RELAX NG Compact Syntax Tutorial

Consider 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 addressBook {
  element card {
    element name { text },
    element email { text }
  }*
}

If the addressBook is required to be non-empty, then we can use + instead of *:

element addressBook {
  element card {
    element name { text },
    element email { text }
  }+
}

Now let's change it to allow each card to have an optional note element:

element addressBook {
  element card {
    element name { text },
    element email { text },
    element note { text }?
  }*
}

Note that the text pattern matches arbitrary text, including empty text. Note also that whitespace separating tags is ignored when matching against a pattern.

Comments start with a # and continue to the end of the line. Comments starting with ## are treated specially; see Section 13, “Annotations”.

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</email>
  </card>
  <card>
    <name>Fred Bloggs</name>
    <email>fb@example.net</email>
  </card>
</addressBook>

We can use the following pattern:

element addressBook {
  element card {
    (element name { text }
     | (element givenName { text },
        element familyName { text })),
    element email { text },
    element note { text }?
  }*
}

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)>
]>

Just as with DTDs, there is no implicit precedence between connectors. For example, x|y,z is not allowed; the precedence must be made explicit by using (x|y),z or x|(y,z) must be used.

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 addressBook {
  element card {
    attribute name { text },
    attribute email { text }
  }*
}

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 card {
  element name { text },
  element email { text }
}

would not match

<card><email>js@example.com</email><name>John Smith</name></card>

Note that an attribute pattern by itself indicates a required attribute, just as an element pattern by itself indicates a required element. To specify an optional attribute, use ? just as with element:

element addressBook {
  element card {
    attribute name { text },
    attribute email { text },
    attribute note { text }?
  }*
}

The , and | connectors can be applied to attribute patterns 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 addressBook {
  element card {
    (attribute name { text }
     | (attribute givenName { text },
        attribute familyName { text })),
    attribute email { text }
  }*
}

The , and | connectors can combine element and attribute patterns without restriction. 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 addressBook {
  element card {
    (element name { text }
     | attribute name { text }),
    (element email { text }
     | attribute email { text })
  }*
}

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.

When an element pattern does not contain any patterns matching attributes, then an element that matches the pattern cannot have any attributes. Similarly, when any element pattern does not contain any patterns matching elements or strings, then an element that matches the pattern cannot have any children. This can be made more explicit by using the empty pattern. For example,

element card {
  attribute email { text },
  empty
}

is equivalent to

element card {
  attribute email { text }
}

The use of the empty pattern is necessary only when an element has neither attributes nor children. For example,

element addressBook {
  element card {
    element name { text },
    element email { text },
    element prefersHTML { empty }?
  }*
}

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 addressBook {
  element card {
    element name { text },
    element email { text }
  }*
}

we can write

grammar {
  start =
    element addressBook {
      element card { cardContent }*
    }
  cardContent =
    element name { text },
    element email { text }
}

A grammar pattern contains one or more definitions. Each definition associates a name with a pattern. Inside a grammar, a pattern consisting of just a name references the definition of that name in the grammar. The name start is special. A grammar pattern is matched by matching the definition of start. A grammar pattern must define start.

We can use the grammar pattern to write RELAX NG in a style similar to DTDs:

grammar {
  start = AddressBook
  AddressBook = element addressBook { Card* }
  Card = element card { Name, Email }
  Name = element name { text }
  Email = element email { text }
}

The opening grammar { and closing } are required only when a grammar pattern is nested within another pattern. In the typical case, when the grammar pattern is the outermost pattern, they can omitted. For example, the above pattern can be written as:

start = AddressBook
AddressBook = element addressBook { Card* }
Card = element card { Name, Email }
Name = element name { text }
Email = element email { text }

Recursive references are allowed. For example,

inline =
  (text
   | element bold { inline }
   | element italic { inline }
   | element span {
       attribute style { text }?,
       inline
     })*

However, recursive references must be within an element. Thus, the following is not allowed:

inline =
  (text
   | element bold { inline }
   | element italic { inline }
   | element span {
       attribute style { text }?,
       inline
     }),
  inline?

To use a keyword such as element, attribute, text, empty, grammar as the name of a definition, it must be quoted with \. For example,

start = \element
\element = element element { text }

is equivalent to

start = e
e = element element { text }

Note that keywords need not be quoted when specifying element or attribute names. A complete list of keywords is in Appendix A, List of keywords.

RELAX NG allows patterns to reference externally-defined datatypes. RELAX NG implementations may differ in what datatypes they support. You can only use datatypes that are supported by the implementation you plan to use. The most commonly used datatypes are those defined by [W3C XML Schema Datatypes].

A pattern consisting of a name qualified with a prefix matches a string that represents a value of a named datatype. The prefix identifies the library of datatypes being used and the rest of the name specifies the name of the datatype in that library. The prefix xsd identifies the datatype library defined by [W3C XML Schema Datatypes]. Assuming your RELAX NG implementation supports this library (most do), you could use:

element number { xsd:integer }

If the children of an element or an attribute match a datatype pattern, then the complete content of the element or attribute must match that datatype pattern. It is not permitted to have a pattern which allows part of the content to match a datatype pattern, and another part to match another pattern. For example, the following pattern is not allowed:

element bad {
  xsd:int,
  element note { text }
}

However, this would be fine:

element ok {
  xsd:int,
  attribute note { text }
}

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. In the case of [W3C XML Schema Datatypes], the applicable parameters correspond to the facets defined in [W3C XML Schema Datatypes] with the exception of the enumeration and whiteSpace facets. Parameters are specified by following the datatype name with a list of one or more name=value parameter assignments in braces. For example, the following constrains the email element to contain a string at least 6 characters long and at most 127 characters long:

element email {
  xsd:string { minLength = "6" maxLength = "127" }
}

The value of a parameter is a string literal. As in XML, string literals can be delimited using either " or '.

A companion document, [Guidelines], describes exactly how the datatypes defined in [W3C XML Schema Datatypes] can be used as a RELAX NG datatype library.

To use a datatype pattern with a prefix other than xsd, a datatypes declaration must be added to the beginning of the file. The datatypes declaration associates the prefix with the URI of a datatype library. The URI of datatype library identified by xsd prefix is http://www.w3.org/2001/XMLSchema-datatypes. So, for example:

datatypes xs = "http://www.w3.org/2001/XMLSchema-datatypes"

element number { xs:integer }

is equivalent to

element number { xsd:integer }

Many markup vocabularies have attributes whose value is constrained to be one of a set of specified strings. A pattern consisting of a literal string matches that string. For example,

element card {
  attribute name { text },
  attribute email { text },
  attribute preferredFormat { "html" | "text" }
}

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>
]>

Literal string patterns are not restricted to attribute values. For example, the following is allowed:

element card {
  element name { text },
  element email { text },
  element preferredFormat { "html" | "text" }
}

The prohibition against a datatype pattern's matching only part of the content of an element also applies to literal string patterns.

By default, a literal string 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 whitespace characters, and collapses sequences of one or more whitespace 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" preferredFormat="  html  "/>

The way that a literal string pattern compares the pattern string with the document string can be controlled by preceding the literal string with a prefixed name, which identifies a datatype in the same way as for the datatype pattern. The pattern string matches the document string if they both represent the same value of the specified datatype. Thus, whereas a datatype pattern matches an arbitrary value of a datatype, a literal string pattern matches a specific value of a datatype.

There are two datatypes built-in to every RELAX NG implementation. These are named string and token: token corresponds to the default comparison behavior of a literal string pattern; string compares strings without any whitespace normalization (other than the end-of-line and attribute value normalization automatically performed by XML). For example,

element card {
  attribute name { text },
  attribute email { text },
  attribute preferredFormat { string "html" | string "text" }
}

will not match

<card name="John Smith" email="js@example.com" preferredFormat="  html  "/>

The list pattern matches a whitespace-separated sequence of tokens; it contains a pattern that the sequence of individual tokens must match. The list pattern splits a string into a list of strings, and then matches the resulting list of strings against the pattern inside the list pattern.

For example, suppose we want to have a vector element that contains two floating point numbers separated by whitespace. We could use list as follows:

element vector {
  list { xsd:float, xsd:float }
}

Or suppose we want the vector element to contain a list of one or more floating point numbers separated by whitespace:

element vector {
  list { xsd:double+ }
}

Or suppose we want a path element containing an even number of floating point numbers:

element path {
  list { (xsd:double, xsd:double)+ }
}

In addition to the , and | connectors, RELAX NG provides the & connector. This is useful when child elements are allowed in any order. For example, the following would allow the card element to contain the name and email elements in any order:

element addressBook {
  element card {
    element name { text }
    & element email { text }
  }*
}

The & connector is called the interleave connector 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:

head = element head { title & base? & style* & script* & link* & meta* }

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

meta*

and a sequence of one title element, which matches the child pattern

title

The semantics of the & connector are that a sequence of elements matches a pattern x & y if it is an interleaving of a sequence that matches x and a sequence that matches y. 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 interleaving 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 pattern is a shorthand for this.

mixed { p }

is short for

text & p

In all the examples up to now, the element and attribute keywords have been followed by a name, possibly qualified with a prefix. However, in general, the element and attribute keywords are followed by a name-class. A name is one particular simple kind of a name-class: a name specifies a name-class with that name as its only member. An element or attribute pattern will only match an element or attribute in the XML document if the name of the element of attribute is a member of the name-class in the pattern. Another simple kind of name-class is * which contains all names, regardless of their local name and namespace URI. For example, the following pattern matches any well-formed XML document:

start = anyElement
anyElement =
  element * {
    (attribute * { text }
     | text
     | anyElement)*
  }

A name-class ns:* contains all names with the namespace URI declared for the prefix ns.

Name-classes can be combined using the | connector. A name-class x | y contains the union of x and y. In other words, a name is a member of x | y if it is a member of x and/or a member of y.

Name-classes can also be combined using the - connector. A name-class x - y contains the difference of x and y. In other words, a name is a member of x - y if it is a member of x but not a member of y. The left-hand name-class to be combined with the - connector must be a * or ns:* name class. As with patterns, there is no implicit precedence between connectors and parentheses must be used to make precedence explicit. For example,

namespace local = ""
default namespace ex = "http://www.example.com"

element card {
  attribute * - (ex:* | local:*) { text }*,
  text
}

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, * must be used.

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 use the - connector. 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 example {
  attribute * { text }*,
  attribute xml:space { "default" | "preserve" }?
}

because an xml:space attribute with a value other than default or preserve would match

attribute * { text }

even though it did not match

attribute xml:space { "default" | "preserve" }

The solution is to use the - connector:

element example {
  attribute * - xml:space { text }*,
  attribute xml:space { "default" | "preserve" }?
}

Note that definitions cannot define name-classes; they can only define patterns.

In the absence of externally supplied information, a RELAX NG Compact Syntax file will be assumed to be in Unicode using either the UTF-8 or UTF-16 encoding. RELAX NG processors can automatically choose between UTF-8 and UTF-16 by using the byte order mark that almost all text editors automatically put at the beginning of a UTF-16 file. Although particular RELAX NG processors may allow you to use a legacy encoding, it is best to use UTF-8 or UTF-16 for interchange.

Unicode characters can be entered using an escape sequence of the form \x{N}, where N is the hex code of the character. For example, \x{A9} can be used to represent represent the copyright sign. Unlike XML character references, the \x escape sequence can be used anywhere, even in names of elements, attributes and definitions. For example,

element \x{E14}\x{E35} { empty }

When a RELAX NG pattern is to be used for purposes other than validation, it is often desirable to be able to annotate it with additional information. For example, if a RELAX NG pattern is intended to be read by a human, it is desirable to be able to annotate it with documentation; when a RELAX NG pattern is converted into another schema language, it is desirable to be able to annotate it with information to guide the conversion.

namespace doc = "http://www.example.com/documentation"

[doc:href="address.html#addressBook"]
element addressBook {
  [doc:href="address.html#card"]
  element card {
    [doc:href="address.html#name"]
    element name { text },
    [doc:href="address.html#email"]
    element email { text }
  }*
}

namespace a = "http://www.example.com/annotation"

element addressBook {
  [ a:documentation [ xml:lang="en" "Information about a single address." ] ]
  element card {
    element name { text },
    element email { text }
  }*
}

"A string can contain both '" ~ 'and ".'

"""A string can contain both ' and "."""

"Line 1\x{A}" ~
"Line 2"

'''Line 1
Line 2'''

## Represents an
## address book.
element addressBook {
  element card {
    element name { text },
    element email { text }
  }*
}

namespace a = "http://relaxng.org/ns/compatibility/annotations/1.0"

[
  a:documentation [
    "Represents an\x{A}" ~
    "address book.
  ]
]
element addressBook {
  element card {
    element name { text },
    element email { text }
  }*
}

namespace m = "http://www.example.com/module"

[ m:name = "inline" ]
div {
  code = pattern
  em = pattern
  var = pattern
}
[ m:name = "block" ]
div {
  p = pattern
  ul = pattern
  ol = pattern
}

There is no prohibition against nesting grammar patterns. A name refers to the definition from the innermost containing grammar pattern. There is also a parent pattern that escapes out of the current grammar and references a definition from the parent of the current grammar. A parent pattern consists of the parent keyword followed by the name of the definition.

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.rnc as follows:

cell.content = notAllowed
start =
  element table {
    element tr {
      element td { cell.content }+
    }+
  }

Patterns that include table.rnc must redefine cell.content. By using a nested grammar pattern containing a parent pattern, 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:

start =
  element doc {
    (element p { inline }
     | grammar {
         include "table.rnc" {
           cell.content = parent inline
         }
       })*
  }
inline =
  (text
   | element em { inline })*

Of course, in a trivial case like this, there is no advantage in nesting the grammars: we could simply have included table.rnc within the outer grammar pattern. 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 html {
  element head {
    element title { text }
  },
  element body {
    element table {
      attribute class { "addressBook" },
      element tr {
        attribute class { "card" },
        element td {
          attribute class { "name" },
          mixed {
            element span {
              attribute class { "givenName" },
              text
            }?,
            element span {
              attribute class { "familyName" },
              text
            }?
          }
        },
        element td {
          attribute class { "email" },
          text
        }
      }+
    }
  }
}

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>

This section describes advanced features, which most users will probably not need. These features exist primarily to ensure equivalence between the XML and compact syntaxes.

namespace ab = inherit

element ab:addressBook {
  element ab:card {
    element ab:name { text },
    element ab:email { text }
  }*
}

default namespace = "http://www.example.com/address"

external "address.rnc"

namespace ab = "http://www.example.com/address"

element ab:addressBook {
  element ab:card {
    element ab:name { text },
    element ab:email { text }
  }*
}

default namespace = inherit

namespace ab = inherit

element ab:addressBook {
  element ab:card {
    element ab:name { text },
    element ab:email { text }
  }*
}

namespace a = "http://www.example.com/address"

external "address.rnc" inherit = a

namespace ab = "http://www.example.com/address"

element ab:addressBook {
  element ab:card {
    element ab:name { text },
    element ab:email { text }
  }*
}

namespace x = "http://www.example.com"

start = foo

x:entity [ name="picture" systemId="picture.jpeg" notation="jpeg" ]

foo = element foo { empty }

namespace eg = "http://www.example.com"

element foo {
  text >> x[] >> y[]
}

<element name="foo" xmlns:eg="http://www.example.com">
  <text/>
  <eg:x/>
  <eg:y/>
</element>

The definitive specification of RELAX NG is [Specification], which uses the XML syntax. [Compact] is the definitive specification for the compact syntax, which defines the compact syntax by mapping it to the XML syntax.

A tutorial for the XML syntax is available separately [Tutorial].

[Guidelines] defines how to use the datatypes defined in [W3C XML Schema Datatypes] as a RELAX NG datatype library.