RELAX NG

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RELAX NG
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What is RELAX NG?

• RELAX NG is a schema language for XML
• It is an alternative to DTDs and XML Schemas
• It is based on earlier schema languages, RELAX
  and TREX
• It is not a W3C standard, but is an OASIS
  standard
• OASIS is the Organization for the Advancement of
  Structured Information Standards
• ebXML (Enterprise Business XML) is a joint effort
  of OASIS and UN/CEFACT (United Nations Centre for
  Trade Facilitation and Electronic Business)
• OASIS developed the highly popular DocBook DTD
  for describing books, articles, and technical
  documents
• RELAX NG has recently been adopted as an ISO/IEC
  standard

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Design goals

• Simple and easy to learn
• Uses XML syntax
• But there is also a concise (non-XML) syntax
• Does not change the information set of an XML
  document
• (Im not sure what this means)
• Supports XML namespaces
• Treats attributes uniformly with elements so far
  as possible
• Has unrestricted support for unordered content
• Has unrestricted support for mixed content
• Has a solid theoretical basis
• Can make use of a separate datatyping language
  (such W3C XML Schema Datatypes)

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RELAX NG tools

• Assorted validators and translators are now
  available for RELAX NG
• See http//relaxng.org

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Basic structure

• A RELAX NG specification is written in XML, so it
  obeys all XML rules
• The RELAX NG specification has one root element
• The document it describes also has one root
  element
• The root element of the specification is element
• If the root element of your document is book,
  then the RELAX NG specifications begins
• ltelement name"book" xmlns"http//relaxng.org/
  ns/structure/1.0"gt
• and ends
• lt/elementgt

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Data elements

• RELAX NG makes a clear separation between
• the structure of a document (which it describes)
• the datatypes used in the document (which it gets
  from somewhere else, such as from XML Schemas)
• For starters, we will use the two (XML-defined)
  elements
• lttextgt ... lt/textgt (usually written lttext/gt)
• Plain character data, not containing other
  elements
• ltemptygtlt/emptygt (usually written ltempty/gt)
• Does not contain anything
• Other datatypes, such as ltdoublegt...lt/doublegtare
  not defined in RELAX NG
• To inherit datatypes from XML Schemas,
  usedatatypeLibrary"http//www.w3.org/2001/XMLSc
  hema-datatypes"as an attribute of the root
  element

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Data types from XML Schemas

• Here are some of the predefined numeric types

xsdecimal xspositiveInteger xsbyte xsnegat
iveInteger xsshort xsnonPositiveInteger xsint
xsnonNegativeInteger xslong
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Predefined date and time types

• xsdate -- A date in the format CCYY-MM-DD, for
  example, 2002-11-05
• xstime -- A date in the format hhmmss (hours,
  minutes, seconds)
• xsdateTime -- Format is CCYY-MM-DDThhmmss
• The T is part of the syntax
• Allowable restrictions on dates and times
• enumeration, minInclusive, minExclusive,
  maxInclusive, maxExclusive, pattern, whiteSpace

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Defining tags

• To define a tag (and specify its content),
  use ltelement name"myElement"gt lt!-- Content
  goes here --gt lt/elementgt
• Example The DTD lt!ELEMENT name (firstName,
  lastName)gt lt!ELEMENT firstName
  (PCDATA)gt lt!ELEMENT lastName (PCDATA)gt
• Translates to ltelement name"name"gt
  ltelement name"firstName"gt lttext/gt lt/elementgt
  ltelement name"lastName"gt lttext/gt
  lt/elementgt lt/elementgt
• Note As in the DTD, the components must occur in
  order

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RELAX NG describes patterns

• Your RELAX NG document specifies a pattern that
  matches your valid XML documents
• For example, the pattern
• ltelement name"name"gt ltelement
  name"firstName"gt lttext/gt lt/elementgt ltelement
  name"lastName"gt lttext/gt lt/elementgtlt/elementgt
• Will match the XML
• ltnamegt ltfirstNamegtDavidlt/firstNamegt
  ltlastNamegtMatuszeklt/lastNamegtlt/namegt

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Easy tags

• ltzeroOrMoregt ... lt/zeroOrMoregt
• The enclosed content occurs zero or more times
• ltoneOrMoregt ... lt/oneOrMoregt
• The enclosed content occurs one or more times
• ltoptionalgt ... lt/optionalgt
• The enclosed content occurs once or not at all
• ltchoicegt ... lt/choicegt
• Any one of the enclosed elements may occur
• lt!-- This is an XML comment it is not a
  container, and it may not contain two
  consecutive hyphens --gt

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Example

• ltelement name"addressList"gt ltzeroOrMoregt
  ltelement name"name"gt ltelement
  name"firstName"gt lttext/gt lt/elementgt
  ltelement name"lastName"gt lttext/gt lt/elementgt
  lt/elementgt ltelement name"address"gt
  ltchoicegt ltelement name"emailgt
  lttext/gt lt/elementgt ltelement
  name"USPost"gt lttext/gt lt/elementgt
  lt/choicegt lt/elementgt lt/zeroOrMoregtlt/elem
  entgt

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Enumerations

• The ltvaluegt...lt/valuegt pattern matches a
  specified value
• Exampleltelement name"gender"gt ltchoicegt
  ltvaluegtmalelt/valuegt
  ltvaluegtfemalelt/valuegt lt/choicegtlt/elementgt
• The contents of ltvaluegt are subject to whitespace
  normalization
• Leading and trailing whitespace is removed
• Internal sequences of whitespace characters are
  collapsed to a single blank

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More about data

• To inherit datatypes from XML Schemas, add this
  attribute to the root elementdatatypeLibrary
  "http//www.w3.org/2001/XMLSchema-datatypes"
• You can access the inherited types with the
  ltdatagt tag, for instance, ltdata type"doublegt
• The ltdatagt pattern must match the entire content
  of the enclosing tag, not just part of it
• ltelement name"illegalUse"gt lt!-- Don't do this!
  --gt ltdata type"double"/gt ltelement
  name"moreStuff"gt lttext/gt lt/elementgtlt/elementgt
• If you don't specify a datatype library, RELAX NG
  defines the following for you (along with lttext/gt
  and ltempty/gt)
• ltstring/gt No whitespace normalization is done
• lttoken/gt A sequence of characters containing no
  whitespace

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ltgroupgt

• ltgroupgt...lt/groupgt is used as fat parentheses
• Example
• ltchoicegt ltelement name"name"gt lttext/gt
  ltelementgt ltgroupgt ltelement
  name"firstName"gt lttext/gt
  lt/elementgt ltelement name"lastName"gt
  lttext/gt lt/elementgt
  lt/groupgtlt/choicegt

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Attributes

• Attributes are defined practically the same way
  as elements
• ltattribute name"attributeName"gt...lt/attributegt
• Example
• ltelement name"name"gt ltattribute name"title"gt
  lttext/gt lt/attributegt ltelement
  name"firstName"gt lttext/gt lt/elementgt ltelement
  name"lastName"gt lttext/gt lt/elementgtlt/elementgt
• Matches
• ltname title"Dr."gt ltfirstNamegtDavidlt/firstNamegt
  ltlastNamegtMatuszeklt/lastNamegtlt/namegt

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More about attributes

• With attributes, as with elements, you can use
  ltoptionalgt, ltchoicegt, and ltgroupgt
• It doesnt make sense to use ltoneOrMoregt or
  ltzeroOrMoregt with attributes
• In keeping with the usual XML rules,
• The order in which you list elements is
  significant
• The order in which you list attributes is not
  significant

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Still more about attributes

• ltattribute name"attributeName"gt lttext/gt
  lt/attributegt can be (and usually is)
  abbreviated asltattribute name"attributeName"/gt
• However,ltelement name"elementName"gt lttext/gt
  lt/elementgt can not be abbreviated asltelement
  name"elementName"/gt
• If an element has no attributes and no content,
  you must use ltempty/gt explicitly

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ltlistgt

• ltlistgt pattern lt/listgt matches a
  whitespace-separated list of tokens, and applies
  the pattern to those tokens
• Examplelt!-- A floating-point number and some
  integers --gtltelement name"vector"gt ltlistgt
  ltdata type"float"/gt ltoneOrMoregt
  ltdata type"int"/gt lt/oneOrMoregt
  lt/listgtlt/elementgt

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ltinterleavegt

• ltinterleavegt ... lt/interleavegt allows the
  contained elements to occur in any order
• ltinterleavegt is more sophisticated than you might
  expect
• If a contained element can occur more than once,
  the various instances do not need to occur
  together

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Interleave example

• ltelement name"contactInformation"gt
  ltinterleavegt ltzeroOrMoregt
  ltelement name"phone"gt lttext/gt lt/elementgt
  lt/zeroOrMoregt ltoneOrMoregt
  ltelement name"email"gt lttext/gt lt/elementgt
  lt/oneOrMoregt lt/interleavegtlt/elementgt
• ltcontactInformationgt ltemailgtdave_at_acm.orglt/ema
  ilgt ltphonegt215-898-8122lt/phonegt
  ltemailgtdavebiz_at_matuszek.orglt/emailgtlt/contactInfor
  mationgt

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ltmixedgt

• ltmixedgt allows mixed content, that is, both text
  and patterns
• If pattern is a RELAX NG pattern, then ltmixedgt
  pattern lt/mixedgtis shorthand for ltinterleavegt
  lttext/gt pattern lt/interleavegt

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Example of ltmixedgt

• Pattern
• ltelement name"words"gt ltmixedgt
  ltzeroOrMoregt ltchoicegt
  ltelement name"bold"gt lttext/gt lt/elementgt
  ltelement name"italic"gt lttext/gt
  lt/elementgt lt/choicegt
  lt/zeroOrMoregt lt/mixedgtlt/elementgt
• Matches
• ltwordsgtThis is ltitalicgtnotlt/italicgt a
  ltboldgtgreatlt/boldgt example, ltitalicgtbutlt/italicgt
  it should suffice.lt/wordsgt

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The need for named patterns

• So far, we have defined elements exactly at the
  point that they can be used
• There is no equivalent of
• lt!ELEMENT person (name)gtlt!ELEMENT name
  (firstName, lastName)gt...use person several
  places in the DTD...
• With the RELAX NG we have discussed so far, each
  time we want to include a person, we would need
  to explicitly define both person and name at that
  point
• ltelement name"person"gt ltelement
  name"firstName"gt lttext/gt lt/elementgt ltelement
  name"lastName"gt lttext/gt lt/elementgtlt/elementgt
• The ltgrammargt element solves this problem

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Syntax of ltgrammargt

• ltgrammar xmlns"http//relaxng.org/ns/structure/1
  .0"gt
• ltstartgt
• ...usual RELAX NG elements, which may include
• ltref name"DefinedName"/gt
• lt/startgt
• lt!-- One or more of the following --gt
• ltdefine name"DefinedName"gt
• ...usual RELAX NG elements, attributes, groups,
  etc.
• lt/definegt
• lt/grammargt

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Use of ltgrammargt

• To write a ltgrammargt,
• Make ltgrammargt the root element of your
  specification
• Hence it should say xmlns"http//relaxng.org/ns/s
  tructure/1.0"
• Use, as the ltstartgt element, a pattern that
  matches the entire (valid) XML document
• In each ltdefinegt element, write a pattern that
  you want to use other places in the specification
• Wherever you want to use a defined element,
  putltref name"NameOfDefinedElement"gt
• Note that defined elements may be used in
  definitions, not just in the ltstartgt element
• Definitions may even be recursive, but
• Recursive references must be in an element, not
  an attribute

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Long example of ltgrammargt

• lt!ELEMENT name (firstName, lastName)gt
• ltgrammar xmlns"http//relaxng.org/ns/structure/1
  .0"gt ltstartgt ltref name"Name"/gt
  lt/startgt ltdefine name"Name"gt
  ltelement name"name"gt ltelement
  name"firstName"gt lttext/gt lt/elementgt
  ltelement name"lastName"gt ltref
  name"LastName"gt lt/elementgt
  lt/elementgt lt/definegt ltdefine
  name"LastName"gt ltelement
  name"lastName"gt lttext/gt lt/elementgt
  lt/definegtlt/grammargt

XML is case sensitive--Note that defined terms
are capitalized differently
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Common usage I

• A typical way to use RELAX NG is to use a
  ltgrammargt with just the root element in ltstartgt
  and every element described by a ltdefinegt
• ltgrammar xmlns"http//relaxng.org/ns/structure/1
  .0"gt ltstartgt ltref name"NOVEL"gt
  lt/startgt ltdefine name"NOVEL"gt
  ltelement name"novel"gt ltref
  name"TITLE"/gt ltref name"AUTHOR"/gt
  ltoneOrMoregt ltref
  name"CHAPTER"/gt lt/oneOrMoregt
  lt/elementgt lt/definegt ...more...

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Common usage II

• ltdefine name"TITLE"gt ltelement name"title"gt
  lttext/gt lt/elementgtlt/definegt
• ltdefine name"AUTHOR"gt ltelement
  name"author"gt lttext/gt
  lt/elementgtlt/definegt

• ltdefine name"CHAPTER"gt ltelement
  name"chapter"gt ltoneOrMoregt
  ltref name"PARAGRAPH"/gt lt/oneOrMoregt
  lt/elementgtlt/definegt
• ltdefine name"PARAGRAPH"gt ltelement
  name"paragraph"gt lttext/gt
  lt/elementgt lt/definegt
• lt/grammargt

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Replacing DTDs

• With ltgrammargt and multiple ltdefinegts, we can do
  essentially the same things as a DTD
• Advantages
• RELAX NG is more expressive than a DTD we can
  interleave elements, specify data types, allow
  specific data values, use namespaces, and control
  the mixing of data and patterns
• RELAX NG is written in XML
• RELAX NG is relatively easy to understand
• Disadvantages
• RELAX NG is extremely verbose
• But there is a compact syntax that is much
  shorter
• RELAX NG is not (yet) nearly as well known
• Hence there are fewer tools to work with it
• This situation seems to be changing

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The End
So by this maxim be impressed, USE THE TOOLS THAT
WORK THE BEST. Do not yield your sovereign
judgment, To any sort of political fudgement. The
criterion of sound design Should be, must be,
your guideline. And if you're designing
documents, Try RNG. We charge no rents.
--
John Cowan