[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [Elist Home]
Subject: Tutorial
Attached is a RELAX NG tutorial based on my TREX tutorial. Send comments to the list, or, if they're just typos, to me. JamesTitle: RELAX NG tutorial
Copyright © 2001 OASIS
RELAX NG is a simple schema language for XML, based on RELAX and TREX. A RELAX NG schema specifies a pattern for the structure and content of an XML document. A RELAX NG schema thus identifies a class of XML documents consisting of those documents that match the pattern. A RELAX NG schema is itself an XML document.
ns attributeConsider a simple XML representation of an email address book:
<addressBook>
<card>
<name>John Smith</name>
<email>js@example.com</email>
</card>
<card>
<name>Fred Bloggs</name>
<email>fb@example.net</email>
</card>
</addressBook>
The DTD would be as follows:
<!DOCTYPE addressBook [ <!ELEMENT addressBook (card*)> <!ELEMENT card (name, email)> <!ELEMENT name (#PCDATA)> <!ELEMENT email (#PCDATA)> ]>
A RELAX NG pattern for this could be written as follows:
<element name="addressBook" xmlns="http://relaxng.org/main/ns/0.1">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</zeroOrMore>
</element>
If the addressBook is required to be non-empty, then
we can use oneOrMore instead of
zeroOrMore:
<element name="addressBook" xmlns="http://relaxng.org/main/ns/0.1">
<oneOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</oneOrMore>
</element>
Now let's change it to allow each card to have an
optional note element.
<element name="addressBook" xmlns="http://relaxng.org/main/ns/0.1">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<optional>
<element name="note">
<text/>
</element>
</optional>
</element>
</zeroOrMore>
</element>
Note that the text pattern matches arbitrary text,
including empty text. Note also that whitespace separating tags is
ignored when matching against a pattern.
All the elements specifying the pattern must be namespace qualified by the namespace URI:
http://relaxng.org/main/ns/0.1
The examples above use a default namespace declaration
xmlns="http://relaxng.org/main/ns/0.1" for this. A
namespace prefix is equally acceptable:
<rng:element name="addressBook" xmlns:rng="http://relaxng.org/main/ns/0.1">
<rng:zeroOrMore>
<rng:element name="card">
<rng:element name="name">
<rng:text/>
</rng:element>
<rng:element name="email">
<rng:text/>
</rng:element>
</rng:element>
</rng:zeroOrMore>
</rng:element>
</rng:div>
For the remainder of this document, the default namespace declaration will be left out of examples.
Now suppose we want to allow the name to be broken
down into a givenName and a familyName,
allowing an addressBook like this:
<addressBook>
<card>
<givenName>John</givenName>
<familyName>Smith</familyName>
<email>js@example.com</name>
</card>
<card>
<name>Fred Bloggs</name>
<email>fb@example.net</email>
</card>
</addressBook>
We can use the following pattern:
<element name="addressBook">
<zeroOrMore>
<element name="card">
<choice>
<element name="name">
<text/>
</element>
<group>
<element name="givenName">
<text/>
</element>
<element name="familyName">
<text/>
</element>
</group>
</choice>
<element name="email">
<text/>
</element>
<optional>
<element name="note">
<text/>
</element>
</optional>
</element>
</zeroOrMore>
</element>
This corresponds to the following DTD:
<!DOCTYPE addressBook [ <!ELEMENT addressBook (card*)> <!ELEMENT card ((name | (givenName, familyName)), email, note?)> <!ELEMENT name (#PCDATA)> <!ELEMENT email (#PCDATA)> <!ELEMENT givenName (#PCDATA)> <!ELEMENT familyName (#PCDATA)> <!ELEMENT note (#PCDATA)> ]>
Suppose we want the card element to have attributes
rather than child elements. The DTD might look like this
<!DOCTYPE addressBook [ <!ELEMENT addressBook (card*)> <!ELEMENT card EMPTY> <!ATTLIST card name CDATA #REQUIRED email CDATA #REQUIRED> ]>
Just change each element pattern to an
attribute pattern:
<element name="addressBook">
<zeroOrMore>
<element name="card">
<attribute name="name">
<text/>
</attribute>
<attribute name="email">
<text/>
</attribute>
</element>
</zeroOrMore>
</element>
In XML, the order of attributes is traditionally not significant. RELAX NG follows this tradition. The above pattern would match both
<card name="John Smith" email="js@example.com"/>
and
<card email="js@example.com" name="John Smith"/>
In contrast, the order of elements is significant. The pattern
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
would not match:
<card><email>js@example.com</email><name>John Smith</name></card>
Note that an attribute element by itself indicates a
required attribute, just as an element element by itself
indicates a required element. To specify an optional attribute, use
optional just as with element:
<element name="addressBook">
<zeroOrMore>
<element name="card">
<attribute name="name">
<text/>
</attribute>
<attribute name="email">
<text/>
</attribute>
<optional>
<attribute name="note">
<text/>
</attribute>
</optional>
</element>
</zeroOrMore>
</element>
The group and choice patterns can be
applied to attribute elements in the same way they are
applied to element patterns. For example, if we wanted
to allow either a name attribute or both a
givenName and a familyName attribute, we can
specify this in the same way that we would if we were using
elements:
<element name="addressBook">
<zeroOrMore>
<element name="card">
<choice>
<attribute name="name">
<text/>
</attribute>
<group>
<attribute name="givenName">
<text/>
</attribute>
<attribute name="familyName">
<text/>
</attribute>
</group>
</choice>
<attribute name="email">
<text/>
</attribute>
</element>
</zeroOrMore>
</element>
There are no restrictions on how element elements and
attribute elements can be combined. For example, the
following pattern would allow a choice of elements and attributes
independently for both the name and the
email part of a card:
<element name="addressBook">
<zeroOrMore>
<element name="card">
<choice>
<element name="name">
<text/>
</element>
<attribute name="name">
<text/>
</attribute>
</choice>
<choice>
<element name="email">
<text/>
</element>
<attribute name="email">
<text/>
</attribute>
</choice>
</element>
</zeroOrMore>
</element>
As usual, the relative order of elements is significant, but the relative order of attributes is not. Thus the above would match any of:
<card name="John Smith" email="js@example.com"/> <card email="js@example.com" name="John Smith"/> <card email="js@example.com"><name>John Smith</name></card> <card name="John Smith"><email>js@example.com</email></card> <card><name>John Smith</name><email>js@example.com</email></card>
However, it would not match
<card><email>js@example.com</email><name>John Smith</name></card>
because the pattern for card requires any
email child element to follow any name child
element.
There is one difference between attribute and
element patterns: <text/>
is the default for the content of an attribute pattern,
whereas an element pattern is not allowed to be
empty. For example,
<attribute name="email"/>
is short for
<attribute name="email"> <text/> </attribute>
It might seem natural that
<element name="x"/>
matched an x element with no attributes and no
content. However, this would make the meaning of empty content
inconsistent between the element pattern and the
attribute pattern, so RELAX NG does not allow the
element pattern to be empty. A pattern that matches an
element with no attributes and no children must use
<empty/> explicitly:
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<optional>
<element name="prefersHTML">
<empty/>
</element>
</optional>
</element>
</zeroOrMore>
</element>
For a non-trivial RELAX NG pattern, it is often convenient to be able to give names to parts of the pattern. Instead of
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</zeroOrMore>
</element>
we can write
<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<ref name="cardContent"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="cardContent">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</define>
</grammar>
A grammar element has a single start
child element, and zero or more define child elements.
The start and define elements contain
patterns. These patterns can contain ref elements that
refer to patterns defined by any of the define elements
in that grammar element. A grammar pattern
is matched by matching the pattern contained in the start
element.
We can use the grammar element to write patterns in a
style similar to DTDs:
<grammar>
<start>
<ref name="AddressBook"/>
</start>
<define name="AddressBook">
<element name="addressBook">
<zeroOrMore>
<ref name="Card"/>
</zeroOrMore>
</element>
</define>
<define name="Card">
<element name="card">
<ref name="Name"/>
<ref name="Email"/>
</element>
</define>
<define name="Name">
<element name="name">
<text/>
</element>
</define>
<define name="Email">
<element name="email">
<text/>
</element>
</define>
</grammar>
Recursive references are allowed. For example
<define name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
<element name="italic">
<ref name="inline"/>
</element>
<element name="span">
<optional>
<attribute name="style"/>
</optional>
<ref name="inline"/>
</element>
</choice>
</zeroOrMore>
</define>
However, recursive references must be within an
element. Thus, the following is not
allowed:
<define name="inline">
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
<element name="italic">
<ref name="inline"/>
</element>
<element name="span">
<optional>
<attribute name="style"/>
</optional>
<ref name="inline"/>
</element>
</choice>
<optional>
<ref name="inline"/>
</optional>
</define>
A start element may also have a name
attribute. This is a shorthand for a define with that
name together with a start element
referencing that definition. For example
<grammar>
<start name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
</choice>
</zeroOrMore>
</start>
</grammar>
is short for
<grammar>
<start>
<ref name="inline"/>
</start>
<define name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
</choice>
</zeroOrMore>
</define>
</grammar>
RELAX NG does not have any system of datatypes built in. Rather it expects to partner with a datatyping vocabulary, such as Part 2 of the W3C's XML Schema language. RELAX NG implementations may differ in the datatyping vocabularies they support. You must pick a datatyping vocabulary that is supported by the implementation you plan to use.
The data pattern matches a string that represents a
value of a named datatype. The type attribute contains
the qualified name of the datatype. For example, if a RELAX NG
implementation supported the built-in datatypes of the W3C's XML
Schema Language, you could use:
<element name="number" xmlns:xsd="http://www.w3.org/2001/XMLSchema-datatypes"> <data type="xsd:integer"/> </element>
The data can use an ns attribute to
specify explicitly the namespace URI of the datatype, instead of using
a prefix within the value of the type attribute.
<element name="number"> <data type="integer" ns="http://www.w3.org/2001/XMLSchema-datatypes"/> </element>
If the children of an element or an attribute match a
data pattern, then complete content of the element or
attribute must match that data pattern. It is not
permitted to have a pattern which allows part of the content to match
a data pattern, and another part to match another
pattern. For example, the following pattern is not
allowed:
<element name="bad">
<data type="xsd:int"/>
<element name="note">
<text/>
</element>
</element>
However, this would be fine:
<element name="ok">
<data type="xsd:int"/>
<attribute name="note">
<text/>
</attribute>
</element>
Note that this restriction does not apply to the
text pattern.
Datatypes may have parameters. For example, a string datatype may
have a parameter controlling the length of the string. The parameters
applicable to any particular datatype are determined by the datatyping
vocabulary. Parameters are specified by adding one or more
param elements as children of the data
element. For example, the following constrains the email
element to contain a string at most 127 characters long:
<element name="email">
<data type="xsd:string">
<param name="maxLength">127</param>
</data>
</element>
Many markup vocabularies have attributes whose value is constrained
to be one of set of specified values. The value pattern
matches a string that has a specified value. For example,
<element name="card">
<attribute name="name"/>
<attribute name="email"/>
<attribute name="preferredFormat">
<choice>
<value>html</value>
<value>text</value>
</choice>
</attribute>
</element>
allows the preferredFormat attribute to have the value
html or text. This corresponds to the
DTD
<!DOCTYPE card [ <!ELEMENT card EMPTY> <!ATTLIST card name CDATA #REQUIRED email CDATA #REQUIRED preferredFormat (html|text) #REQUIRED> ]>
The value pattern is not restricted to attribute
values. For example, the following is allowed:
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<element name="preferredFormat">
<choice>
<value>html</value>
<value>text</value>
</choice>
</element>
</element>
The prohibition against a data pattern's matching
only part of the content of an element also applies to
value patterns.
By default, the value pattern will consider the string
in the pattern to match the string in the document if the two strings
are the same after the whitespace in both strings is normalized.
Whitespace normalization strips leading and trailing white-space
characters, and collapses sequences of one or more white-space
characters to a single space character. This corresponds to the
behaviour of an XML parser for an attribute that is declared as other
than CDATA. Thus the above pattern will match any of
<card name="John Smith" email="js@example.com" preferredFormat="html"/>
<card name="John Smith" email="js@example.com" prefersFormat=" html "/>
The way that the value pattern compares the pattern
string with the document string can be controlled by specifying a
type attribute specifying a datatype. The
type attribute contains a qualified name identifying the
datatype. The pattern string matches the document string if they both
represent the same value of the specified datatype. Thus, whereas the
data pattern matches an arbitrary value of a datatype,
the value pattern matches a specific value of a
datatype.
RELAX NG provides two builtin datatypes that are useful with the
value pattern. These datatypes are specified by using an
unprefixed name as the value of the type attribute. The
two builtin datatypes are string and token.
The builtin datatype token corresponds to the default
comparison behavior of the value pattern. The builtin
datatype string compares strings without any
normalization (other than that performed by XML). For example,
<element name="card">
<attribute name="name"/>
<attribute name="email"/>
<attribute name="preferredFormat">
<choice>
<value type="string">html</value>
<value type="string">text</value>
</choice>
</attribute>
</element>
will not match
<card name="John Smith" email="js@example.com" prefersHTML=" html "/>
The oneOrMoreTokens and zeroOrMoreTokens
patterns match a whitespace-separated sequence of tokens; they each
contain a pattern that the individual tokens must match. For example,
the extension-element-prefixes attribute in XSLT contains
a whitespace-separated list of zero or more namespace prefixes, where
each namespace prefix is either an NCName or the special value
#default:
<attribute name="extension-element-prefixes">
<zeroOrMoreTokens>
<choice>
<data type="xsd:NCName"/>
<value>#default</value>
</choice>
</zeroOrMoreTokens>
</attribute>
The oneOrMoreTokens and zeroOrMoreTokens
patterns must not contain element or
attribute patterns.
The interleave pattern allows child elements to occur
in any order. For example, the following would allow the
card element to contain the name and
email elements in any order:
<element name="addressBook">
<zeroOrMore>
<element name="card">
<interleave>
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</interleave>
</element>
</zeroOrMore>
</element>
The pattern is called interleave because of how it
works with patterns that match more than one element. Suppose we want
to write a pattern for the HTML head element which
requires exactly one title element, at most one
base element and zero or more style,
script, link and meta elements
and suppose we are writing a grammar pattern that has one
definition for each element. Then we could define the pattern for
head as follows:
<define name="head">
<element name="head">
<interleave>
<ref name="title"/>
<optional>
<ref name="base"/>
</optional>
<zeroOrMore>
<ref name="style"/>
</zeroOrMore>
<zeroOrMore>
<ref name="script"/>
</zeroOrMore>
<zeroOrMore>
<ref name="link"/>
</zeroOrMore>
<zeroOrMore>
<ref name="meta"/>
</zeroOrMore>
</interleave>
</element>
</define>
Suppose we had a head element that contained a
meta element, followed by a title element,
followed by a meta element. This would match the pattern
because it is an interleaving of a sequence of two meta
elements, which match the child pattern
<zeroOrMore>
<ref name="meta"/>
</zeroOrMore>
and a sequence of one title element, which matches
the child pattern
<ref name="title"/>
The semantics of the interleave pattern are that a
sequence of elements matches an interleave pattern if it
is an interleaving of sequences that match the child patterns of the
interleave pattern. Note that this is different from the
& connector in SGML: A* & B matches
the sequence of elements A A B or the sequence of
elements B A A but not the sequence of elements A B
A.
One special case of interleave is very common:
interleaving <text/> with a pattern
p represents a pattern that matches what p
matches but also allows characters to occur as children. The
mixed element is a shorthand for this.
<mixed> p </mixed>
is short for
<interleave> <text/> p </interleave>
The include element can be used to allow a pattern to
be divided amongst multiple files. The include element
has a required href attribute that specifies the URL of a
file to be included in place of the include element.
The include element can be used as a pattern. In this
case, it will match if the pattern contained in the specified URL
matches. Suppose for example, you have a RELAX NG pattern that matches
HTML inline content stored in inline.rng:
<grammar>
<start name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="code">
<ref name="inline"/>
</element>
<element name="em">
<ref name="inline"/>
</element>
<!-- etc -->
</choice>
</zeroOrMore>
</start>
</grammar>
Then we could allow the note element to contain
inline HTML markup by using include as follows:
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<optional>
<element name="note">
<include href="inline.rng"/>
</element>
</optional>
</element>
</zeroOrMore>
</element>
For another example, suppose you have two RELAX NG patterns stored in
files pattern1.rng and pattern2.rng. Then
the following is a pattern that which match anything matched
by one of those patterns:
<choice> <include href="pattern1.rng"/> <include href="pattern2.rng"/> </choice>
The include element is also allowed as a child of a
grammar pattern. In this case the specified URL must
contain a grammar pattern, and the included
grammar will be merged with the including
grammar.
Normally, duplicate definitions (two definitions with the same
name) result in an error. However, define elements may
be put inside the include element to indicate that they
are to replace definitions in the included grammar
pattern.
Suppose the file addressBook.rng contains the
following grammar pattern:
<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<ref name="card.local"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="card.local">
<empty/>
</define>
</grammar>
Another pattern could customize addressBook.rng as
follows:
<grammar>
<include href="addressBook.rng">
<define name="card.local">
<optional>
<element name="note">
<text/>
</element>
</optional>
</define>
</include>
</grammar>
This would be equivalent to:
<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<ref name="card.local"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="card.local">
<optional>
<element name="note">
<text/>
</element>
</optional>
</define>
</grammar>
which is equivalent to
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<optional>
<element name="note">
<text/>
</element>
</optional>
</element>
</zeroOrMore>
</element>
It is also possible to combine together duplicate definitions from
separate files by adding a combine attribute to the
define elements. The combine attribute
specifies how the definitions should be combined; it may have the
value choice or interleave. For example, we
could have written our customization as:
<grammar>
<include href="addressBook.rng"/>
<define name="card.local" combine="choice">
<!-- no optional element needed this time -->
<element name="note">
<text/>
</element>
</define>
</grammar>
This would be equivalent to:
<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<ref name="card.local"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="card.local">
<choice>
<empty/>
<element name="note">
<text/>
</element>
</choice>
</define>
</grammar>
This has the same meaning as before, since an optional pattern is equivalent to a choice between the pattern and empty.
We could also have used combine="interleave" here:
<grammar>
<include href="addressBook.rng"/>
<define name="card.local" combine="interleave">
<optional>
<element name="note">
<text/>
</element>
</optional>
</define>
</grammar>
This would be equivalent to:
<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<ref name="card.local"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="card.local">
<interleave>
<empty/>
<optional>
<element name="note">
<text/>
</element>
</optional>
</interleave>
</define>
</grammar>
This has the same meaning as before, since adding an
empty pattern to the content of a interleave
pattern does not make any difference to what the
interleave pattern matches.
@@@ Add example of combine="interleave" with attributes.
The notAllowed pattern never matches anything. Just
as adding empty to a group makes no
difference, so adding notAllowed to a choice
makes no difference. It is typically used in a definition that is
referenced in a choice element to allow an including
pattern to specify additional choices. For example, suppose a RELAX NG
pattern inline.rng provides a pattern for inline
content, which allows bold and italic
elements arbitrarily nested:
<grammar>
<start name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
<element name="italic">
<ref name="inline"/>
</element>
<ref name="local.inline"/>
</choice>
</zeroOrMore>
</start>
<define name="local.inline">
<notAllowed/>
</define>
</grammar>
Another RELAX NG pattern could use inline.rng and add
code and em to the set of inline elements as
follows:
<grammar>
<include href="inline.rng">
<define name="local.inline">
<choice>
<element name="code">
<ref name="inline">
</element>
<element name="em">
<ref name="inline">
</element>
</choice>
</define>
</include>
<start>
<element name="doc">
<zeroOrMore>
<element name="p">
<ref name="inline"/>
</element>
</zeroOrMore>
</element>
</start>
</grammar>
We could instead have used combine="choice". In this
case, inline.rng would need to separate out the choices
as a separate definition:
<grammar>
<start name="inline">
<zeroOrMore>
<ref name="inline.class"/>
</zeroOrMore>
</start>
<define name="inline.class">
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
<element name="italic">
<ref name="inline"/>
</element>
</choice>
</define>
</grammar>
and the customization would add to those choices:
<grammar>
<include href="inline.rng"/>
<start>
<element name="doc">
<zeroOrMore>
<element name="p">
<ref name="inline"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="inline.class" combine="choice">
<choice>
<element name="code">
<ref name="inline">
</element>
<element name="em">
<ref name="inline">
</element>
</choice>
</define>
</grammar>
RELAX NG is namespace-aware. Thus, it considers an element or attribute to have both a local name and a namespace URI which together constitute the name of that element or attribute.
ns attribute
The element pattern uses an ns attribute
to specify the namespace URI of the elements that it matches. For
example
<element name="foo" ns="http://www.example.com"> <empty/> </element>
would match any of
<foo xmlns="http://www.example.com"/>
<e:foo xmlns:e="http://www.example.com"/>
<example:foo xmlns:example="http://www.example.com"/>
but not any of
<foo/>
<e:foo xmlns:e="http://WWW.EXAMPLE.COM"/>
<example:foo xmlns:example="http://www.example.net"/>
A value of an empty string for the ns attribute
indicates a null or absent namespace URI (just as with the
xmlns attribute). Thus, the pattern
<element name="foo" ns=""> <empty/> </element>
matches any of
<foo xmlns=""/>
<foo/>
but not any of
<foo xmlns="http://www.example.com"/>
<e:foo xmlns:e="http://www.example.com"/>
It is tedious and error-prone to specify the ns
attribute on every element, so RELAX NG allows it to be
defaulted. If an element pattern does not specify an
ns attribute, then it defaults to the value of the
ns attribute of the nearest ancestor that has an
ns attribute, or the empty string if there is no such
ancestor. Thus
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</zeroOrMore>
</element>
is equivalent to
<element name="addressBook" ns="">
<zeroOrMore>
<element name="card" ns="">
<element name="name" ns="">
<text/>
</element>
<element name="email" ns="">
<text/>
</element>
</element>
</zeroOrMore>
</element>
and
<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</zeroOrMore>
</element>
is equivalent to
<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card" ns="http://www.example.com">
<element name="name" ns="http://www.example.com">
<text/>
</element>
<element name="email" ns="http://www.example.com">
<text/>
</element>
</element>
</zeroOrMore>
</element>
The attribute pattern also takes an ns
attribute. However, there is a difference in how it defaults. This
is because of the fact that the XML Namespaces Recommendation does not
apply the default namespace to attributes. If an ns
attribute is not specified on the attribute pattern, then
it defaults to the empty string. Thus
<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card">
<attribute name="name"/>
<attribute name="email"/>
</element>
</zeroOrMore>
</element>
is equivalent to
<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card" ns="http://www.example.com">
<attribute name="name" ns=""/>
<attribute name="email" ns=""/>
</element>
</zeroOrMore>
</element>
and so will match
<addressBook xmlns="http://www.example.com"> <card name="John Smith" email="js@example.com"/> </addressBook>
or
<example:addressBook xmlns:example="http://www.example.com"> <example:card name="John Smith" email="js@example.com"/> </example:addressBook>
but not
<example:addressBook xmlns:example="http://www.example.com"> <example:card example:name="John Smith" example:email="js@example.com"/> </example:addressBook>
To match this last example, the attribute patterns
must specify global="true":
<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card">
<attribute name="name" global="true"/>
<attribute name="email" global="true"/>
</element>
</zeroOrMore>
</element>
This is equivalent to:
<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card" ns="http://www.example.com">
<attribute name="name" ns="http://www.example.com"/>
<attribute name="email" ns="http://www.example.com"/>
</element>
</zeroOrMore>
</element>
Thus, specifying global="true" on an
attribute pattern makes the ns attribute
default in the same way that it does on an element
pattern.
The ns attribute is allowed on any element in a RELAX NG
pattern. The global attribute is allowed only on an
attribute pattern.
When a pattern matches elements and attributes from multiple
namespaces, using the ns attribute would require
repeating namespace URIs in different places in the pattern. This is
error-prone and hard to maintain, so RELAX NG also allows the
element and attribute patterns to use a
prefix in the value of the name attribute to specify the
namespace URI. In this case, the prefix specifies the namespace URI to
which that prefix is bound by the namespace declarations in scope on
the element or attribute pattern. Thus
<element name="e:addressBook" xmlns:e="http://www.example.com">
<zeroOrMore>
<element name="e:card">
<element name="e:name">
<text/>
</element>
<element name="e:email">
<text/>
</element>
</element>
</zeroOrMore>
</element>
is equivalent to
<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card" ns="http://www.example.com">
<element name="name" ns="http://www.example.com">
<text/>
</element>
<element name="email" ns="http://www.example.com">
<text/>
</element>
</element>
</zeroOrMore>
</element>
If a prefix is specified in the value of the name
attribute of an element or attribute
pattern, then that prefix determines the namespace URI of the elements
or attributes that will be matched by that pattern, regardless of
the value of any ns attribute.
Note that the XML default namespace (as specified by the
xmlns attribute) is not used in determining the namespace
URI of elements and attributes that element and
attribute patterns match.
Normally, the name of the element to be matched by an
element element is specified by a name
attribute. An element element can instead start with an
element specifying a name-class. In this case, the
element pattern will only match an element if the name of
the element is a member of the name-class. The simplest name-class is
anyName, which any name at all is a member of, regardless
of its local name and its namespace URI. For example, the following
pattern matches any well-formed XML document:
<grammar>
<start name="anyElement">
<element>
<anyName/>
<zeroOrMore>
<choice>
<attribute>
<anyName/>
</attribute>
<text/>
<ref name="anyElement"/>
</choice>
</zeroOrMore>
</element>
</start>
</grammar>
The nsName name-class contains any name with the
namespace URI specified by the ns attribute, which
defaults in the same way as the ns attribute on the
element pattern.
The choice name-class matches any name that is a
member of any of its child name-classes.
The not name-classes contains any name that is not
a member of the child name-class.
For example
<element name="card" ns="http://www.example.com">
<zeroOrMore>
<attribute>
<not>
<choice>
<nsName/>
<nsName ns=""/>
</choice>
</not>
</attribute>
</zeroOrMore>
<text/>
</element>
would allow the card element to have any number of
namespace-qualified attributes provided that they were qualified with
namespace other than that of the card element.
Note that an attribute pattern matches a single
attribute even if it has a name-class that contains multiple names.
To match zero or more attributes, the zeroOrMore element
must be used.
The difference name-class contains any name that is a
member of the first child name-class, but not a member of any of the
following name-classes. The not name-class is, in
fact, a shorthand for difference:
<not> name-class </not>
is short for
<difference> <anyName/> name-class </difference>
The name name-class contains a single name. The
content of the name element specifies the name in the
same way as the name attribute of the
element pattern. The ns attribute specifies
the namespace URI in the same way as the element
pattern.
Some schema languages have a concept of lax validation,
where an element or attribute is validated against a definition only
if there is one. We can implement this concept in RELAX NG with name
classes that uses difference and name.
Suppose, for example, we wanted to allow an element to have any
attribute with a qualified name, but we still wanted to ensure that if
there was an xml:space attribute, it had the value
default or preserve. It wouldn't work to
use:
<element name="example">
<zeroOrMore>
<attribute>
<anyName/>
</attribute>
</zeroOrMore>
<optional>
<attribute name="xml:space">
<choice>
<value>default</value>
<value>preserve</value>
</choice>
</attribute>
</optional>
</element>
because an xml:space attribute with a value
other than default or preserve
would match
<attribute>
<anyName/>
</attribute>
even though it did not match
<attribute name="xml:space">
<choice>
<value>default</value>
<value>preserve</value>
</choice>
</attribute>
The solution is to use name together with
difference:
<element name="example">
<zeroOrMore>
<attribute>
<difference>
<anyName/>
<name>xml:space</name>
</difference>
</attribute>
</zeroOrMore>
<optional>
<attribute name="xml:space">
<choice>
<value>default</value>
<value>preserve</value>
</choice>
</attribute>
</optional>
</element>
Note that the define element cannot contain a
name-class; it can only contain a pattern.
RELAX NG generalizes the ID/IDREF feature of XML. A
data pattern may have either a key or a
keyRef attribute. A data pattern with a
key attribute behaves like an XML ID; a data
pattern with a keyRef attribute type behaves like an XML
IDREF. Whereas XML has a single symbol-space of IDs and IDREFs, RELAX
NG has an unlimited number of named symbol-spaces. The value of the
key or keyRef is an unprefixed name
identifying the symbol-space. An element or attribute that matches a
data pattern with a key attribute is called
a key; an element or attribute that matches a data
pattern with a keyRef attribute is called a
key-reference. A document is invalid if it has two distinct
keys in the same symbol-space with same value; it is also invalid if
it contains a key-reference that does not have a corresponding key in
the same symbol-space in the same document with the same value.
Whereas in XML IDs and IDREFs must be names, in RELAX NG keys and
key-references may have any datatype; whether an element or attribute
is a key or key-reference is orthogonal to its datatype.The values of
keys and key-references are compared using the datatype specified by
the data pattern. All data patterns sharing
the same symbol space must specify the same value for the
type attribute.
For example, suppose a document contains termref
elements referencing defined terms:
<element name="termref"> <data type="token" keyRef="term"/> </element>
For each such defined term, there is a corresponding
dt, dd pair in a glossary
element:
<element name="glossary">
<zeroOrMore>
<element name="dt">
<data type="token" key="term"/>
</element>
<element name="dd">
<text/>
</element>
</zeroOrMore>
</element>
The above example is using the builtin token datatype
introduced in the Enumerations
section.
It must be possible to determine for any element or attribute whether it is a key or key reference and, if so, the symbol space of the key or key reference, by examining just the name of the element or attribute and the names of the ancestors of that element or attribute. For example, it is not permitted to have the pattern:
<element name="bad">
<choice>
<data type="string" key="x"/>
<data type="string" key="y"/>
</choice>
</element>
If a RELAX NG element has an attribute or child element with a namespace URI other than the RELAX NG namespace, then that attribute or element is ignored. Thus, you can add annotations to RELAX NG patterns simply by using an attribute or element in a separate namespace:
<element name="addressBook" xmlns="http://relaxng.org/main/ns/0.1" xmlns:a="http://www.example.com/annotation">
<zeroOrMore>
<element name="card">
<a:documentation>Information about a single email address.</a:documentation>
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</zeroOrMore>
</element>
There is no prohibition against nesting grammar patterns. A
ref pattern refers to the definition from nearest
grammar ancestor. However, by putting a
parent="true" attribute on ref, it is
possible to escape out of the current grammar and reference its parent
grammar.
Imagine the problem of writing a pattern for tables. The pattern
for tables only cares about the structure of tables; it doesn't care
about what goes inside a table cell. First, we create a RELAX NG pattern
table.rng as follows:
<grammar>
<define name="cell.content">
<notAllowed/>
</define>
<start>
<element name="table">
<oneOrMore>
<element name="tr">
<oneOrMore>
<element name="td">
<ref name="cell.content"/>
</element>
</oneOrMore>
</element>
</oneOrMore>
</element>
</start>
</grammar>
Patterns that include table.rng must redefine
cell.content. By using a nested grammar
pattern containing a ref pattern with
parent="true", the including pattern can redefine
cell.content to be a pattern defined in the including
pattern's grammar, thus effectively importing a pattern from the
parent grammar into the child grammar:
<grammar>
<start>
<element name="doc">
<zeroOrMore>
<choice>
<element name="p">
<ref name="inline"/>
</element>
<grammar>
<include href="table.rng"/>
<define name="cell.content" combine="replace">
<ref name="inline" parent="true"/>
</define>
</grammar>
</choice>
</zeroOrMore>
</element>
</start>
<define name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="em">
<ref name="inline"/>
</element>
</choice>
</zeroOrMore>
</define>
</grammar>
Of course, in a trivial case like this, there is no advantage in
nesting the grammars: we could simply have have included
table.rng within the outer grammar element.
However, when the included grammar has many definitions, nesting it
avoids the possibility of name conflicts between the including grammar
and the included grammar.
RELAX NG does not require patterns to be "deterministic" or "unambiguous".
Suppose we wanted to write the email address book in HTML, but use class attributes to specify the structure.
<element name="html">
<element name="head">
<element name="title">
<text/>
</element>
</element>
<element name="body">
<element name="table">
<attribute name="class">
<value>addressBook</value>
</attribute>
<oneOrMore>
<element name="tr">
<attribute name="class">
<value>card</value>
</attribute>
<element name="td">
<attribute name="class">
<value>name</value>
</attribute>
<interleave>
<text/>
<optional>
<element name="span">
<attribute name="class">
<value>givenName</value>
</attribute>
<text/>
</element>
</optional>
<optional>
<element name="span">
<attribute name="class">
<value>familyName</value>
</attribute>
<text/>
</element>
</optional>
</interleave>
</element>
<element name="td">
<attribute name="class">
<value>email</value>
</attribute>
<text/>
</element>
</element>
</oneOrMore>
</element>
</element>
</element>
This would match a XML document such as:
<html>
<head>
<title>Example Address Book</title>
</head>
<body>
<table class="addressBook">
<tr class="card">
<td class="name">
<span class="givenName">John</span>
<span class="familyName">Smith</span>
</td>
<td class="email">js@example.com</td>
</tr>
</table>
</body>
</html>
but not
<html>
<head>
<title>Example Address Book</title>
</head>
<body>
<table class="addressBook">
<tr class="card">
<td class="name">
<span class="givenName">John</span>
<!-- Note the incorrect class attribute -->
<span class="givenName">Smith</span>
</td>
<td class="email">js@example.com</td>
</tr>
</table>
</body>
</html>
The role of RELAX NG is simply to specify a class of documents, not to assist in interpretation of the documents belonging to the class. It does not change the infoset of the document. In particular, RELAX NG
Also RELAX NG does not define a way for an XML document to associate itself with a RELAX NG pattern.
concur pattern has been removed
string pattern has been replaced by the
value pattern
anyString pattern has been renamed to
textdata pattern can have parameters specified by
param child elements
oneOrMoreTokens and zeroOrMoreTokens
patterns have been added for matching whitespace-separated sequences
of tokens
data pattern can have a key or
keyRef attribute
replace and group values for the
combine attribute have been removed
include element in a grammar may contain
define elements that replace included definitions
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output encoding="iso-8859-1" method="html"/>
<xsl:template match="/|*|@*|comment()">
<xsl:copy>
<xsl:apply-templates select="@*"/>
<xsl:apply-templates/>
</xsl:copy>
</xsl:template>
<xsl:template match="@xml:space"/>
<xsl:template match="h1">
<xsl:copy-of select="."/>
</xsl:template>
<xsl:template match="h2|h3|h4">
<xsl:copy>
<a name="{translate(.,' ','_')}"/>
<xsl:number format="1.1" count="div" level="multiple"/>
<xsl:text> </xsl:text>
<xsl:apply-templates/>
</xsl:copy>
</xsl:template>
<xsl:template match="h2|h3|h4" mode="toc">
<xsl:apply-templates select="." mode="indent"/>
<xsl:number format="1.1" count="div" level="multiple"/>
<xsl:text> </xsl:text>
<a href="#{translate(.,' ','_')}">
<xsl:apply-templates/>
</a>
<br/>
</xsl:template>
<xsl:template match="h2" mode="indent"></xsl:template>
<xsl:template match="h3" mode="indent">  </xsl:template>
<xsl:template match="h4" mode="indent">    </xsl:template>
<xsl:template match="body">
<xsl:copy>
<xsl:copy-of select="@*"/>
<xsl:apply-templates select="*[not(self::div)]"/>
<div>
<h2>Table of contents</h2>
<xsl:apply-templates mode="toc" select="div//h2|div//h3|div//h4"/>
</div>
<xsl:apply-templates select="div"/>
</xsl:copy>
</xsl:template>
<xsl:template match="p[@class='abstract']">
<div class="abstract">
<h2>Abstract</h2>
<p><xsl:apply-templates/></p>
</div>
</xsl:template>
</xsl:stylesheet>
<?xml version="1.0"?>
<!-- $Id: tutorial.xml,v 1.9 2001/05/24 15:43:25 jjc Exp $ -->
<?xml-stylesheet type="text/xsl" href="toc.xsl"?>
<html xml:space="preserve">
<head>
<title>RELAX NG tutorial</title>
</head>
<body>
<h1>RELAX NG<br/>
Tutorial</h1>
<p class="author"><b>Editor: </b><br/>
    James Clark <<a href="mailto:jjc@jclark.com">jjc@jclark.com</a>><br/>
<b>Date:</b><br/>
    2001-05-24</p>
<p>Copyright © 2001 OASIS</p>
<p class="abstract">RELAX NG is a simple schema language for XML,
based on <a href="http://www.xml.gr.jp/relax/">RELAX</a> and <a
href="http://www.thaiopensource.com/trex/">TREX</a>. A RELAX NG schema
specifies a pattern for the structure and content of an XML
document. A RELAX NG schema thus identifies a class of XML documents
consisting of those documents that match the pattern. A RELAX NG
schema is itself an XML document.</p>
<div>
<h2>Getting started</h2>
<p>Consider a simple XML representation of an email address book:</p>
<pre><![CDATA[<addressBook>
<card>
<name>John Smith</name>
<email>js@example.com</email>
</card>
<card>
<name>Fred Bloggs</name>
<email>fb@example.net</email>
</card>
</addressBook>]]></pre>
<p>The DTD would be as follows:</p>
<pre><![CDATA[<!DOCTYPE addressBook [
<!ELEMENT addressBook (card*)>
<!ELEMENT card (name, email)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT email (#PCDATA)>
]>]]></pre>
<p>A RELAX NG pattern for this could be written as follows:</p>
<pre><![CDATA[<element name="addressBook" xmlns="http://relaxng.org/main/ns/0.1">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</zeroOrMore>
</element>]]></pre>
<p>If the <code>addressBook</code> is required to be non-empty, then
we can use <code>oneOrMore</code> instead of
<code>zeroOrMore</code>:</p>
<pre><![CDATA[<element name="addressBook" xmlns="http://relaxng.org/main/ns/0.1">
<oneOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</oneOrMore>
</element>]]></pre>
<p>Now let's change it to allow each <code>card</code> to have an
optional <code>note</code> element.</p>
<pre><![CDATA[<element name="addressBook" xmlns="http://relaxng.org/main/ns/0.1">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<optional>
<element name="note">
<text/>
</element>
</optional>
</element>
</zeroOrMore>
</element>]]></pre>
<p>Note that the <code>text</code> pattern matches arbitrary text,
including empty text. Note also that whitespace separating tags is
ignored when matching against a pattern.</p>
<p>All the elements specifying the pattern must be namespace qualified
by the namespace URI:</p>
<pre>http://relaxng.org/main/ns/0.1</pre>
<p>The examples above use a default namespace declaration
<code>xmlns="http://relaxng.org/main/ns/0.1"</code> for this. A
namespace prefix is equally acceptable:</p>
<pre><![CDATA[<rng:element name="addressBook" xmlns:rng="http://relaxng.org/main/ns/0.1">
<rng:zeroOrMore>
<rng:element name="card">
<rng:element name="name">
<rng:text/>
</rng:element>
<rng:element name="email">
<rng:text/>
</rng:element>
</rng:element>
</rng:zeroOrMore>
</rng:element>
</rng:div>]]></pre>
<p>For the remainder of this document, the default namespace
declaration will be left out of examples.</p>
</div>
<div>
<h2>Choice</h2>
<p>Now suppose we want to allow the <code>name</code> to be broken
down into a <code>givenName</code> and a <code>familyName</code>,
allowing an <code>addressBook</code> like this:</p>
<pre><![CDATA[<addressBook>
<card>
<givenName>John</givenName>
<familyName>Smith</familyName>
<email>js@example.com</name>
</card>
<card>
<name>Fred Bloggs</name>
<email>fb@example.net</email>
</card>
</addressBook>]]></pre>
<p>We can use the following pattern:</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<choice>
<element name="name">
<text/>
</element>
<group>
<element name="givenName">
<text/>
</element>
<element name="familyName">
<text/>
</element>
</group>
</choice>
<element name="email">
<text/>
</element>
<optional>
<element name="note">
<text/>
</element>
</optional>
</element>
</zeroOrMore>
</element>]]></pre>
<p>This corresponds to the following DTD:</p>
<pre><![CDATA[<!DOCTYPE addressBook [
<!ELEMENT addressBook (card*)>
<!ELEMENT card ((name | (givenName, familyName)), email, note?)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT email (#PCDATA)>
<!ELEMENT givenName (#PCDATA)>
<!ELEMENT familyName (#PCDATA)>
<!ELEMENT note (#PCDATA)>
]>]]></pre>
</div>
<div>
<h2>Attributes</h2>
<p>Suppose we want the <code>card</code> element to have attributes
rather than child elements. The DTD might look like this</p>
<pre><![CDATA[<!DOCTYPE addressBook [
<!ELEMENT addressBook (card*)>
<!ELEMENT card EMPTY>
<!ATTLIST card
name CDATA #REQUIRED
email CDATA #REQUIRED>
]>]]></pre>
<p>Just change each <code>element</code> pattern to an
<code>attribute</code> pattern:</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<attribute name="name">
<text/>
</attribute>
<attribute name="email">
<text/>
</attribute>
</element>
</zeroOrMore>
</element>]]></pre>
<p>In XML, the order of attributes is traditionally not significant.
RELAX NG follows this tradition. The above pattern would match both</p>
<pre><![CDATA[<card name="John Smith" email="js@example.com"/>]]></pre>
<p>and</p>
<pre><![CDATA[<card email="js@example.com" name="John Smith"/>]]></pre>
<p>In contrast, the order of elements is significant. The pattern</p>
<pre><![CDATA[<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>]]></pre>
<p>would <em>not</em> match:</p>
<pre><![CDATA[<card><email>js@example.com</email><name>John Smith</name></card>]]></pre>
<p>Note that an <code>attribute</code> element by itself indicates a
required attribute, just as an <code>element</code> element by itself
indicates a required element. To specify an optional attribute, use
<code>optional</code> just as with <code>element</code>:</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<attribute name="name">
<text/>
</attribute>
<attribute name="email">
<text/>
</attribute>
<optional>
<attribute name="note">
<text/>
</attribute>
</optional>
</element>
</zeroOrMore>
</element>]]></pre>
<p>The <code>group</code> and <code>choice</code> patterns can be
applied to <code>attribute</code> elements in the same way they are
applied to <code>element</code> patterns. For example, if we wanted
to allow either a <code>name</code> attribute or both a
<code>givenName</code> and a <code>familyName</code> attribute, we can
specify this in the same way that we would if we were using
elements:</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<choice>
<attribute name="name">
<text/>
</attribute>
<group>
<attribute name="givenName">
<text/>
</attribute>
<attribute name="familyName">
<text/>
</attribute>
</group>
</choice>
<attribute name="email">
<text/>
</attribute>
</element>
</zeroOrMore>
</element>]]></pre>
<p>There are no restrictions on how <code>element</code> elements and
<code>attribute</code> elements can be combined. For example, the
following pattern would allow a choice of elements and attributes
independently for both the <code>name</code> and the
<code>email</code> part of a <code>card</code>:</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<choice>
<element name="name">
<text/>
</element>
<attribute name="name">
<text/>
</attribute>
</choice>
<choice>
<element name="email">
<text/>
</element>
<attribute name="email">
<text/>
</attribute>
</choice>
</element>
</zeroOrMore>
</element>]]></pre>
<p>As usual, the relative order of elements is significant, but the
relative order of attributes is not. Thus the above would match any
of:</p>
<pre><![CDATA[<card name="John Smith" email="js@example.com"/>
<card email="js@example.com" name="John Smith"/>
<card email="js@example.com"><name>John Smith</name></card>
<card name="John Smith"><email>js@example.com</email></card>
<card><name>John Smith</name><email>js@example.com</email></card>]]></pre>
<p>However, it would not match</p>
<pre><![CDATA[<card><email>js@example.com</email><name>John Smith</name></card>]]></pre>
<p>because the pattern for <code>card</code> requires any
<code>email</code> child element to follow any <code>name</code> child
element.</p>
<p>There is one difference between <code>attribute</code> and
<code>element</code> patterns: <code><![CDATA[<text/>]]></code>
is the default for the content of an <code>attribute</code> pattern,
whereas an <code>element</code> pattern is not allowed to be
empty. For example,</p>
<pre><![CDATA[<attribute name="email"/>]]></pre>
<p>is short for</p>
<pre><![CDATA[<attribute name="email">
<text/>
</attribute>]]></pre>
<p>It might seem natural that</p>
<pre><![CDATA[<element name="x"/>]]></pre>
<p>matched an <code>x</code> element with no attributes and no
content. However, this would make the meaning of empty content
inconsistent between the <code>element</code> pattern and the
<code>attribute</code> pattern, so RELAX NG does not allow the
<code>element</code> pattern to be empty. A pattern that matches an
element with no attributes and no children must use
<code><![CDATA[<empty/>]]></code> explicitly:</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<optional>
<element name="prefersHTML">
<empty/>
</element>
</optional>
</element>
</zeroOrMore>
</element>]]></pre>
</div>
<div>
<h2>Named patterns</h2>
<p>For a non-trivial RELAX NG pattern, it is often convenient to be able
to give names to parts of the pattern. Instead of</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</zeroOrMore>
</element>]]></pre>
<p>we can write</p>
<pre><![CDATA[<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<ref name="cardContent"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="cardContent">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</define>
</grammar>]]></pre>
<p>A <code>grammar</code> element has a single <code>start</code>
child element, and zero or more <code>define</code> child elements.
The <code>start</code> and <code>define</code> elements contain
patterns. These patterns can contain <code>ref</code> elements that
refer to patterns defined by any of the <code>define</code> elements
in that <code>grammar</code> element. A <code>grammar</code> pattern
is matched by matching the pattern contained in the <code>start</code>
element.</p>
<p>We can use the <code>grammar</code> element to write patterns in a
style similar to DTDs:</p>
<pre><![CDATA[<grammar>
<start>
<ref name="AddressBook"/>
</start>
<define name="AddressBook">
<element name="addressBook">
<zeroOrMore>
<ref name="Card"/>
</zeroOrMore>
</element>
</define>
<define name="Card">
<element name="card">
<ref name="Name"/>
<ref name="Email"/>
</element>
</define>
<define name="Name">
<element name="name">
<text/>
</element>
</define>
<define name="Email">
<element name="email">
<text/>
</element>
</define>
</grammar>]]></pre>
<p>Recursive references are allowed. For example</p>
<pre><![CDATA[<define name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
<element name="italic">
<ref name="inline"/>
</element>
<element name="span">
<optional>
<attribute name="style"/>
</optional>
<ref name="inline"/>
</element>
</choice>
</zeroOrMore>
</define>]]></pre>
<p>However, recursive references must be within an
<code>element</code>. Thus, the following is <em>not</em>
allowed:</p>
<pre><![CDATA[<define name="inline">
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
<element name="italic">
<ref name="inline"/>
</element>
<element name="span">
<optional>
<attribute name="style"/>
</optional>
<ref name="inline"/>
</element>
</choice>
<optional>
<ref name="inline"/>
</optional>
</define>]]></pre>
<p>A <code>start</code> element may also have a <code>name</code>
attribute. This is a shorthand for a <code>define</code> with that
<code>name</code> together with a <code>start</code> element
referencing that definition. For example</p>
<pre><![CDATA[<grammar>
<start name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
</choice>
</zeroOrMore>
</start>
</grammar>]]></pre>
<p>is short for</p>
<pre><![CDATA[<grammar>
<start>
<ref name="inline"/>
</start>
<define name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
</choice>
</zeroOrMore>
</define>
</grammar>]]></pre>
</div>
<div>
<h2>Datatyping</h2>
<p>RELAX NG does not have any system of datatypes built in. Rather it
expects to partner with a datatyping vocabulary, such as Part 2 of the
W3C's XML Schema language. RELAX NG implementations may differ in the
datatyping vocabularies they support. You must pick a datatyping
vocabulary that is supported by the implementation you plan to
use.</p>
<p>The <code>data</code> pattern matches a string that represents a
value of a named datatype. The <code>type</code> attribute contains
the qualified name of the datatype. For example, if a RELAX NG
implementation supported the built-in datatypes of the W3C's XML
Schema Language, you could use:</p>
<pre><![CDATA[<element name="number" xmlns:xsd="http://www.w3.org/2001/XMLSchema-datatypes">
<data type="xsd:integer"/>
</element>]]></pre>
<p>The <code>data</code> can use an <code>ns</code> attribute to
specify explicitly the namespace URI of the datatype, instead of using
a prefix within the value of the <code>type</code> attribute.</p>
<pre><![CDATA[<element name="number">
<data type="integer" ns="http://www.w3.org/2001/XMLSchema-datatypes"/>
</element>]]></pre>
<p>If the children of an element or an attribute match a
<code>data</code> pattern, then complete content of the element or
attribute must match that <code>data</code> pattern. It is not
permitted to have a pattern which allows part of the content to match
a <code>data</code> pattern, and another part to match another
pattern. For example, the following pattern is <em>not</em>
allowed:</p>
<pre><![CDATA[<element name="bad">
<data type="xsd:int"/>
<element name="note">
<text/>
</element>
</element>]]></pre>
<p>However, this would be fine:</p>
<pre><![CDATA[<element name="ok">
<data type="xsd:int"/>
<attribute name="note">
<text/>
</attribute>
</element>]]></pre>
<p>Note that this restriction does not apply to the
<code>text</code> pattern.</p>
<p>Datatypes may have parameters. For example, a string datatype may
have a parameter controlling the length of the string. The parameters
applicable to any particular datatype are determined by the datatyping
vocabulary. Parameters are specified by adding one or more
<code>param</code> elements as children of the <code>data</code>
element. For example, the following constrains the <code>email</code>
element to contain a string at most 127 characters long:</p>
<pre><![CDATA[<element name="email">
<data type="xsd:string">
<param name="maxLength">127</param>
</data>
</element>]]></pre>
</div>
<div>
<h2>Enumerations</h2>
<p>Many markup vocabularies have attributes whose value is constrained
to be one of set of specified values. The <code>value</code> pattern
matches a string that has a specified value. For example,</p>
<pre><![CDATA[<element name="card">
<attribute name="name"/>
<attribute name="email"/>
<attribute name="preferredFormat">
<choice>
<value>html</value>
<value>text</value>
</choice>
</attribute>
</element>]]></pre>
<p>allows the <code>preferredFormat</code> attribute to have the value
<code>html</code> or <code>text</code>. This corresponds to the
DTD</p>
<pre><![CDATA[<!DOCTYPE card [
<!ELEMENT card EMPTY>
<!ATTLIST card
name CDATA #REQUIRED
email CDATA #REQUIRED
preferredFormat (html|text) #REQUIRED>
]>]]></pre>
<p>The <code>value</code> pattern is not restricted to attribute
values. For example, the following is allowed:</p>
<pre><![CDATA[<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<element name="preferredFormat">
<choice>
<value>html</value>
<value>text</value>
</choice>
</element>
</element>]]></pre>
<p>The prohibition against a <code>data</code> pattern's matching
only part of the content of an element also applies to
<code>value</code> patterns.</p>
<p>By default, the <code>value</code> pattern will consider the string
in the pattern to match the string in the document if the two strings
are the same after the whitespace in both strings is normalized.
Whitespace normalization strips leading and trailing white-space
characters, and collapses sequences of one or more white-space
characters to a single space character. This corresponds to the
behaviour of an XML parser for an attribute that is declared as other
than CDATA. Thus the above pattern will match any of</p>
<pre><![CDATA[<card name="John Smith" email="js@example.com" preferredFormat="html"/>]]><br/>
<![CDATA[<card name="John Smith" email="js@example.com" prefersFormat=" html "/>]]></pre>
<p>The way that the <code>value</code> pattern compares the pattern
string with the document string can be controlled by specifying a
<code>type</code> attribute specifying a datatype. The
<code>type</code> attribute contains a qualified name identifying the
datatype. The pattern string matches the document string if they both
represent the same value of the specified datatype. Thus, whereas the
<code>data</code> pattern matches an arbitrary value of a datatype,
the <code>value</code> pattern matches a specific value of a
datatype.</p>
<p>RELAX NG provides two builtin datatypes that are useful with the
<code>value</code> pattern. These datatypes are specified by using an
unprefixed name as the value of the <code>type</code> attribute. The
two builtin datatypes are <code>string</code> and <code>token</code>.
The builtin datatype <code>token</code> corresponds to the default
comparison behavior of the <code>value</code> pattern. The builtin
datatype <code>string</code> compares strings without any
normalization (other than that performed by XML). For example,</p>
<pre><![CDATA[<element name="card">
<attribute name="name"/>
<attribute name="email"/>
<attribute name="preferredFormat">
<choice>
<value type="string">html</value>
<value type="string">text</value>
</choice>
</attribute>
</element>]]></pre>
<p>will <em>not</em> match</p>
<pre><![CDATA[<card name="John Smith" email="js@example.com" prefersHTML=" html "/>]]></pre>
</div>
<div>
<h2>Lists</h2>
<p>The <code>oneOrMoreTokens</code> and <code>zeroOrMoreTokens</code>
patterns match a whitespace-separated sequence of tokens; they each
contain a pattern that the individual tokens must match. For example,
the <code>extension-element-prefixes</code> attribute in XSLT contains
a whitespace-separated list of zero or more namespace prefixes, where
each namespace prefix is either an NCName or the special value
<code>#default</code>:</p>
<pre><![CDATA[<attribute name="extension-element-prefixes">
<zeroOrMoreTokens>
<choice>
<data type="xsd:NCName"/>
<value>#default</value>
</choice>
</zeroOrMoreTokens>
</attribute>]]></pre>
<p>The <code>oneOrMoreTokens</code> and <code>zeroOrMoreTokens</code>
patterns must not contain <code>element</code> or
<code>attribute</code> patterns.</p>
</div>
<div>
<h2>Interleaving</h2>
<p>The <code>interleave</code> pattern allows child elements to occur
in any order. For example, the following would allow the
<code>card</code> element to contain the <code>name</code> and
<code>email</code> elements in any order:</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<interleave>
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</interleave>
</element>
</zeroOrMore>
</element>]]></pre>
<p>The pattern is called <code>interleave</code> because of how it
works with patterns that match more than one element. Suppose we want
to write a pattern for the HTML <code>head</code> element which
requires exactly one <code>title</code> element, at most one
<code>base</code> element and zero or more <code>style</code>,
<code>script</code>, <code>link</code> and <code>meta</code> elements
and suppose we are writing a <code>grammar</code> pattern that has one
definition for each element. Then we could define the pattern for
<code>head</code> as follows:</p>
<pre><![CDATA[<define name="head">
<element name="head">
<interleave>
<ref name="title"/>
<optional>
<ref name="base"/>
</optional>
<zeroOrMore>
<ref name="style"/>
</zeroOrMore>
<zeroOrMore>
<ref name="script"/>
</zeroOrMore>
<zeroOrMore>
<ref name="link"/>
</zeroOrMore>
<zeroOrMore>
<ref name="meta"/>
</zeroOrMore>
</interleave>
</element>
</define>]]></pre>
<p>Suppose we had a <code>head</code> element that contained a
<code>meta</code> element, followed by a <code>title</code> element,
followed by a <code>meta</code> element. This would match the pattern
because it is an interleaving of a sequence of two <code>meta</code>
elements, which match the child pattern</p>
<pre><![CDATA[ <zeroOrMore>
<ref name="meta"/>
</zeroOrMore>]]></pre>
<p>and a sequence of one <code>title</code> element, which matches
the child pattern</p>
<pre><![CDATA[ <ref name="title"/>]]></pre>
<p>The semantics of the <code>interleave</code> pattern are that a
sequence of elements matches an <code>interleave</code> pattern if it
is an interleaving of sequences that match the child patterns of the
<code>interleave</code> pattern. Note that this is different from the
<code>&</code> connector in SGML: <code>A* & B</code> matches
the sequence of elements <code>A A B</code> or the sequence of
elements <code>B A A</code> but not the sequence of elements <code>A B
A</code>.</p>
<p>One special case of <code>interleave</code> is very common:
interleaving <code><![CDATA[<text/>]]></code> with a pattern
<var>p</var> represents a pattern that matches what <var>p</var>
matches but also allows characters to occur as children. The
<code>mixed</code> element is a shorthand for this.</p>
<pre><mixed> <var>p</var> </mixed></pre>
<p>is short for</p>
<pre><interleave> <text/> <var>p</var> </interleave></pre>
</div>
<div>
<h2>Modularity</h2>
<p>The <code>include</code> element can be used to allow a pattern to
be divided amongst multiple files. The <code>include</code> element
has a required <code>href</code> attribute that specifies the URL of a
file to be included in place of the <code>include</code> element.</p>
<div>
<h3>Including patterns</h3>
<p>The <code>include</code> element can be used as a pattern. In this
case, it will match if the pattern contained in the specified URL
matches. Suppose for example, you have a RELAX NG pattern that matches
HTML inline content stored in <code>inline.rng</code>:</p>
<pre><![CDATA[<grammar>
<start name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="code">
<ref name="inline"/>
</element>
<element name="em">
<ref name="inline"/>
</element>
<!-- etc -->
</choice>
</zeroOrMore>
</start>
</grammar>]]></pre>
<p>Then we could allow the <code>note</code> element to contain
inline HTML markup by using <code>include</code> as follows:</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<optional>
<element name="note">
<include href="inline.rng"/>
</element>
</optional>
</element>
</zeroOrMore>
</element>]]></pre>
<p>For another example, suppose you have two RELAX NG patterns stored in
files <code>pattern1.rng</code> and <code>pattern2.rng</code>. Then
the following is a pattern that which match anything matched
by one of those patterns:</p>
<pre><![CDATA[<choice>
<include href="pattern1.rng"/>
<include href="pattern2.rng"/>
</choice>]]></pre>
</div>
<div>
<h3>Merging grammars</h3>
<p>The <code>include</code> element is also allowed as a child of a
<code>grammar</code> pattern. In this case the specified URL must
contain a <code>grammar</code> pattern, and the included
<code>grammar</code> will be merged with the including
<code>grammar</code>.</p>
<p>Normally, duplicate definitions (two definitions with the same
name) result in an error. However, <code>define</code> elements may
be put inside the <code>include</code> element to indicate that they
are to replace definitions in the included <code>grammar</code>
pattern.</p>
<p>Suppose the file <code>addressBook.rng</code> contains the
following grammar pattern:</p>
<pre><![CDATA[<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<ref name="card.local"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="card.local">
<empty/>
</define>
</grammar>]]></pre>
<p>Another pattern could customize <code>addressBook.rng</code> as
follows:</p>
<pre><![CDATA[<grammar>
<include href="addressBook.rng">
<define name="card.local">
<optional>
<element name="note">
<text/>
</element>
</optional>
</define>
</include>
</grammar>]]></pre>
<p>This would be equivalent to:</p>
<pre><![CDATA[<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<ref name="card.local"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="card.local">
<optional>
<element name="note">
<text/>
</element>
</optional>
</define>
</grammar>]]></pre>
<p>which is equivalent to</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<optional>
<element name="note">
<text/>
</element>
</optional>
</element>
</zeroOrMore>
</element>]]></pre>
<p>It is also possible to combine together duplicate definitions from
separate files by adding a <code>combine</code> attribute to the
<code>define</code> elements. The <code>combine</code> attribute
specifies how the definitions should be combined; it may have the
value <code>choice</code> or <code>interleave</code>. For example, we
could have written our customization as:</p>
<pre><![CDATA[<grammar>
<include href="addressBook.rng"/>
<define name="card.local" combine="choice">
<!-- no optional element needed this time -->
<element name="note">
<text/>
</element>
</define>
</grammar>]]></pre>
<p>This would be equivalent to:</p>
<pre><![CDATA[<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<ref name="card.local"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="card.local">
<choice>
<empty/>
<element name="note">
<text/>
</element>
</choice>
</define>
</grammar>]]></pre>
<p>This has the same meaning as before, since an optional pattern is
equivalent to a choice between the pattern and empty.</p>
<p>We could also have used <code>combine="interleave"</code> here:</p>
<pre><![CDATA[<grammar>
<include href="addressBook.rng"/>
<define name="card.local" combine="interleave">
<optional>
<element name="note">
<text/>
</element>
</optional>
</define>
</grammar>]]></pre>
<p>This would be equivalent to:</p>
<pre><![CDATA[<grammar>
<start>
<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<ref name="card.local"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="card.local">
<interleave>
<empty/>
<optional>
<element name="note">
<text/>
</element>
</optional>
</interleave>
</define>
</grammar>]]></pre>
<p>This has the same meaning as before, since adding an
<code>empty</code> pattern to the content of a <code>interleave</code>
pattern does not make any difference to what the
<code>interleave</code> pattern matches.</p>
<p>@@@ Add example of combine="interleave" with attributes.</p>
<p>The <code>notAllowed</code> pattern never matches anything. Just
as adding <code>empty</code> to a <code>group</code> makes no
difference, so adding <code>notAllowed</code> to a <code>choice</code>
makes no difference. It is typically used in a definition that is
referenced in a <code>choice</code> element to allow an including
pattern to specify additional choices. For example, suppose a RELAX NG
pattern <code>inline.rng</code> provides a pattern for inline
content, which allows <code>bold</code> and <code>italic</code>
elements arbitrarily nested:</p>
<pre><![CDATA[<grammar>
<start name="inline">
<zeroOrMore>
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
<element name="italic">
<ref name="inline"/>
</element>
<ref name="local.inline"/>
</choice>
</zeroOrMore>
</start>
<define name="local.inline">
<notAllowed/>
</define>
</grammar>]]></pre>
<p>Another RELAX NG pattern could use <code>inline.rng</code> and add
<code>code</code> and <code>em</code> to the set of inline elements as
follows:</p>
<pre><![CDATA[<grammar>
<include href="inline.rng">
<define name="local.inline">
<choice>
<element name="code">
<ref name="inline">
</element>
<element name="em">
<ref name="inline">
</element>
</choice>
</define>
</include>
<start>
<element name="doc">
<zeroOrMore>
<element name="p">
<ref name="inline"/>
</element>
</zeroOrMore>
</element>
</start>
</grammar>]]></pre>
<p>We could instead have used <code>combine="choice"</code>. In this
case, <code>inline.rng</code> would need to separate out the choices
as a separate definition:</p>
<pre><![CDATA[<grammar>
<start name="inline">
<zeroOrMore>
<ref name="inline.class"/>
</zeroOrMore>
</start>
<define name="inline.class">
<choice>
<text/>
<element name="bold">
<ref name="inline"/>
</element>
<element name="italic">
<ref name="inline"/>
</element>
</choice>
</define>
</grammar>]]></pre>
<p>and the customization would add to those choices:</p>
<pre><![CDATA[<grammar>
<include href="inline.rng"/>
<start>
<element name="doc">
<zeroOrMore>
<element name="p">
<ref name="inline"/>
</element>
</zeroOrMore>
</element>
</start>
<define name="inline.class" combine="choice">
<choice>
<element name="code">
<ref name="inline">
</element>
<element name="em">
<ref name="inline">
</element>
</choice>
</define>
</grammar>]]></pre>
</div>
</div>
<div>
<h2>Namespaces</h2>
<p>RELAX NG is namespace-aware. Thus, it considers an element or attribute
to have both a local name and a namespace URI which together
constitute the name of that element or attribute.</p>
<div>
<h3>Using the <code>ns</code> attribute</h3>
<p>The <code>element</code> pattern uses an <code>ns</code> attribute
to specify the namespace URI of the elements that it matches. For
example</p>
<pre><![CDATA[<element name="foo" ns="http://www.example.com">
<empty/>
</element>]]></pre>
<p>would match any of</p>
<pre><![CDATA[<foo xmlns="http://www.example.com"/>]]><br/>
<![CDATA[<e:foo xmlns:e="http://www.example.com"/>]]><br/>
<![CDATA[<example:foo xmlns:example="http://www.example.com"/>]]></pre>
<p>but not any of</p>
<pre><![CDATA[<foo/>]]><br/>
<![CDATA[<e:foo xmlns:e="http://WWW.EXAMPLE.COM"/>]]><br/>
<![CDATA[<example:foo xmlns:example="http://www.example.net"/>]]></pre>
<p>A value of an empty string for the <code>ns</code> attribute
indicates a null or absent namespace URI (just as with the
<code>xmlns</code> attribute). Thus, the pattern</p>
<pre><![CDATA[<element name="foo" ns="">
<empty/>
</element>]]></pre>
<p>matches any of</p>
<pre><![CDATA[<foo xmlns=""/>]]><br/>
<![CDATA[<foo/>]]></pre>
<p>but not any of</p>
<pre><![CDATA[<foo xmlns="http://www.example.com"/>]]><br/>
<![CDATA[<e:foo xmlns:e="http://www.example.com"/>]]></pre>
<p>It is tedious and error-prone to specify the <code>ns</code>
attribute on every <code>element</code>, so RELAX NG allows it to be
defaulted. If an <code>element</code> pattern does not specify an
<code>ns</code> attribute, then it defaults to the value of the
<code>ns</code> attribute of the nearest ancestor that has an
<code>ns</code> attribute, or the empty string if there is no such
ancestor. Thus</p>
<pre><![CDATA[<element name="addressBook">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</zeroOrMore>
</element>]]></pre>
<p>is equivalent to</p>
<pre><![CDATA[<element name="addressBook" ns="">
<zeroOrMore>
<element name="card" ns="">
<element name="name" ns="">
<text/>
</element>
<element name="email" ns="">
<text/>
</element>
</element>
</zeroOrMore>
</element>]]></pre>
<p>and</p>
<pre><![CDATA[<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
</element>
</zeroOrMore>
</element>]]></pre>
<p>is equivalent to</p>
<pre><![CDATA[<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card" ns="http://www.example.com">
<element name="name" ns="http://www.example.com">
<text/>
</element>
<element name="email" ns="http://www.example.com">
<text/>
</element>
</element>
</zeroOrMore>
</element>]]></pre>
<p>The <code>attribute</code> pattern also takes an <code>ns</code>
attribute. However, there is a difference in how it defaults. This
is because of the fact that the XML Namespaces Recommendation does not
apply the default namespace to attributes. If an <code>ns</code>
attribute is not specified on the <code>attribute</code> pattern, then
it defaults to the empty string. Thus</p>
<pre><![CDATA[<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card">
<attribute name="name"/>
<attribute name="email"/>
</element>
</zeroOrMore>
</element>]]></pre>
<p>is equivalent to</p>
<pre><![CDATA[<element name="addressBook" ns="http://www.example.com">
<zeroOrMore>
<element name="card" ns="http://www.example.com">
<attribute name="name" ns=""/>
<attribute name="email" ns=""/>
</element>
</zeroOrMore>
</element>]]></pre>
<p>and so will match</p>
<pre><![CDATA[<addressBook xmlns="http://www.example.com">
<card name="John Smith" email="js@example.com"/>
</addressBook>]]></pre>
<p>or</p>
<pre><![CDATA[<example:addressBook xmlns:example="http://www.example.com">
<example:card name="John Smith" email="js@example.com"/>
</example:addressBook>]]></pre>
<p>but