is also possible to construct values of various types by using a cast
expression. For example:
9 cast as
hatsize
returns the atomic value 9
whose type is hatsize
.
Every variable reference must match a name in the The A variable may be bound by an XPath expression. for
expressions (
Every variable binding has a static scope. The scope defines where
references to the variable can validly occur.
It is a
If a variable reference matches two or more variable bindings that are in scope, then the reference is taken as referring to the inner binding, that is, the one whose scope is smaller. At evaluation time, the value of a variable reference is the value of the expression to which the relevant variable is bound. The scope of a variable binding is defined separately for each kind of expression that can bind variables.
Parentheses may be used to enforce a particular evaluation order in
expressions that contain multiple operators. For example, the expression (2 + 4)
* 5
evaluates to thirty, since the parenthesized expression (2 + 4)
is evaluated first and its result is multiplied by five. Without
parentheses, the expression 2 + 4 * 5
evaluates to twenty-two, because the multiplication operator has higher
precedence than the addition operator.
Empty parentheses are used to denote an empty sequence, as
described in
A fn:doc("bib.xml")/books/book[fn:count(./author)>1]
)
or an atomic value (as in the expression (1 to
100)[. mod 5 eq 0]
).
If the
A
If the
A function call is evaluated as follows:
Argument expressions are evaluated, producing argument
values. The order of argument evaluation is
Each argument value is converted by applying the function conversion rules listed below.
The function is evaluated using the converted argument values. The result is either an instance of the function's declared return type or a dynamic error. The
The
If true
and an
argument is not of the expected type, then the
following conversions are
applied sequentially to the argument value V:
If the expected type calls for a single item or optional single item (examples: xs:string
, xs:string?
, xs:untypedAtomic
, xs:untypedAtomic?
, node()
, node()?
, item()
, item()?
), then the value V is effectively replaced by V[1].
If the expected type is
xs:string
or xs:string?
,
then the value V
is effectively
replaced by
fn:string(V)
.
If
the expected type is xs:double
or xs:double?
, then the value V
is effectively replaced by
fn:number(V)
.
If the
expected type is a sequence of an atomic type
(possibly with an occurrence indicator *
,
+
, or ?
), the following
conversions are applied:
Each item in the atomic
sequence that is of type
xs:untypedAtomic
is cast to the expected
atomic type. For xs:untypedAtomic
are cast to xs:double
.
For each
For each item of type xs:anyURI
in the atomic sequence that can be
If, after the
above conversions, the resulting value does not match
the expected type according to the rules for
Since the arguments of a function call are separated by commas, any
argument expression that contains a top-level
my:three-argument-function(1,
2, 3)
denotes a function call with three arguments.
my:two-argument-function((1,
2), 3)
denotes a function call with two arguments, the first of which is a
sequence of two values.
my:two-argument-function(1,
())
denotes a function call with two arguments, the second of which is an
empty sequence.
my:one-argument-function((1, 2,
3))
denotes a function call with one argument that is a sequence of three
values.
my:one-argument-function(( ))
denotes a function call with one argument that is an empty sequence.
my:zero-argument-function( )
denotes a function call with zero arguments.
/
" or
"//
", and optionally beginning with
"/
" or "//
"./
" or "//
" is an abbreviation for
one or more initial steps that are implicitly added to the
beginning of the path expression, as described below.
A
path expression consisting of a single step is evaluated as
described in
A "/
"
at the beginning of a path expression is an abbreviation for
the initial step
(however, if the
"/
" is the entire path expression, the trailing "/
" is omitted from the expansion.) The effect
of this initial step is to begin the path at the root node of
the tree that contains the context node. If the context item
is not a node, a
A "//
" at the beginning of a path expression
is an abbreviation for the initial steps
(however, "//
" by itself is not a valid path expression
The descendants of a node do not include attribute
nodes
Each
non-initial occurrence of "//
" in a path expression is
expanded as described in /
". This sequence
of steps is then evaluated from left to right. Each operation
E1/E2
is evaluated as follows:
Expression E1
is evaluated,
and if the result is not a (possibly empty) sequence of nodes, a E1
then serves in turn to provide an E2
, as
described in E2
are combined as follows:
If every evaluation of E2
returns a (possibly empty) sequence of
nodes, these sequences are combined, and duplicate nodes are eliminated
based on node identity.
If every evaluation of E2
returns a (possibly empty) sequence of
atomic values, these sequences are concatenated
If the multiple evaluations of E2
return at least
one node and at least one atomic value, a
Since each step in a path provides context nodes for the following step, in effect, only the last step in a path is allowed to return a sequence of atomic values.
As an example of a path expression, child::div1/child::para
selects the
para
element children of the div1
element children of the context node, or, in other words, the
para
element grandchildren of the context node
that have div1
parents.
The "/
" character can be used either as a complete path expression or as the beginning of a longer path expression such as "/*
". Also, "*
" is both the multiply operator and a wildcard in path expressions. This can cause
parsing difficulties when "/
" appears on the left hand side of "*
". This is resolved using the /*
" and "/ *
" are valid path
expressions containing wildcards, but "/*5
" and "/ * 5
" raise syntax errors. Parentheses must be used when
"/
" is used on the left hand side of an operator,
as in "(/) * 5
". Similarly, "4 + / * 5
" raises a syntax error, but "4 + (/) * 5
" is a valid expression. The expression "4 + /
" is also valid, because /
does not occur on the left hand side of the operator.
In the
The unabbreviated syntax for an axis step consists of the axis name
and node test separated by a double colon. The result of the step consists of the nodes
reachable from the context node via the specified axis that have the node kind, name,
and/or child::para
selects the para
element children of the context node: child
is the name of the axis, and para
is the name of the element nodes to be selected on this axis. The available axes are described in
XPath defines a full set of
The child
axis
contains the children of the context
node, which are the nodes returned by
the dm:children
accessor
in
Only document
nodes and element
nodes have
children. If the
context node is any
other kind of node,
or if the context
node is an empty
document or element
node, then the child
axis is an empty
sequence. The
children of a
document node or
element node may be
element, processing
instruction,
comment, or text
nodes. Attribute
the descendant
axis is defined as the transitive closure of
the child axis; it contains the descendants
of the context node (the children, the children of the children, and so on)
the parent
axis contains the sequence
returned by the
dm:parent
accessor in
An attribute node may have an element node as its parent, even though the attribute node is not a child of the element node.
the
ancestor
axis is
defined as the transitive
closure of the parent axis; it
contains the ancestors of the
context node (the parent, the
parent of the parent, and so
on)
The ancestor axis includes the root node of the tree in which the context node is found, unless the context node is the root node.
the following-sibling
axis contains the context node's following
siblings, those children of the context
node's parent that occur after the context
node in following-sibling
axis is
empty
the preceding-sibling
axis contains the context node's preceding
siblings, those children of the context
node's parent that occur before the context
node in preceding-sibling
axis is
empty
the following
axis
contains all nodes that are
descendants of the root of the tree in
which the context node is found, are
not descendants of the context node,
and occur after the context node in
the preceding
axis
contains all nodes that are
descendants of the root of the tree in
which the context node is found, are
not ancestors of the context node, and
occur before the context node in
the attribute
axis
contains the attributes of the context node,
which are the nodes returned by the
dm:attributes
accessor in
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-or-self axis will always include the root node
the namespace
axis
contains the namespace nodes of the
context node, which are the nodes
returned by the
dm:namespace-nodes
accessor in
namespace
axis is
deprecated in XPath 2.0. If true
, the namespace
axis must be supported. If false
, then support for the
namespace
axis is
namespace
axis when false
must raise
a fn:in-scope-prefixes
and
fn:namespace-uri-for-prefix
defined in
Axes can be categorized as
The parent
, ancestor
, ancestor-or-self
, preceding
, and preceding-sibling
axes are reverse axes; all other axes are forward axes. The ancestor
, descendant
, following
, preceding
and self
axes partition a document (ignoring attribute
For the attribute axis, the principal node kind is attribute.
For the namespace axis, the principal node kind is namespace.
For all other axes, the principal node kind is element.
eq
operator) to the
child::para
selects the para
element children of
the context node; if the context node has no
para
children, it selects an empty set
of nodes. attribute::abc:href
selects
the attribute of the context node with the QName
abc:href
; if the context node has no
such attribute, it selects an empty set of
nodes.
A QName in a name test is resolved into an
A name test is not satisfied by an element node whose name does not match the
A node test *
is true for any node of the 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
A node test can also
have the form *:NCName
. In this case,
the node test is true for any node of the
node()
matches any
node.
text()
matches
any text
node.
comment()
matches any comment
node.
element()
matches any element
node.
schema-element(person)
matches any element node whose name is
person
(or is in the person
), and whose type
annotation is the same as (or is derived from) the declared type of the person
element in the
element(person)
matches any element node whose name is
person
, regardless of its type annotation.
element(person, surgeon)
matches any non-nilled element node whose name
is person
, and whose type
annotation is
surgeon
or is derived from surgeon
.
element(*,
surgeon)
matches any non-nilled element node whose type
annotation is surgeon
(or is derived from surgeon
), regardless of
its
name.
attribute()
matches any
attribute node.
attribute(price)
matches
any attribute whose name is price
,
regardless of its type annotation.
attribute(*,
xs:decimal)
matches any attribute whose type
annotation is xs:decimal
(or is derived from xs:decimal
), regardless of
its
name.
document-node()
matches any document
node.
document-node(element(book))
matches any document node whose content consists of
a single element node that satisfies the element(book)
, interleaved with zero or more
comments and processing
instructions.
For each item in the input sequence, the predicate expression is evaluated
using an
For each item in the input sequence, the result of the predicate
expression is coerced to an xs:boolean
value, called the true
are retained, and those for which the predicate truth value is false
are discarded.
The predicate truth value is derived by applying the following rules, in order:
If the value of the predicate expression is a true
if the value of the predicate expression is equal (by the eq
operator) to the false
otherwise.
Otherwise, the predicate truth value is the
Here are some examples of
This example selects the second chapter
element that is a child
of the context node:
This example selects all the descendants of the
context node that are elements named
"toy"
and whose color
attribute has the value "red"
:
This example selects all the employee
children of the context node
that have both a secretary
child element and an assistant
child element:
When using preceding::foo[1]
returns the first qualifying foo
element in (preceding::foo)[1]
returns the first qualifying foo
element in (preceding::foo)
to be parsed as a ancestor::*[1]
returns the nearest ancestor element, because the ancestor
axis is a
reverse axis, whereas (ancestor::*)[1]
returns the root element (first ancestor in document order).
The fact that a reverse-axis step assigns context positions in reverse document order for the purpose of evaluating predicates does not alter the fact that the final result of the step is always in document order.
This section provides a number of examples of path expressions in which the
axis is explicitly specified in each
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. Note that no attribute nodes are returned, because attributes are not children.
attribute::name
selects the name
attribute of the context node
attribute::*
selects all the attributes of the context node
parent::node()
selects the parent of the context node. If the context node is an attribute node, this expression returns the element node (if any) to which the attribute node is attached.
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 returns an empty sequence
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 root of the tree that contains the context node, but raises a dynamic error if this root is not a document node
/descendant::para
selects all the para
elements in the same document as the context node
/descendant::list/child::member
selects all
the member
elements that have a list
parent and that are in the same document as the context node
child::para[fn:position() = 1]
selects the first para
child of the context node
child::para[fn:position() = fn:last()]
selects the last para
child of the context node
child::para[fn:position() = fn:last()-1]
selects the last but one para
child of the context node
child::para[fn:position() > 1]
selects all the para
children of the context node other than the first para
child of the context node
following-sibling::chapter[fn:position() = 1]
selects the next chapter
sibling of the context node
preceding-sibling::chapter[fn:position() = 1]
selects the previous chapter
sibling of the context node
/descendant::figure[fn:position() = 42]
selects the forty-second figure
element in the document containing the context node
/child::book/child::chapter[fn:position() = 5]/child::section[fn:position() = 2]
selects the
second section
of the fifth chapter
of the book
whose parent is the document node that contains the context node
child::para[attribute::type eq "warning"]
selects
all para
children of the context node that have a type
attribute with value warning
child::para[attribute::type eq 'warning'][fn:position() = 5]
selects the fifth para
child of the context node that has a type
attribute with value warning
child::para[fn:position() = 5][attribute::type eq "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 whose 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][fn:position() = fn:last()]
selects the
last chapter
or appendix
child of the context node
The abbreviated syntax permits the following abbreviations:
The attribute axis attribute::
can be
abbreviated by @
. For example, a path expression 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
.
If the axis name is omitted from an child
unless the axis step contains an attribute
. For example, the path expression section/para
is an abbreviation for child::section/child::para
, and the path expression section/@id
is an abbreviation for child::section/attribute::id
. Similarly, section/attribute(id)
is an abbreviation for child::section/attribute::attribute(id)
. Note that the latter expression contains both an axis specification and a
Each non-initial occurrence of //
is effectively replaced by /descendant-or-self::node()/
during processing of a path expression. For example, div1//para
is
short for child::div1/descendant-or-self::node()/child::para
and so will select all para
descendants of div1
children.
The path expression //para[1]
does /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 respective parents.
A step consisting
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.
The expression .
, known as a
Here are some examples of path expressions that use the 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[fn:last()]
selects the last para
child of the context node
*/para
selects
all para
grandchildren of the context node
/book/chapter[5]/section[2]
selects the
second section
of the fifth chapter
of the book
whose parent is the document node that contains the context node
chapter//para
selects the para
element descendants of the chapter
element children of the context node
//para
selects all
the para
descendants of the root document node and thus selects all para
elements in the same document as the context node
//@version
selects all the version
attribute nodes that are in the same document as the context node
//list/member
selects all the member
elements in the same document as the context node that have a list
parent
.//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 whose 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
book/(chapter|appendix)/section
selects
every section
element that has a parent that is either a chapter
or an appendix
element, that in turn is a child of a book
element that is a child of the context node.
If E
is any expression that returns a sequence of nodes, then the expression E/.
returns the same nodes in
XPath supports operators to construct, filter, and combine
1
, (2, 3)
, and ( )
into a single sequence results
in the sequence (1, 2, 3)
.
A sequence may contain duplicate atomic values or nodes, but a sequence is never an item in another sequence. When a new sequence is created by concatenating two or more input sequences, the new sequence contains all the items of the input sequences and its length is the sum of the lengths of the input sequences.
In places where the grammar calls for
Here are some examples of expressions that construct sequences:
The result of this expression is a sequence of five integers:
This expression combines four sequences of length one, two, zero, and two, respectively, into a single sequence of length five. The result of this expression is the sequence 10, 1, 2, 3, 4
.
The result of this expression is a sequence containing
all salary
children of the context node followed by all bonus
children.
Assuming that $price
is bound to
the value 10.50
, the result of this expression is the sequence 10.50, 10.50
.
A to
operator is
converted as though it was an argument of a function with the expected
parameter type xs:integer?
.
If either operand is an empty sequence, or if the integer derived from the first operand is greater than the integer derived from the second operand, the result of the range expression is an empty sequence. If the two operands convert to the same integer, the result of the range expression is that integer. Otherwise, the result is a sequence containing the two integer operands and
every integer between the two operands, in increasing order.
This example uses a range expression as one operand in constructing a sequence. It evaluates to the sequence 10, 1, 2, 3, 4
.
This example constructs a sequence of length one containing the single integer 10
.
The result of this example is a sequence of length zero.
This example uses the fn:reverse
function to construct a sequence of six integers in decreasing order. It evaluates to the sequence 15, 14, 13, 12, 11, 10
.
Here are some examples of filter expressions:
Given a sequence of products in a variable, return only those products whose price is greater than 100.
List all the integers from 1 to 100 that are divisible by 5. (See to
operator.)
The result of the following expression is the integer 25:
The following example returns the fifth through ninth items in the sequence bound to variable $orders
.
The following example illustrates the use of a filter expression as a $book
:
The following example also illustrates the use of a filter expression as a tiger
:
XPath provides the following operators for combining sequences of nodes:
The union
and |
operators are equivalent. They take two node sequences as operands and
return a sequence containing all the nodes that occur in either of the
operands.
The intersect
operator takes two node sequences as operands and returns a sequence
containing all the nodes that occur in both operands.
The except
operator takes two node sequences as operands and returns a sequence
containing all the nodes that occur in the first operand but not in the second
operand.
All these operators eliminate duplicate nodes from their result sequences based on node identity.
If an operand
of union
, intersect
, or except
contains an item that is not a node, a
Here are some examples of expressions that combine sequences. Assume the existence of three element nodes that we will refer to by symbolic names A, B, and C. Assume that the variables $seq1
, $seq2
and $seq3
are bound to the following sequences of these nodes:
$seq1
is bound to (A, B)
$seq2
is bound to (A, B)
$seq3
is bound to (B, C)
Then:
$seq1 union $seq2
evaluates to the sequence (A, B).
$seq2 union $seq3
evaluates to the sequence (A, B, C).
$seq1 intersect $seq2
evaluates to the sequence (A, B).
$seq2 intersect $seq3
evaluates to the sequence containing B only.
$seq1 except $seq2
evaluates to the empty sequence.
$seq2 except $seq3
evaluates to the sequence containing A only.
In addition to the sequence operators described here,
XPath provides arithmetic operators for addition, subtraction, multiplication, division, and modulus, in their usual binary and unary forms.
A subtraction operator must be preceded by whitespace if
it could otherwise be interpreted as part of the previous token. For
example, a-b
will be interpreted as a
name, but a - b
and a -b
will be interpreted as arithmetic expressions. (See
The first step in evaluating an arithmetic expression is to evaluate its operands. The order in which the operands are evaluated is
If true
, each operand is evaluated by applying the following steps, in order:
If the atomized operand is an empty sequence, the result of
the arithmetic expression is the xs:double
value NaN
, and the implementation
need not evaluate the other operand or apply the operator. However,
an implementation may choose to evaluate the other operand in order
to determine whether it raises an error.
If the atomized operand is a sequence of length greater than one, any items after the first item in the sequence are discarded.
If the atomized operand is now an instance of type xs:boolean
, xs:string
,
xs:decimal
(including xs:integer
), xs:float
, or xs:untypedAtomic
, then it
is converted to the type xs:double
by applying the fn:number
function. (Note that fn:number
returns the value NaN
if its operand cannot be converted to a number.)
false
, each
If the atomized operand is an empty sequence, the result of the arithmetic expression is an empty sequence, and the implementation need not evaluate the other operand or apply the operator. However, an implementation may choose to evaluate the other operand in order to determine whether it raises an error.
If the atomized operand is a sequence of
length greater than one, a
If the atomized operand is of type xs:untypedAtomic
, it is cast to xs:double
. If
the cast fails, a
After evaluation of the operands, if the types of the operands are a valid combination
for the given arithmetic operator, the operator is applied to the operands,
resulting in an atomic value or a
If the types of the operands, after evaluation, are not a valid combination for the given operator, according to the rules in
XPath supports two division operators named div
and idiv
. Each of these operators accepts two operands of any $arg1 idiv $arg2
is equivalent to ($arg1 div $arg2) cast as xs:integer?
except for error cases.
Here are some examples of arithmetic expressions:
The first expression below returns the xs:decimal
value -1.5
, and the second expression returns the xs:integer
value -1
:
Subtraction of two date values results in a value of type xs:dayTimeDuration
:
This example illustrates the difference between a subtraction operator and a hyphen:
Unary operators have higher precedence than binary operators, subject of course to the use of parentheses. Therefore, the following two examples have different meanings:
Multiple consecutive unary arithmetic operators are permitted by XPath for compatibility with
Comparison expressions allow two values to be compared. XPath provides three kinds of comparison expressions, called value comparisons, general comparisons, and node comparisons.
When an XPath expression is written
within an XML document, the XML escaping rules for special characters
must be followed; thus "<
" must be written as
"<
".
The value comparison operators are eq
, ne
, lt
, le
, gt
, and ge
. Value comparisons are used for comparing single values.
The first step in evaluating a value comparison is to evaluate its operands. The order in which the operands are evaluated is
If the atomized operand is an empty sequence, the result of the value comparison is an empty sequence, and the implementation need not evaluate the other operand or apply the operator. However, an implementation may choose to evaluate the other operand in order to determine whether it raises an error.
If the atomized operand is a sequence of
length greater than one, a
If the atomized operand is of type xs:untypedAtomic
, it is cast to xs:string
.
The purpose of this rule is to make value comparisons transitive. Users should be aware that the general comparison operators have a different rule for casting of xs:untypedAtomic
operands. Users should also be aware that transitivity of value comparisons may be compromised by loss of precision during type conversion (for example, two xs:integer
values that differ slightly may both be considered equal to the same xs:float
value because xs:float
has less precision than xs:integer
).
Next, if possible, the two operands are converted to their least common
type by a combination of hatsize
(derived from xs:integer
) and
shoesize
(derived from xs:float
), their least common type is xs:float
.
Finally, if the types of the operands are a valid combination for the
given operator, the operator is applied to the operands. The combinations of atomic types
that are accepted by the various value comparison operators, and their
respective result types, are listed in
Informally, if both atomized operands consist of exactly one atomic
value, then the result of the comparison is true
if the value of the
first operand is (equal, not equal, less than, less than or equal,
greater than, greater than or equal) to the value of the second
operand; otherwise the result of the comparison is false
.
If the types of the operands, after evaluation, are not a valid combination for the given operator, according to the rules in
Here are some examples of value comparisons:
The following comparison atomizes the node(s) that are returned by the expression $book/author
. The comparison is true only if the result of atomization is the value "Kennedy" as an instance of xs:string
or xs:untypedAtomic
. If the result of atomization is an empty sequence, the result of the comparison is an empty sequence. If the result of atomization is a sequence containing more than one value, a
The following weight
subelement, the value of the predicate is the empty sequence, and the product is not selected. This example assumes that weight
is a validated element with a numeric type.
The following comparison is true if my:hatsize
and my:shoesize
are both user-defined types that are derived by restriction from a primitive
The following comparison is true. The eq
operator compares two QNames by performing codepoint-comparisons of their namespace URIs and their local names, ignoring their namespace prefixes.
The general comparison operators are =
, !=
, <
, <=
, >
, and >=
. General comparisons are existentially quantified comparisons that may be applied to operand sequences of any length. The result of a general comparison that does not raise an error is
always true
or false
.
If true
, a general comparison is evaluated by applying the following rules, in order:
If either operand is a single atomic value that is an instance of
xs:boolean
, then the other operand is converted to xs:boolean
by taking its
If the comparison operator is <
, <=
, >
, or >=
, then each item in both of the
operand sequences is converted to the type xs:double
by applying the
fn:number
function. (Note that fn:number
returns the value NaN
if its operand cannot be converted to a number.)
The result of the comparison is true
if and only if there is a pair of
atomic values, one in the first operand sequence and the other in the second operand sequence, that have the required
false
. The cast
operation called for by these rules is not successful, a dynamic error is raised. [err:FORG0001]
If at least one of the two atomic values is an instance of
a xs:double
by applying
the fn:number
function.
If both atomic values are instances of xs:untypedAtomic
,
then the values are cast to the type xs:string
.
If exactly one of the atomic values is an instance of
xs:untypedAtomic
, and the previous rule does not apply (that
is, the other value is not numeric), then it is cast to a type depending
on the other value's dynamic type T according to the following rules,
in which V denotes the value to be cast:
If T is xs:dayTimeDuration
or is derived from
xs:dayTimeDuration
,
then V is cast to xs:dayTimeDuration
.
If T is xs:yearMonthDuration
or is derived from
xs:yearMonthDuration
,
then V is cast to xs:yearMonthDuration
.
In all other cases, V is cast to the primitive base type of T.
The special treatment of the duration types is required to avoid
errors that may arise when comparing the primitive type
xs:duration
with any duration type.
After performing the conversions described above, the atomic values are
compared using one of the value comparison operators eq
, ne
, lt
, le
, gt
, or
ge
, depending on whether the general comparison operator was =
, !=
, <
, <=
,
>
, or >=
. The values have the required true
.
false
, a
The result of the comparison is true
if and only if there is a pair of
atomic values, one in the first operand sequence and the other in the second operand sequence, that have the required
false
. The cast
operation called for by these rules is not successful, a dynamic error is raised. [err:FORG0001]
If both atomic values are instances of xs:untypedAtomic
,
then the values are cast to the type xs:string
.
If exactly one of the atomic values is an instance of
xs:untypedAtomic
, it is cast to a type depending on
the other value's dynamic type T according to the following rules,
in which V denotes the value to be cast:
If T is a numeric type or is derived from a numeric type,
then V is cast to xs:double
.
If T is xs:dayTimeDuration
or is derived from
xs:dayTimeDuration
,
then V is cast to xs:dayTimeDuration
.
If T is xs:yearMonthDuration
or is derived from
xs:yearMonthDuration
,
then V is cast to xs:yearMonthDuration
.
In all other cases, V is cast to the primitive base type of T.
The special treatment of the duration types is required to avoid
errors that may arise when comparing the primitive type
xs:duration
with any duration type.
After performing the conversions described above, the
atomic values are compared using one of the value comparison
operators eq
, ne
, lt
,
le
, gt
, or ge
, depending on
whether the general comparison operator was =
,
!=
, <
, <=
,
>
, or >=
. The values have the
required true
.
When evaluating a general comparison in which either operand is a sequence of items, an implementation may return true
as soon as it finds an item in the first operand and an item in the second operand that have the required
Here are some examples of general comparisons:
The following comparison is true if the author
subelement of $book1
is "Kennedy" as an instance of xs:string
or xs:untypedAtomic
:
The following example contains three general comparisons. The value of the first two comparisons is true
, and the value of the third comparison is false
. This example illustrates the fact that general comparisons are not transitive.
The following example contains two general comparisons, both of which are true
. This example illustrates the fact that the =
and !=
operators are not inverses of each other.
Suppose that $a
, $b
, and $c
are bound to element nodes with type annotation xs:untypedAtomic
, with 1
", "2
", and "2.0
" respectively. Then ($a, $b) = ($c, 3.0)
returns false
, because $b
and $c
are compared as strings. However, ($a, $b) = ($c, 2.0)
returns true
, because $b
and 2.0
are compared as numbers.
Node comparisons are used to compare two nodes, by their identity or by their
The operands of a node comparison are evaluated in
If either operand is an empty sequence, the result of the comparison is an empty sequence, and the implementation need not evaluate the other operand or apply the operator. However, an implementation may choose to evaluate the other operand in order to determine whether it raises an error.
Each operand must be either a single node or an empty sequence; otherwise
a
A comparison with the is
operator is true
if the two operand nodes have the same identity, and are thus the same node; otherwise it
is false
. See
A comparison with the <<
operator returns true
if the left operand node precedes the right operand node in
false
.
A comparison with the >>
operator returns true
if the left operand node follows the right operand node in
false
.
Here are some examples of node comparisons:
The following comparison is true only if the left and right sides each evaluate to exactly the same single node:
The following comparison is true only if the node identified by the left side occurs before the node identified by the right side in document order:
A true
or false
.
The first step in evaluating a logical expression is to find the
The value of an and-expression is determined by the effective boolean values (EBV's) of its operands, as shown in the following table:
AND: | EBV2 =
true | EBV2 = false | error in EBV2 |
EBV1 =
true | true | false | error |
EBV1
= false | false | false | true , then false ; otherwise either false or error. |
error in EBV1 | error | true , then error; otherwise either false or error. | error |
The value of an or-expression is determined by the effective boolean values (EBV's) of its operands, as shown in the following table:
OR: | EBV2 =
true | EBV2 = false | error in EBV2 |
EBV1 =
true | true | true | true , then true ; otherwise either true or error. |
EBV1 =
false | true | false | error |
error in EBV1 | true , then error; otherwise either true or error. | error | error |
If true
, the order in which the operands of a logical expression are evaluated is effectively prescribed. Specifically, it is defined that when there is no
need to evaluate the second operand in order to determine the result, then
no error can occur as a result of evaluating the second operand.
false
, the
order in which the operands of a logical expression are evaluated is
true
if the first
expression evaluated is true, and it can raise an error if evaluation
of the first expression raises an error. Similarly, an and-expression
can return false
if the first expression evaluated is
false, and it can raise an error if evaluation of the first expression
raises an error. As a result of these rules, a logical expression is
not deterministic in the presence of errors, as illustrated in the examples
below.
Here are some examples of logical expressions:
The following expressions return
true
:
The following
expression may return either false
or raise a false
)
The
following expression may return either true
or raise a
true
)
The
following expression must raise a
In addition to and- and or-expressions, XPath provides a
function named fn:not
that takes a general sequence as
parameter and returns a boolean value. The fn:not
function
is defined in fn:not
function reduces its parameter to an true
if the effective boolean value of its parameter is
false
, and false
if the effective boolean
value of its parameter is true
. If an error is
encountered in finding the effective boolean value of its operand,
fn:not
raises the same error.
XPath provides an iteration facility called a
A for
expression is evaluated as follows:
If the for
expression uses multiple variables, it is first expanded to a set of nested for
expressions, each of which uses only one variable. For example, the expression
for $x in X, $y in Y return $x + $y
is expanded to
for $x in X return
for $y in Y return $x + $y
.
In a single-variable for
expression, the variable is called the in
keyword is called the return
keyword is called the for
expression is obtained by evaluating the return
expression once for each item in the binding sequence, with the range variable bound to that item. The resulting sequences are concatenated (as if by the
The following example illustrates the use of a for
expression in restructuring an input document. The example is based on the following
input:
The following example transforms the input document into a list in
which each author's name appears only once, followed by a list of
titles of books written by that author. This example assumes that the
context item is the bib
element in the input
document.
The result of the above
expression consists of the following sequence of elements. The
titles of books written by a given author are listed after the
name of the author.
The ordering of author
elements in the result is fn:distinct-values
function.
The following example illustrates a for
expression containing more than one variable:
The result of the above expression, expressed as a sequence of numbers, is as follows: 11, 12, 21, 22
The scope of a variable bound in a for
expression comprises all subexpressions of the for
expression
that appear after the variable binding. The scope does not
include the expression to which the variable is bound. The following example illustrates how a variable binding may reference another variable bound earlier in the same for
expression:
The focus for evaluation of the return
clause of a for
expression
is the same as the focus for evaluation of the for
expression itself. The
following example, which attempts to find the total value of a set of
order-items, is therefore incorrect:
Instead, the expression must be written to use the variable bound in the for
clause:
XPath supports a conditional expression based on the keywords if
, then
, and else
.
The expression following the if
keyword is called the then
and else
keywords are called the
The first step in processing a conditional expression is to find
the
The value of a conditional expression is defined as follows: If the
effective boolean value of the test expression is true
, the value of the then-expression is returned. If the
effective boolean value of the test expression is false
,
the value of the else-expression is returned.
Conditional expressions have a special rule for propagating true
, the conditional expression ignores (does not raise) any dynamic errors encountered in the else-expression. In this case, since the else-expression can have no observable effect, it need not be evaluated. Similarly, if the effective value of the test expression is false
, the conditional expression ignores any
Here are some examples of conditional expressions:
In this example, the test expression is a comparison expression:
In this example, the test expression tests for the existence of an attribute
named discounted
, independently of its value:
Quantified expressions support existential and universal quantification. The
value of a quantified expression is always true
or false
.
A some
or every
, followed by one or more in-clauses that are used to bind variables,
followed by the keyword satisfies
and a test expression. Each in-clause associates a variable with an
expression that returns a sequence of items, called the
If the quantifier is some
, the quantified expression is true
if at least one evaluation of the test expression has the true
; otherwise the quantified expression is false
. This rule implies that, if the in-clauses generate zero binding
tuples, the value of the quantified expression is false
.
If the quantifier is every
, the quantified expression is true
if every evaluation of the test expression has the true
; otherwise the quantified expression is false
. This rule implies that, if the in-clauses generate zero binding
tuples, the value of the quantified
expression is true
.
The scope of a variable bound in a quantified expression comprises all subexpressions of the quantified expression that appear after the variable binding. The scope does not include the expression to which the variable is bound.
The order in which test expressions are evaluated for the various binding
tuples is some
, an implementation may
return true
as soon as it finds one binding tuple for which the test expression has
an true
, and it may raise a every
, an implementation may return false
as soon as it finds one binding tuple for which the test expression has
an false
, and it may raise a
Here are some examples of quantified expressions:
This expression is true
if every part
element has a discounted
attribute (regardless of the values of these attributes):
This expression is true
if at least
one employee
element satisfies the given comparison expression:
In the following examples, each quantified expression evaluates its test
expression over nine tuples of variable bindings, formed from the Cartesian
product of the sequences (1, 2, 3)
and (2, 3, 4)
. The expression beginning with some
evaluates to true
, and the expression beginning with every
evaluates to false
.
This quantified expression may either return true
or raise a true
for one variable binding
and raises a
This quantified expression may either return false
or raise a false
for one variable binding and raises a
instance of
, cast
, castable
, and treat
expressions.
The boolean
operator instance of
returns true
if the value of its first operand matches
the false
. For example:
5 instance of xs:integer
This example returns true
because the given value is an instance of the given type.
5 instance of xs:decimal
This example returns true
because the given value is an integer literal, and xs:integer
is derived by restriction from xs:decimal
.
(5, 6) instance of xs:integer+
This example returns true
because the given sequence contains two integers, and is a valid instance of the specified type.
. instance of element()
This example returns true
if the context item is an element node or false
if the context item is defined but is not an element node. If the context item is
Occasionally
it is necessary to convert a value to a specific datatype. For this
purpose, XPath provides a cast
expression that
creates a new value of a specific type based on an existing value. A
cast
expression takes two operands: an xs:NOTATION
or xs:anyAtomicType
?
" denotes that an empty
sequence is permitted. If the target type has no namespace prefix, it
is considered to be in the cast
expression
are as follows:
If the result of atomization is a
sequence of more than one atomic value, a
If the result of atomization is an empty sequence:
If
?
is specified after the target type, the result of the
cast
expression is an empty sequence.
If ?
is not specified after the target type, a
If the result of atomization is a single atomic value, the result
of the cast expression depends on the input type and the target
type. In general, the cast expression attempts to create a new value
of the target type based on the input value. Only certain combinations
of input type and target type are supported. A summary of the rules
are listed below— the normative definition of these rules is
given in
cast
is supported for the combinations of
input type and target type listed in xs:string
can be cast into
the schema type xs:decimal
. For each of these built-in
combinations, the semantics of casting are specified in
If the target type of a
cast
expression is xs:QName
, or is a type
that is derived from xs:QName
or
xs:NOTATION
, and if the base type of the input is not
the same as the base type of the target type, then the input
expression must be a string literal
The reason for this rule is that
construction of an instance of one of these target types from a
string requires knowledge about namespace bindings. If the input
expression is a non-literal string, it might be derived from an
input document whose namespace bindings are different from the
cast
is supported if the input type is a
non-primitive atomic type that is derived by restriction from the
target type. In this case, the input value is mapped into the value
space of the target type, unchanged except for its type. For
example, if shoesize
is derived by restriction from
xs:integer
, a value of type shoesize
can
be cast into the schema type xs:integer
.
cast
is supported if the target type is a
non-primitive atomic type and the input type is
xs:string
or xs:untypedAtomic
. The
input value is first converted to a value in the lexical space of
the target type by applying the whitespace normalization rules
for the target type (as defined in
cast
is supported if the target type is a
non-primitive atomic type that is derived by restriction from the
input type. The input value must satisfy all the facets of the
target type (in the case of the pattern facet, this is checked by
generating a string representation of the input value, using the
rules for casting to xs:string
). The resulting value
is the same as the input value, but with a different
If a primitive type P1 can be cast into a primitive type P2, then any type derived by restriction from P1 can be cast into any type derived by restriction from P2, provided that the facets of the target type are satisfied. First the input value is cast to P1 using rule (b) above. Next, the value of type P1 is cast to the type P2, using rule (a) above. Finally, the value of type P2 is cast to the target type, using rule (d) above.
For any combination of input type and target type that is
not in the above list, a cast
expression raises a
If casting from the input type to the target type is supported but nevertheless it is not possible to cast the input value into the value space of the target type, a "2003-02-31" cast as xs:date
would raise a
XPath
provides an expression that tests whether a given value
is castable into a given target type. The target
type must be an atomic type that is in the xs:NOTATION
or xs:anyAtomicType
?
" denotes that an empty
sequence is permitted.
The expression E castable
as T
returns true
if the result of evaluating E
can
be successfully cast into the target type T
by using a
cast
expression; otherwise it returns
false
. If evaluation of E
fails with a dynamic error, the castable
expression as a whole fails. The castable
expression can be used as a
If the target type of a castable
expression is xs:QName
, or is a type that is derived from xs:QName
or xs:NOTATION
, and the input argument of the expression is of type xs:string
but it is not a literal string, the result of the castable
expression is false
.
For every atomic type in the xs:NOTATION
and xs:anyAtomicType
, which are not instantiable), a
T($arg)
are defined to be equivalent to the expression (($arg) cast as T?)
.
The constructor functions for xs:QName
and for types derived from xs:QName
and xs:NOTATION
require their arguments to be string literals or to have a base type that is the same as the base type of the target type; otherwise a type error cast
expressions for these types, as defined in
The following examples illustrate the use of constructor functions:
This
example is equivalent to ("2000-01-01" cast as
xs:date?)
.
This
example is equivalent to
(($floatvalue * 0.2E-5) cast as xs:decimal?)
.
This example returns a
xs:dayTimeDuration
value equal to 21 days. It is
equivalent to ("P21D" cast as xs:dayTimeDuration?)
.
If
usa:zipcode
is a user-defined atomic type
in the ("12345" cast as
usa:zipcode?)
.
An instance of an atomic type that is not in a namespace can be constructed in either of the following ways:
By using a cast
expression, if the
By using a constructor function, if the
XPath provides an
expression called treat
that can be used to modify the
Like cast
, the treat
expression takes two operands: an expression and a cast
, however, treat
does not change the
treat
is to ensure that an expression has an expected
dynamic type at evaluation time.
The semantics of expr1
treat as
type1
During static analysis:
The
treat
expression is type1
type1
During expression evaluation:
If expr1
type1
treat
expression returns the value of
expr1
expr1
treat
expression ensures that the value of
its expression operand conforms to the expected type at
run-time.
Example:
The
$myaddress
may be element(*, Address)
, a
less specific type than element(*, USAddress)
. However,
at run-time, the value of $myaddress
must match the type
element(*, USAddress)
using rules for
The grammar of XPath uses the same simple Extended Backus-Naur Form
(EBNF) notation as
All named symbols have a name that begins with an uppercase letter.
It adds a notation for referring to productions in external specs.
Comments or extra-grammatical constraints on grammar productions are between '/*' and '*/' symbols.
A 'xgc:' prefix is an extra-grammatical constraint, the
details of which are explained in
A 'ws:' prefix explains the whitespace rules for the production, the
details of which are explained in
A 'gn:'
prefix means a 'Grammar Note', and is meant as a clarification for
parsing rules, and is explained in
The terminal symbols for this grammar include the quoted
strings used in the production rules below, and the terminal
symbols defined in section
The EBNF notation is described in more detail in
To increase readability, the EBNF in the main body of this document omits some of these notational features. This appendix is the normative version of the EBNF.
The following definitions will be helpful in defining precisely this exposition.
matches any
matches any
matches any
matches the sequence of characters that appear inside the double quotes.
matches the sequence of characters that appear inside the single quotes.
matches any string matched by the production defined in the external specification as per the provided reference.
Patterns (including the above constructs) can be combined with grammatical operators to form more complex patterns, matching more complex sets of character strings. In the examples that follow, A and B represent (sub-)patterns.
A
is treated as a unit and may be combined as described in this list.
matches A
or nothing; optional A
.
matches A
followed by B
. This operator has higher precedence than alternation; thus A B | C D
is identical to (A B) | (C D)
.
matches A
or B
but not both.
matches any string that matches A
but does not match B
.
matches one or more occurrences of A
. Concatenation has higher precedence than alternation; thus A+ | B+
is identical to (A+) | (B+)
.
matches zero or more occurrences of A
. Concatenation has higher precedence than alternation; thus A* | B*
is identical to (A*) | (B*)
This section contains constraints on the EBNF productions, which are required to parse legal sentences. The notes below are referenced from the right side of the production, with the notation:
A single slash may appear either as a complete path expression or as the first
part of a path expression in which it is followed by a *
token and keywords like
union
could be either an operator or a / * 5
is
easily taken to be a complete expression, / *
, which has a very different
interpretation (the child nodes of /
).
Therefore to reduce the need for lookahead, if the token immediately following
a slash can form the start of a
A single slash may be used as the left-hand argument of an operator by
parenthesizing it: (/) * 5
. The expression 5 * /
,
on the other hand, is legal without parentheses.
An implementation's choice to support the prefix : localname
is not a valid QName for purposes
of this specification, just as it is not permitted in a XML document.
Also, comments are not permissible on either side of the colon. Also extra-grammatical constraints such as well-formedness constraints must be taken into account.
Unprefixed function names spelled the same way as language
keywords could make the language harder to recognize. For
instance, if(foo)
could be taken either as a
A function named "if" can be called by binding its namespace to a prefix and using the prefixed form: "library:if(foo)" instead of "if(foo)".
As written, the grammar in 4 treat as
item() + - 5
must be interpreted as (4 treat as item()+) - 5
,
taking the '+' as an OccurrenceIndicator and the
'-' as a subtraction operator. To force the interpretation
of "+" as an addition operator (and the corresponding
interpretation of the "-" as a unary minus), parentheses
may be used: the form (4 treat as item()) + -5
surrounds
the
This rule has as a consequence that certain forms which
would otherwise be legal and unambiguous are not
recognized: in "4 treat as item() + 5", the "+" is
taken as an
This section contains general notes on the EBNF productions, which may be helpful in understanding how to interpret and implement the EBNF. These notes are not normative. The notes below are referenced from the right side of the production, with the notation:
Look-ahead is required to distinguish address (: this may be empty :)
may be mistaken for a call to a function named "address" unless this lookahead is employed. Another example is for (: whom the bell :) $tolls in 3 return $tolls
, where the keyword "for" must not be mistaken for a function name.
Comments are allowed everywhere that
A comment can contain nested comments, as long as all "(:" and ":)" patterns are balanced, no matter where they occur within the outer comment.
Lexical analysis may typically handle nested comments by incrementing a counter for each "(:" pattern, and decrementing the counter for each ":)" pattern. The comment does not terminate until the counter is back to zero.
Some illustrative examples:
(: commenting out a (: comment :) may be confusing, but often helpful :)
is a legal Comment, since balanced nesting of comments is allowed.
"this is just a string :)"
is a legal expression. However, (: "this is just a string :)" :)
will cause a syntax error. Likewise, "this is another string (:"
is a legal expression, but (: "this is another string (:" :)
will cause a syntax error. It is a limitation of nested comments that literal content can cause unbalanced nesting of comments.
for (: set up loop :) $i in $x return $i
is syntactically legal, ignoring the comment.
5 instance (: strange place for a comment :) of xs:integer
is also syntactically legal.
The terminal symbols assumed by the grammar above are described in this section.
Quoted strings appearing in production rules are terminal symbols.
Other terminal symbols are defined in
A
When tokenizing, the longest possible match that is valid in the current context is used.
All keywords are case sensitive. Keywords are not reserved—that is, any QName may duplicate a keyword except as noted in
The following symbols are used only in the definition of
terminal symbols; they are not terminal symbols in the
grammar of
XPath 2.0 expressions consist of
Terminal symbols that are not used exclusively in
It is customary to separate consecutive terminal symbols by
The XPath processor must behave as if it normalized all line breaks on input, before parsing. The normalization should be done according to the choice to support either
For
the two-character sequence #xD #xA
any #xD character that is not immediately followed by #xA.
For
the two-character sequence #xD #xA
the two-character sequence #xD #x85
the single character #x85
the single character #x2028
any #xD character that is not immediately followed by #xA or #x85.
foo- foo
results in a syntax error. "foo-" would be recognized as a QName.
foo -foo
is syntactically equivalent to foo - foo
, two QNames separated by a subtraction operator.
foo(: This is a comment :)- foo
is syntactically equivalent to foo - foo
. This is because the comment prevents the two adjacent terminals from being recognized as one.
foo-foo
is syntactically equivalent to single QName. This is because "-" is a valid character in a QName.
When used as an operator after the characters of a name, the "-"
must be separated from the name, e.g. by using whitespace or
parentheses.
10div 3
results in a syntax error.
10 div3
also results in a syntax error.
10div3
also results in a syntax error.
Explicit whitespace notation is specified with the EBNF productions, when it is different from the default rules, using the notation shown below. This notation is not inherited. In other words, if an EBNF rule is marked as /* ws: explicit */, the notation does not automatically apply to all the 'child' EBNF productions of that rule.
/* ws: explicit */ means that the EBNF
notation explicitly notates, with S
or otherwise, where
The following names are not allowed as function names in an unprefixed form because expression syntax takes precedence.
attribute
comment
document-node
element
empty-sequence
if
item
node
processing-instruction
schema-attribute
schema-element
text
typeswitch
Although the keyword typeswitch
is not used in XPath, it is considered a reserved function name for compatibility with XQuery.
The grammar in
# | Operator | Associativity |
---|---|---|
1 | , (comma) | left-to-right |
3 | left-to-right | |
4 | left-to-right | |
5 | left-to-right | |
6 | left-to-right | |
7 | left-to-right | |
8 | left-to-right | |
9 | left-to-right | |
10 | left-to-right | |
11 | left-to-right | |
12 | left-to-right | |
13 | left-to-right | |
14 | left-to-right | |
15 | left-to-right | |
16 | right-to-left | |
17 | left-to-right | |
18 | left-to-right | |
19 | left-to-right |
Parentheses can be used to override the operator precedence in the usual way. Square brackets in an expression such as A[B] serve two roles: they act as an operator causing B to be evaluated once for each item in the value of A, and they act as parentheses enclosing the expression B.
Numeric type promotion:
A value of type xs:float
(or any type
derived by restriction from xs:float
) can be promoted to
the type xs:double
. The result is the
xs:double
value that is the same as the original
value.
A value of type xs:decimal
(or any type derived
by restriction from xs:decimal
) can be promoted to either
of the types xs:float
or xs:double
. The
result of this promotion is created by casting the original value to
the required type. This kind of promotion may cause loss of
precision.
URI type promotion: A value of type xs:anyURI
(or any type derived by restriction from xs:anyURI
) can be promoted to the type xs:string
. The result of this promotion is created by casting the original value to the type xs:string
.
Since xs:anyURI
values can be promoted to xs:string
, functions and operators that compare strings using the xs:anyURI
values using the xs:anyURI
values, or any combination of the two types are consistent and well-defined.
Note that
A function that expects a parameter $p
of type xs:float
can be invoked with a value of type xs:decimal
. This is an example of $p instance of xs:decimal
returns false
.
A function that expects a parameter $p
of type xs:decimal
can be invoked with a value of type xs:integer
. This is an example of $p instance of xs:integer
returns true
.
The operator mapping tables in this section list the
combinations of types for which the various operators of XPath
are defined.
The and
and
or
operators are defined directly in the main body of
this document, and do not occur in the operator mapping tables.
If an operator in the operator mapping tables expects an operand of type
gt
operator may
be applied to two xs:date
operands, returning
xs:boolean
. Therefore, the gt
operator may
also be applied to two (possibly different) subtypes of
xs:date
, also returning xs:boolean
.
xs:integer
, xs:decimal
,
xs:float
, and xs:double
.+
operator might be
thought of as representing the following four operators:
Operator | First operand type | Second operand type | Result type |
+ | xs:integer | xs:integer | xs:integer |
+ | xs:decimal | xs:decimal | xs:decimal |
+ | xs:float | xs:float | xs:float |
+ | xs:double | xs:double | xs:double |
A numeric operator may be validly applied to an operand of type (xs:integer, xs:decimal, xs:float, xs:double)
into which all operands can be converted by hatsize
is derived from xs:integer
and the type shoesize
is derived from xs:float
. Then if the +
operator is invoked with operands of type hatsize
and shoesize
, it returns a result of type xs:float
. Similarly, if +
is invoked with two operands of type hatsize
it returns a result of type xs:integer
.
xs:gYearMonth
, xs:gYear
,
xs:gMonthDay
, xs:gDay
, and
xs:gMonth
.xs:gDay
, the other
operand must be of type xs:gDay
.)
Operator | Type(A) | Type(B) | Function | Result type |
---|---|---|---|---|
A + B | numeric | numeric | op:numeric-add(A, B) | numeric |
A + B | xs:date | xs:yearMonthDuration | op:add-yearMonthDuration-to-date(A, B) | xs:date |
A + B | xs:yearMonthDuration | xs:date | op:add-yearMonthDuration-to-date(B, A) | xs:date |
A + B | xs:date | xs:dayTimeDuration | op:add-dayTimeDuration-to-date(A, B) | xs:date |
A + B | xs:dayTimeDuration | xs:date | op:add-dayTimeDuration-to-date(B, A) | xs:date |
A + B | xs:time | xs:dayTimeDuration | op:add-dayTimeDuration-to-time(A, B) | xs:time |
A + B | xs:dayTimeDuration | xs:time | op:add-dayTimeDuration-to-time(B, A) | xs:time |
A + B | xs:dateTime | xs:yearMonthDuration | op:add-yearMonthDuration-to-dateTime(A, B) | xs:dateTime |
A + B | xs:yearMonthDuration | xs:dateTime | op:add-yearMonthDuration-to-dateTime(B, A) | xs:dateTime |
A + B | xs:dateTime | xs:dayTimeDuration | op:add-dayTimeDuration-to-dateTime(A, B) | xs:dateTime |
A + B | xs:dayTimeDuration | xs:dateTime | op:add-dayTimeDuration-to-dateTime(B, A) | xs:dateTime |
A + B | xs:yearMonthDuration | xs:yearMonthDuration | op:add-yearMonthDurations(A, B) | xs:yearMonthDuration |
A + B | xs:dayTimeDuration | xs:dayTimeDuration | op:add-dayTimeDurations(A, B) | xs:dayTimeDuration |
A - B | numeric | numeric | op:numeric-subtract(A, B) | numeric |
A - B | xs:date | xs:date | op:subtract-dates(A, B) | xs:dayTimeDuration |
A - B | xs:date | xs:yearMonthDuration | op:subtract-yearMonthDuration-from-date(A, B) | xs:date |
A - B | xs:date | xs:dayTimeDuration | op:subtract-dayTimeDuration-from-date(A, B) | xs:date |
A - B | xs:time | xs:time | op:subtract-times(A, B) | xs:dayTimeDuration |
A - B | xs:time | xs:dayTimeDuration | op:subtract-dayTimeDuration-from-time(A, B) | xs:time |
A - B | xs:dateTime | xs:dateTime | op:subtract-dateTimes(A, B) | xs:dayTimeDuration |
A - B | xs:dateTime | xs:yearMonthDuration | op:subtract-yearMonthDuration-from-dateTime(A, B) | xs:dateTime |
A - B | xs:dateTime | xs:dayTimeDuration | op:subtract-dayTimeDuration-from-dateTime(A, B) | xs:dateTime |
A - B | xs:yearMonthDuration | xs:yearMonthDuration | op:subtract-yearMonthDurations(A, B) | xs:yearMonthDuration |
A - B | xs:dayTimeDuration | xs:dayTimeDuration | op:subtract-dayTimeDurations(A, B) | xs:dayTimeDuration |
A * B | numeric | numeric | op:numeric-multiply(A, B) | numeric |
A * B | xs:yearMonthDuration | numeric | op:multiply-yearMonthDuration(A, B) | xs:yearMonthDuration |
A * B | numeric | xs:yearMonthDuration | op:multiply-yearMonthDuration(B, A) | xs:yearMonthDuration |
A * B | xs:dayTimeDuration | numeric | op:multiply-dayTimeDuration(A, B) | xs:dayTimeDuration |
A * B | numeric | xs:dayTimeDuration | op:multiply-dayTimeDuration(B, A) | xs:dayTimeDuration |
A idiv B | numeric | numeric | op:numeric-integer-divide(A, B) | xs:integer |
A div B | numeric | numeric | op:numeric-divide(A, B) | numeric; but xs:decimal if both operands are xs:integer |
A div B | xs:yearMonthDuration | numeric | op:divide-yearMonthDuration(A, B) | xs:yearMonthDuration |
A div B | xs:dayTimeDuration | numeric | op:divide-dayTimeDuration(A, B) | xs:dayTimeDuration |
A div B | xs:yearMonthDuration | xs:yearMonthDuration | op:divide-yearMonthDuration-by-yearMonthDuration (A, B) | xs:decimal |
A div B | xs:dayTimeDuration | xs:dayTimeDuration | op:divide-dayTimeDuration-by-dayTimeDuration (A, B) | xs:decimal |
A mod B | numeric | numeric | op:numeric-mod(A, B) | numeric |
A eq B | numeric | numeric | op:numeric-equal(A, B) | xs:boolean |
A eq B | xs:boolean | xs:boolean | op:boolean-equal(A, B) | xs:boolean |
A eq B | xs:string | xs:string | op:numeric-equal(fn:compare(A, B), 0) | xs:boolean |
A eq B | xs:date | xs:date | op:date-equal(A, B) | xs:boolean |
A eq B | xs:time | xs:time | op:time-equal(A, B) | xs:boolean |
A eq B | xs:dateTime | xs:dateTime | op:dateTime-equal(A, B) | xs:boolean |
A eq B | xs:duration | xs:duration | op:duration-equal(A, B) | xs:boolean |
A eq B | Gregorian | Gregorian | op:gYear-equal(A, B) etc. | xs:boolean |
A eq B | xs:hexBinary | xs:hexBinary | op:hex-binary-equal(A, B) | xs:boolean |
A eq B | xs:base64Binary | xs:base64Binary | op:base64-binary-equal(A, B) | xs:boolean |
A eq B | xs:anyURI | xs:anyURI | op:numeric-equal(fn:compare(A, B), 0) | xs:boolean |
A eq B | xs:QName | xs:QName | op:QName-equal(A, B) | xs:boolean |
A eq B | xs:NOTATION | xs:NOTATION | op:NOTATION-equal(A, B) | xs:boolean |
A ne B | numeric | numeric | fn:not(op:numeric-equal(A, B)) | xs:boolean |
A ne B | xs:boolean | xs:boolean | fn:not(op:boolean-equal(A, B)) | xs:boolean |
A ne B | xs:string | xs:string | fn:not(op:numeric-equal(fn:compare(A, B), 0)) | xs:boolean |
A ne B | xs:date | xs:date | fn:not(op:date-equal(A, B)) | xs:boolean |
A ne B | xs:time | xs:time | fn:not(op:time-equal(A, B)) | xs:boolean |
A ne B | xs:dateTime | xs:dateTime | fn:not(op:dateTime-equal(A, B)) | xs:boolean |
A ne B | xs:duration | xs:duration | fn:not(op:duration-equal(A, B)) | xs:boolean |
A ne B | Gregorian | Gregorian | fn:not(op:gYear-equal(A, B)) etc. | xs:boolean |
A ne B | xs:hexBinary | xs:hexBinary | fn:not(op:hex-binary-equal(A, B)) | xs:boolean |
A ne B | xs:base64Binary | xs:base64Binary | fn:not(op:base64-binary-equal(A, B)) | xs:boolean |
A ne B | xs:anyURI | xs:anyURI | fn:not(op:numeric-equal(fn:compare(A, B), 0)) | xs:boolean |
A ne B | xs:QName | xs:QName | fn:not(op:QName-equal(A, B)) | xs:boolean |
A ne B | xs:NOTATION | xs:NOTATION | fn:not(op:NOTATION-equal(A, B)) | xs:boolean |
A gt B | numeric | numeric | op:numeric-greater-than(A, B) | xs:boolean |
A gt B | xs:boolean | xs:boolean | op:boolean-greater-than(A, B) | xs:boolean |
A gt B | xs:string | xs:string | op:numeric-greater-than(fn:compare(A, B), 0) | xs:boolean |
A gt B | xs:date | xs:date | op:date-greater-than(A, B) | xs:boolean |
A gt B | xs:time | xs:time | op:time-greater-than(A, B) | xs:boolean |
A gt B | xs:dateTime | xs:dateTime | op:dateTime-greater-than(A, B) | xs:boolean |
A gt B | xs:yearMonthDuration | xs:yearMonthDuration | op:yearMonthDuration-greater-than(A, B) | xs:boolean |
A gt B | xs:dayTimeDuration | xs:dayTimeDuration | op:dayTimeDuration-greater-than(A, B) | xs:boolean |
A gt B | xs:anyURI | xs:anyURI | op:numeric-greater-than(fn:compare(A, B), 0) | xs:boolean |
A lt B | numeric | numeric | op:numeric-less-than(A, B) | xs:boolean |
A lt B | xs:boolean | xs:boolean | op:boolean-less-than(A, B) | xs:boolean |
A lt B | xs:string | xs:string | op:numeric-less-than(fn:compare(A, B), 0) | xs:boolean |
A lt B | xs:date | xs:date | op:date-less-than(A, B) | xs:boolean |
A lt B | xs:time | xs:time | op:time-less-than(A, B) | xs:boolean |
A lt B | xs:dateTime | xs:dateTime | op:dateTime-less-than(A, B) | xs:boolean |
A lt B | xs:yearMonthDuration | xs:yearMonthDuration | op:yearMonthDuration-less-than(A, B) | xs:boolean |
A lt B | xs:dayTimeDuration | xs:dayTimeDuration | op:dayTimeDuration-less-than(A, B) | xs:boolean |
A lt B | xs:anyURI | xs:anyURI | op:numeric-less-than(fn:compare(A, B), 0) | xs:boolean |
A ge B | numeric | numeric | op:numeric-greater-than(A, B) or op:numeric-equal(A, B) | xs:boolean |
A ge B | xs:boolean | xs:boolean | fn:not(op:boolean-less-than(A, B)) | xs:boolean |
A ge B | xs:string | xs:string | op:numeric-greater-than(fn:compare(A, B), -1) | xs:boolean |
A ge B | xs:date | xs:date | fn:not(op:date-less-than(A, B)) | xs:boolean |
A ge B | xs:time | xs:time | fn:not(op:time-less-than(A, B)) | xs:boolean |
A ge B | xs:dateTime | xs:dateTime | fn:not(op:dateTime-less-than(A, B)) | xs:boolean |
A ge B | xs:yearMonthDuration | xs:yearMonthDuration | fn:not(op:yearMonthDuration-less-than(A, B)) | xs:boolean |
A ge B | xs:dayTimeDuration | xs:dayTimeDuration | fn:not(op:dayTimeDuration-less-than(A, B)) | xs:boolean |
A ge B | xs:anyURI | xs:anyURI | op:numeric-greater-than(fn:compare(A, B), -1) | xs:boolean |
A le B | numeric | numeric | op:numeric-less-than(A, B) or op:numeric-equal(A, B) | xs:boolean |
A le B | xs:boolean | xs:boolean | fn:not(op:boolean-greater-than(A, B)) | xs:boolean |
A le B | xs:string | xs:string | op:numeric-less-than(fn:compare(A, B), 1) | xs:boolean |
A le B | xs:date | xs:date | fn:not(op:date-greater-than(A, B)) | xs:boolean |
A le B | xs:time | xs:time | fn:not(op:time-greater-than(A, B)) | xs:boolean |
A le B | xs:dateTime | xs:dateTime | fn:not(op:dateTime-greater-than(A, B)) | xs:boolean |
A le B | xs:yearMonthDuration | xs:yearMonthDuration | fn:not(op:yearMonthDuration-greater-than(A, B)) | xs:boolean |
A le B | xs:dayTimeDuration | xs:dayTimeDuration | fn:not(op:dayTimeDuration-greater-than(A, B)) | xs:boolean |
A le B | xs:anyURI | xs:anyURI | op:numeric-less-than(fn:compare(A, B), 1) | xs:boolean |
A is B | node() | node() | op:is-same-node(A, B) | xs:boolean |
A << B | node() | node() | op:node-before(A, B) | xs:boolean |
A >> B | node() | node() | op:node-after(A, B) | xs:boolean |
A union B | node()* | node()* | op:union(A, B) | node()* |
A | B | node()* | node()* | op:union(A, B) | node()* |
A intersect B | node()* | node()* | op:intersect(A, B) | node()* |
A except B | node()* | node()* | op:except(A, B) | node()* |
A to B | xs:integer | xs:integer | op:to(A, B) | xs:integer* |
A , B | item()* | item()* | op:concatenate(A, B) | item()* |
Operator | Operand type | Function | Result type |
---|---|---|---|
+ A | numeric | op:numeric-unary-plus(A) | numeric |
- A | numeric | op:numeric-unary-minus(A) | numeric |
The tables in this section describe the scope (range of applicability) of the various components in the static context and dynamic context.
The following table describes the components of
the
Component | Scope |
---|---|
XPath 1.0 Compatibility Mode | global |
Statically known namespaces | global |
Default element/type namespace | global |
Default function namespace | global |
In-scope schema types | global |
In-scope element declarations | global |
In-scope attribute declarations | global |
In-scope variables | lexical; for-expressions and quantified expressions can bind new variables |
Context item static type | lexical |
Function signatures | global |
Statically known collations | global |
Default collation | global |
Base URI | global |
Statically known documents | global |
Statically known collections | global |
Statically known default collection type | global |
The following table describes how values are assigned to the various components of the
Component | Scope |
---|---|
Context item | dynamic; changes during evaluation of path expressions and predicates |
Context position | dynamic; changes during evaluation of path expressions and predicates |
Context size | dynamic; changes during evaluation of path expressions and predicates |
Variable values | dynamic; for-expressions and quantified expressions can bind new variables |
Current date and time | global; must be initialized by implementation |
Implicit timezone | global; must be initialized by implementation |
Available documents | global; must be initialized by implementation |
Available collections | global; must be initialized by implementation |
Default collection | global; overwriteable by implementation |
The following items in this specification are
The version of Unicode that is used to construct expressions.
The
The
The circumstances in which
The method by which errors are reported to the external processing environment.
Whether the implementation is based on the rules of
Whether the implementation supports the namespace axis.
Any
Additional
XPath is
intended primarily as a component that can be used by other
specifications. Therefore, XPath relies on specifications that use it
(such as
The specification of such a language may describe it as an extension of XPath provided that every expression that conforms to the XPath grammar behaves as described in this specification.
If an implementation does not support the
In some cases, the static typing rules defined in fn:root
). Some
implementations may wish to support more precise
static typing rules.
A conforming implementation that implements the
It is a
It is a
It is a
It is a
During the analysis phase,
it is a ()
or data(())
is empty-sequence()
.
(Not currently used.)
(Not currently used.)
It is a
An implementation must raise a
It is a
It is a
It is a
It is a
(Not currently used.)
It is a treat
expression does not match the treat
expression. This error might also be raised by a path expression beginning with "/
" or "//
" if the context node is not in a tree that is rooted at a document node. This is because a leading "/
" or "//
" in a path expression is an abbreviation for an initial step that includes the clause treat as document-node()
.
It is a
It is a
cast
or castable
expression is xs:NOTATION
or xs:anyAtomicType
.
It is a
(Not currently used.)
This appendix provides a summary of the areas of incompatibility
between XPath 2.0 and
Three separate cases are considered:
Incompatibilities that exist when source documents have no schema,
and when running with XPath 1.0 compatibility mode set to true. This specification
has been designed to reduce the number of incompatibilities in this situation to
an absolute min
Incompatibilities that arise when XPath 1.0 compatibility mode is set to false. In this case, the number of expressions where compatibility is lost is rather greater.
Incompatibilities that arise when the source document is processed using a schema (whether or not XPath 1.0 compatibility mode is set to true). Processing the document with a schema changes the way that the values of nodes are interpreted, and this can cause an XPath expression to return different results.
The list below contains all known areas, within the scope of this specification, where
an XPath 2.0 processor running with compatibility mode set to true will produce different
results from an XPath 1.0 processor evaluating the same expression, assuming that the expression
was valid in XPath 1.0, and that the nodes in the source document have no type annotations other than
xs:untyped
and xs:untypedAtomic
.
Incompatibilities in the behavior of individual functions are not listed here, but are included
in an appendix of
Since both XPath 1.0 and XPath 2.0 leave some aspects of the specification implementation-defined, there may be incompatiblities in the behavior of a particular implementation that are outside the scope of this specification. Equally, some aspects of the behavior of XPath are defined by the host language.
Consecutive comparison operators such as A < B < C
were
supported in XPath 1.0, but are not permitted by the XPath 2.0 grammar. In most cases such
comparisons in XPath 1.0 did not have the intuitive meaning, so it is unlikely that
they have been widely used in practice. If such a construct is found, an XPath 2.0 processor
will report a syntax error, and the construct can be rewritten as (A < B) < C
When converting strings to numbers (either explicitly when using the number
function,
or implicitly say on a function call), certain strings that converted to the special value NaN
under XPath 1.0 will convert to values other than NaN
under XPath 2.0. These include
any number written with a leading +
sign, any number in exponential floating point
notation (for example 1.0e+9
), and the strings INF
and -INF
.
Furthermore, the strings Infinity
and -Infinity
, which were
accepted by XPath 1.0 as representations of the floating-point values positive and negative
infinity, are no longer recognized. They are converted to NaN
when running under
XPath 2.0 with compatibility mode set to true, and cause a dynamic error when
compatibility mode is set to false.
XPath 2.0 does not allow a token starting with a letter to follow immediately after a numeric
literal, without intervening whitespace. For example, 10div 3
was permitted in XPath 1.0,
but in XPath 2.0 must be written as 10 div 3
.
The namespace axis is deprecated in XPath 2.0. Implementations may support the namespace axis for backward compatibility with XPath 1.0, but they are not required to do so. (XSLT 2.0 requires that if XPath backwards compatibility mode is supported, then the namespace axis must also be supported; but other host languages may define the conformance rules differently.)
If one operand in a general comparison is a single atomic value of type
xs:boolean
, the other operand is converted to xs:boolean
when XPath 1.0
compatibility mode is set to true. In XPath 1.0, if neither operand of a
comparison operation using the <, <=, > or >= operator was a node set,
both operands were converted to numbers. The result of the expression
true() > number('0.5')
is therefore true in XPath 1.0, but is
false in XPath 2.0 even when compatibility mode is set to true.
In XPath 2.0, a type error is raised if, for a PITarget specified in a SequenceType of form processing-instruction(N)
,
In XPath 1.0, the expression -x|y
parsed as -(x|y)
,
and returned the negation of the numeric value of the first node in the union of
x
and y
. In XPath 2.0,
this expression parses as (-x)|y
. When XPath 1.0 Compatibility Mode is true,
this will always cause a type error.
The rules for converting numbers to strings have changed. These may affect
the way numbers are displayed in the output of a stylesheet. For numbers whose
absolute value is in the range 1E-6
to 1E+6
, the result
should be the same, but outside this range, scientific format is used for non-integral
xs:float
and xs:double
values.
Even when the setting of the XPath 1.0 compatibility mode is false, many XPath expressions will still produce the same results under XPath 2.0 as under XPath 1.0. The exceptions are described in this section.
In all cases it is assumed that the expression
in question was valid under XPath 1.0, that XPath 1.0 compatibility mode is false, and that all elements
and attributes are annotated with the types xs:untyped
and xs:untypedAtomic
respectively.
In the description below, the terms
When a node-set containing more than one node is supplied as an argument to a
function or operator that expects a single node or value, the XPath 1.0 rule was that all nodes after the first were
discarded. Under XPath 2.0, a type error occurs if there is more than one node.
The XPath 1.0 behavior can always be restored by using the predicate [1]
to
explicitly select the first node in the node-set.
In XPath 1.0, the <
and >
operators, when applied
to two strings, attempted to convert both the strings to numbers and then made a numeric
comparison between the results. In XPath 2.0, these operators perform a string comparison using the
default collating sequence. (If either value is numeric, however, the results are compatible
with XPath 1.0)
When an empty node-set is supplied as an argument to a
function or operator that expects a number, the value is no longer converted
implicitly to NaN.
The XPath 1.0 behavior can always be restored by using the number
function to perform an explicit conversion.
More generally, the supplied arguments to a function or operator are no longer implicitly converted
to the required type, except in the case where the supplied argument is of type xs:untypedAtomic
(which will commonly be the case when a node in a schemaless document is supplied as the argument).
For example, the function call substring-before(10 div 3, ".")
raises a type error under XPath 2.0, because the arguments
to the substring-before
function must be strings rather than numbers. The XPath 1.0 behavior can be
restored by performing an explicit conversion to the required type using a constructor function
or cast.
The rules for comparing a node-set to a boolean have changed. In XPath 1.0,
an expression such as $node-set = true()
was evaluated by converting the
node-set to a boolean and then performing a boolean comparison: so this expression would return true
if $node-set
was non-empty. In XPath 2.0, this expression is handled in
the same way as other comparisons between a sequence and a singleton: it is true
if
$node-set
contains at least one node whose value, after atomization and conversion
to a boolean using the casting rules, is true
.
This means that if $node-set
is empty, the result under XPath 2.0
will be false
regardless of
the value of the boolean operand, and regardless of which operator is used.
If $node-set
is non-empty, then in most cases the comparison with a boolean is
likely to fail, giving a dynamic error. But if a node has the value "0",
"1", "true", or "false", evaluation of the expression may succeed.
Comparisons of a number to a boolean, a number to a string, or a string to a boolean
are not allowed in XPath 2.0: they result in a type error. In XPath 1.0 such comparisons were
allowed, and were handled by converting one of the operands to the type of the other. So for
example in XPath 1.0 4 = true()
was true; 4 = "+4"
was false (because
the string +4
converts to NaN
), and false = "false"
was
false (because the string "false"
converts to the boolean true
).
In XPath 2.0 all these comparisons are type errors.
Additional numeric types have been introduced, with the effect that arithmetic
may now be done as an integer, decimal, or single- or double-precision floating point calculation
where previously it was always performed as double-precision floating point.
The result of the div
operator when dividing two integers is now a value
of type decimal rather than double. The expression 10 div 0
raises an
error rather than returning positive infinity.
The rules for converting numbers to strings have changed. These may affect the
way numbers are displayed in the output of a stylesheet. For numbers whose absolute value
is in the range 1E-6 to 1E+6, the result should be the same, but outside this range,
scientific format is used for non-integral xs:float
and xs:double
values.
The rules for converting strings to numbers have changed. The implicit
conversion that occurs when passing an xs:untypedAtomic
value as an argument to a function
that expects a number no longer converts unrecognized strings to the value NaN
;
instead, it reports a dynamic error. This is in addition to the differences that apply
when backwards compatibility mode is set to true.
Many operations in XPath 2.0 produce an empty sequence as their result
when one of the arguments or operands is an empty sequence. Where the operation
expects a string, an empty sequence is usually considered equivalent to a zero-length string, which
is compatible with the XPath 1.0 behavior. Where the operation expects a number, however, the
result is not the same. For example, if @width
returns an empty sequence, then
in XPath 1.0 the result of @width+1
was NaN
, while with XPath 2.0
it is ()
. This has the effect that a filter expression such as item[@width+1 != 2]
will select items having no width
attribute under XPath 1.0, and will not select them
under XPath 2.0.
The typed value of a comment node, processing instruction node, or namespace node under
XPath 2.0 is of type xs:string
, not xs:untypedAtomic
. This means that no implicit conversions
are applied if the value is used in a context where a number is expected. If a processing-instruction node is used as an operand of
an arithmetic operator, for example, XPath 1.0 would attempt to convert the string value of the node to a number (and deliver
NaN
if unsuccessful), while XPath 2.0 will report a type error.
In XPath 1.0, it was defined that with an expression of the form A and
B
,
B would not be evaluated if A was false. Similarly in the case of A or
B
, B would not be evaluated if A was true. This is no longer
guaranteed with XPath 2.0: the implementation is free to evaluate the two
operands in either order or in parallel. This change has been made to give
more scope for optimization in situations where XPath expressions are
evaluated against large data collections supported by indexes. Implementations
may choose to retain backwards compatibility in this area, but they are not
obliged to do so.
In XPath 1.0, the expression -x|y
parsed as -(x|y)
,
and returned the negation of the numeric value of the first node in the union of
x
and y
. In XPath 2.0,
this expression parses as (-x)|y
. When XPath 1.0 Compatibility Mode is false,
this will cause a type error, except in the situation where x
evaluates
to an empty sequence. In that situation, XPath 2.0 will return the value of
y
, whereas XPath 1.0 returned the negation of the numeric value of y
.
An XPath expression applied to a document that has been processed against a schema will not always give the same results as the same expression applied to the same document in the absence of a schema. Since schema processing had no effect on the result of an XPath 1.0 expression, this may give rise to further incompatibilities. This section gives a few examples of the differences that can arise.
Suppose that the context node is an element node derived from
the following markup: <background color="red green blue"/>
.
In XPath 1.0, the predicate [@color="blue"]
would return false
.
In XPath 2.0, if the color
attribute is defined in a schema
to be of type xs:NMTOKENS
, the same predicate will return true
.
Similarly, consider the expression @birth < @death
applied to the
element <person birth="1901-06-06" death="1991-05-09"/>
. With XPath 1.0, this
expression would return false, because both attributes are converted to numbers, which returns
NaN
in each case. With XPath 2.0, in the presence of a schema that annotates these
attributes as dates, the expression returns true
.
Once schema validation is applied, elements and attributes cannot be used as operands and arguments
of expressions that expect a different data type. For example, it is no longer possible to apply the substring
function to a date to extract the year component, or to a number to extract the integer part. Similarly, if an attribute is
annotated as a boolean then it is not possible to compare it with the strings "true"
or "false"
.
All such operations lead to type errors. The remedy when such errors occur is to introduce an explicit conversion, or
to do the computation in a different way. For example, substring-after(@temperature, "-")
might be
rewritten as abs(@temperature)
.
In the case of an XPath 2.0 implementation that provides the static typing feature, many further type errors will
be reported in respect of expressions that worked under XPath 1.0. For example, an expression such as
round(../@price)
might lead to a static type error because the processor cannot infer statically that
../@price
is guaranteed to be numeric.
Schema validation will in many cases perform whitespace normalization on the contents of elements (depending on their type).
This will change the result of operations such as the string-length
function.
Schema validation augments the data model by adding default values for omitted attributes and empty elements.
This version of the XPath specification
was created by applying the errata from
Erratum | Bugzilla | Category | Description |
|
| editorial | Spelling mistake: minimum |
|
| editorial | Some incompatibilities from XPath 1.0 are undocumented; others are wrongly classified as applying only when compatibility mode is false. |
|
| editorial | For valid syntax, parentheses need to be added to the expansion for leading "/" and leading "//" in a path expression. |
|
| substantive | This erratum adds more details to the rules defining permissible expression rewrites for optimization and other purposes. |
|
| substantive | This erratum clarifies the conditions under which a castable expression may raise an error. |
|
| editorial | Undocumented incompatibility when the operators <, >, <=, or >= are used to compare a number to a boolean. |
|
| substantive | Specifies that an error results if the PITarget specified in a SequenceType of form processing-instruction(PITarget) is not a syntactically valid NCName. |
|
| editorial | Removes references to error code FORG0001 from description of cast expression. Replaces them with a reference to Functions and Operators for normative description of error behavior. |
|
| editorial | Deletes unnecessary reference to RFC2396 from Normative References. This item is never referenced in the normative text. |
|
| substantive | Specifies that general comparisons cast an untyped operand to the primitive base type of the other operand rather than to the most specific type of the other operand. |
|
| editorial | Corrects a list of examples of primitive atomic types. |
|
| substantive | Allows (and encourages) the use of XML 1.0 editions newer than the Third Edition. |
|
| substantive | Specifies conformance criteria for syntax extensions. |
|
| editorial | Defines the meaning of "undefined" for Data Model properties. |
|
| substantive | Clarifications on parsing leading / in XPath expressions. |
|
| substantive | Corrects the description of precedence with respect to parentheses and square brackets. |
|
| substantive | Specifies that leading and trailing whitespace are stripped from a PITarget specified in a SequenceType of form processing-instruction(PITarget) before it is tested to see if it is a syntactically valid NCName. Also makes the description of the error introduced in E12 more precise. If accepted, this supersedes E12. |