PB138 — Relax NG
What is Relax NG
• Specified by OASIS under OASIS RELAX NG TC
• "Relax New Generation" modeling language for XML
• Simpler and easier to use than XML Schema
• Based on Relax (REgular LAnguage for XML) and
• TREX (Tree Regular Expressions for XML by James Clark)
What is Relax NG (2)
• Having both XML and non-XML syntax improving readability
• published first in 2003
• later became ISO/IEC 19757 standard
• uses .rng and .rnc file extensions
Motivation
XML Schema is an industry standard but:
• too complicated (more than 200 pages of specification)
• ambiguous in some situations
• tries to cover all applications area (documents, databases and all in between)
• hardly fully implemented
• see http://www.xml.com/lpt/a/2002/01/23/relaxng.html for more
More on Relax NG advantages
• is simple
• is easy to learn
• has both an XML syntax (.rng files) or a compact non-XML syntax (.rnc files)
• does not change the information set of an XML document
• supports XML namespaces
• treats attributes uniformly with elements so far as possible
• has unrestricted support for unordered content
• has unrestricted support for mixed content
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• has a solid theoretical basis
• can partner with a separate datatyping language (such W3C XML Schema Datatypes)
Specifications, Resources
Based on RELAX designed by OASIS-OPEN:
• http://www.oasis-open.org/committees/relax-ng
Now ISO standard:
• the ISO/IEC 19757 standard can be downloaded
RELAX NG home page (Key resource to Relax NG!):
• http://relaxng.org
RELAX NG — an excellent book by Eric van der Vlist:
• http://books.xmlschemata.org/relaxng/
Relax NG Basic Tools
Validators:
• online https://validator.nu/
Applications:
• Java-written Jing
• mainly for Linux is the C application libxml2
• RNV - supports the compact syntax only
• See http://relaxng.org/#validators for more.
Relax NG Editors etc.
Editors, other tools:
• Firedocs — a Firefox plug-in (XML editor)
• XML Operator — OSS (BSD Licence)
• XML editors supporting Relax NG such as <oXygen/> xml editor — commercial
• See Relax NG editors for more
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Compact syntax
Following samples are from http://relaxng.org/compact-tutorial-20030326.html:
<addressBook>
  <card>
  <name>John Smith</name>
  <email>js@example.com</email>
  </card>
  <card>
  <name>Fred Bloggs</name>
  <email>fb@example.net</email>
  </card>
</addressBook>
DTD model
• The same model would be expressed in DTD as follows:
<!DOCTYPE addressBook [
<!ELEMENT addressBook (card*)>
<!ELEMENT card (name, email)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT email (#PCDATA)>
]>
RELAX NG pattern
element addressBook {
  element card {
  element name { text },
  element email { text }
  }*
}
Cardinality
If the addressBook is required to be non-empty, then we can use + instead of *:
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element addressBook {
  element card {
  element name { text },
  element email { text }
  }+
}
Optional
Now let’s change it to allow each card to have an optional note 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.
element addressBook {
  element card {
  element name { text },
  element email { text },
  element note { text }?
  }*
}
# and this is a comment
Choice
If we want to have a choice of two (more) alternatives for one content model (eg. in card):
<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>
Choice (2)
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element addressBook {
  element card {
  # here is the option
  (element name { text }
  | (element givenName { text },
  element familyName { text })),
  element email { text },
  element note { text }?
  }*
}
Attributes
Specifies that addressBook has attributes instead of elements:
element addressBook {
  element card {
  attribute name { text },
  attribute email { text }
  }*
}
Alternatively attributes or elements
• 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 })
  }*
}
Then all alternatives are valid
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<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>
Defining a grammar
grammar {
  start = AddressBook
  AddressBook = element addressBook { Card* }
  Card = element card { Name, Email }
  Name = element name { text }
  Email = element email { text }
}
• Uppercase names are type names, while
• lowercase names are element names in this example.
• Elsewhere it could be different but still there can be types and concrete occurencies.
Recursion
• The "inline" content is defined recursively
• This is, of course, allowed for elements only (attributes cannot be nested).
inline =
  (text
  | element bold { inline }
  | element italic { inline }
  | element span {
  attribute style { text }?,
  # here is the recursion
  inline
  })*
Datatyping
• Relax NG allows the use of external typing systems, such as the one from XML Schema W3C
XML Schema Datatypes.
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element number { xsd:integer }
element numberWithNote {
  xsd:int,
  attribute note { text }
}
element email {
  xsd:string { minLength = "6" maxLength = "127" }
}
Enumerations for attributes
element card {
  attribute name { text },
  attribute email { text },
  attribute preferredFormat { "html" | "text" }
}
Enumerations for text content
element card {
  element name { text },
  element email { text },
  element preferredFormat { "html" | "text" }
}
Lists
• lists of primitive values separated by white space(s)
• the cardinality of occurencies can be specified
element list_of_two_floats {
  list { xsd:float, xsd:float }
}
element list_of_some_doubles {
  list { xsd:double+ }
}
element list_of_some_double_pairs {
  list { (xsd:double, xsd:double)+ }
}
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Interleave
• for specification of occurence in any order
• example: name, email
element addressBook {
  element card {
  element name { text }
  & element email { text }
  }*
  }
Modularity
• we can (re)use some external .rnc schema
start = inline
inline =
(text
  | element code { inline }
  | element em { inline }
  # etc
)*
Modularity (2)
Then we could allow the note element to contain inline HTML markup by using external as follows:
element addressBook {
  element card {
  element name { text },
  element email { text },
  element note { external "inline.rnc" }?
  }*
}
Namespaces
• prefixed or default namespaces can be used
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namespace eg = "http://www.example.com"
element eg:foo { empty }
namespace = "http://www.example.com"
element foo { empty }
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