Pitt-Greensburg Digital Studies Logo: I Code!


Maintained by: Elisa E. Beshero-Bondar (ebb8 at pitt.edu) Creative Commons License
Last modified:


When we began writing XML, we learned about well-formedness: that all XML documents must have a single root element, all elements must be properly nested, and element and attribute tags must be written correctly so that a computer can parse a file as XML. But we also code XML documents for human beings wishing to work in an organized and systematic way, and for this reason, we write schema rules and we apply schema files in our projects to check our XML against the rules we create. When we check our XML files against a set of schema rules, we are checking their validity. We can use validity checks to make sure we’re spelling element names properly, writing attribute names and values consistently, and nesting elements in a way that makes sense to us that we want to hold consistent: as for example, always nesting a <title> element inside a <heading> element.

Writing schema rules is closely related to performing a document analysis to record the key features of a text that you wish to mark up in XML. When you write a schema, you formalize those rules. Over the course of a project, people tend to modify their coding rules and frequently revise their schema files. In turn coders on a project apply updated schema rules to update their coding, change their tag names, the order of particular tags, the values they are inputting for attributes, etc. Knowing how to write a schema and knowing how to apply it and update it can thus give you the know-how necessary for managing and streamlining your work, and a team’s work on an XML project.

Relax NG: A Little Background

RELAX NG is an acronym for REgular LAnguage description for XML Next Generation, and as the name implies, it took some “generational” cycles or a long period of time to develop, roughly concurrent with the early history of XML. There are a couple of different “syntaxes” for writing Relax NG, an XML-based version and a Compact version. The XML-based version is written as XML code with elements and attributes, which makes it a little more difficult to write and follow than the Compact version, so we’ve opted for the Compact version for our work. Later on, we’ll be learning an XML-based schema language called Schematron that we can use for specialized, targeted needs in project development. For now, we want you to be aware that there are a few different ways to write schema languages for XML projects, and that we are learning one specific way that was designed from the outset to be “simple and easy to learn,” that is, optimized for organizing and managing XML projects. (Credits: For further reading on the background of Relax NG and its formative goal of being “simple and easy to learn,” see Eric van der Vlist’s online book, and the collection of resources at http://c2.com/cgi/wiki?RelaxNg. We’ve drawn from David Birnbaum’s tutorial on Relax NG to help produce this page. )

Relax NG Compact Syntax: How We Write It

In the Compact Syntax of Relax NG, we write rule statements in the form of named patterns, that look like this:

label = type name {content}

Each named pattern in Compact Syntax takes this basic form: The label is a name for the XML “object” you’re describing; type indicates what kind of object it is, an element or an attribute; name is the literal name of the element or attribute; and {content} indicates what the element or attribute is allowed to contain (such as: other elements, an attribute value of a particular kind, an attribute, text, etc. The label can be anything you find convenient, but it’s usually easiest for us to deal with when the label is the same as the name. Here’s an example in practice, from the very beginning of a file:

start = root
root = element root {head, body}

Our first rule in Relax-NG needs to indicate how we begin, and it always has the label “start,” which is a reserved word for this purpose in Relax-NG. Here, we’re defining the root element for our XML file, and we’re simply using the label and element name “root” for it. (If you wanted to call the root element something else, of course you can, and this is the place to specify that. So if you wanted to call it, “letter,” you’d write:

start = letter
letter = element letter {head, body}

The second line here defines letter as an element named letter, and it holds inside the curly braces { } what the root element is allowed to contain: in this case exactly one head and one body. We need to write more rules to define head and body as elements, and to specify what they will contain.

Defining Content Models: { }

The part of the rule wrapped in curly braces is called the “content model,” and it’s where we essentially write the rules for what an element or attribute is allowed to contain in our nested XML hierarchy. The words inside the curly braces are, themselves, labels which must be defined somewhere in the schema. The way we write the rule specifies what’s allowed inside, in what order, and how many of it we permit. The order we set is determined by our combination indicators, and the quantity we expect is determined by repetition indicators.

Combination Indicators:

We commonly use two combination indicators for multiple kinds of combinations: One kind of combination is a sequence, in which we want elements or attributes to come in a particular order. The second kind of combination is a choice in which we indicate that either one OR another is permitted, but not both. (Sidenote: There’s a third kind of combination indicator called “interleave,” which is signalled with an ampersand ( & ), but we won’t really need to use it once we show you “grouping” below.)

Repetition Indicators

We use four repetition indicators, to indicate HOW MUCH of something we will permit:

Grouping with Parentheses ( )

Using parentheses, we can group content items and apply the combination and repetition indicators above to make very precise rules. When we use parentheses, these are read by the computer parser from the inside out. For example, here is a rule to control how to organize entries in an XML bibliography citation. Notice the grouping around author | editor:

Rules for Elements, Attributes, Plain Text and Other Data Types

So far, we’ve just been showing you rules that involve element names. But schemas need to set rules for all the content of an XML document, and that includes attribute names and values, as well as whether an element contains plain text or only other elements, or nothing at all. When an element is permitted to contain both plain text as well as other elements and/or attributes, it is said to have mixed content. When it contains nothing at all, as a self-closing element, it’s said to be empty.

Rules for Elements with Attributes:

Here is an example demonstrating a rule for element to contain an attribute, followed by a rule defining the attribute and indicating a closed list of its permitted values:

location = element location {type, text}
type = attribute type {"astron" | "atmos" | "ocean" | "river" | "lake" | "island" | "mount" }

Rules for Empty (Self-Closing) Elements:

Elements are empty when they contain no text and no other elements. These are self-closing elements, and here’s an example of how to write a rule for one:

lineBreak = element lineBreak { empty }

This uses a special reserved word in Relax NG called empty, used especially for this purpose. If we wanted to designate an element named “empty” we’d need to “escape” the reserve character. (More on this below.)

Empty elements ARE allowed to have attributes, and if we’re using page breaks or line breaks or other “milestone” self-closing elements, we frequently use attributes to hold information. Here’s an example of a rule designating an empty element with attributes:

pageBreak = element pageBreak {n, empty}
n = attribute n {xsd:int}

In this example, the attribute n is given a special value, xsd:int, which means in must contain an integer (or number). This is an example of using a data type, and we’ll give more examples of this below.

Rules for Text and Mixed Content:

We need to designate some elements to contain the text of a document, and to do that Relax NG has a special reserved word, text. (We’re not allowed, without a workaround, to use that reserved word text as an element name, and we’ll show you a couple of workarounds below.) So, let’s say we have an element that we only want to contain plain text. Here’s how we designate it, using the reserve word:

name = element name {text}

Often, though, we’ll want to designate an element to contain a mix of plain text, an attribute (or more), and other elements. Imagine a prose paragraph you’ve coded, as a mix of text and elements marking off names, dates, locations, grammar, etc. In these situations, we have an alternate way to designate that text can appear at any point, interspersed with element content:

paragraph = element paragraph {mixed {(name | location | date)*}}

This is a more complex syntax than you’ve seen before! Let’s walk through it: First, we say that the element paragraph contains mixed content, with the word mixed in the outer set of curly braces: That word “mixed” indicates there’s plain text floating around in any order with the element content that follows. Notice there’s an inner set of curly braces that we always use after mixed, and that contains the element content. We’ll go on to define name, location, and date as elements with their own rules.

Now, a review of grouping: In the above example, we made this internal grouping: (name | location | date)*. This means that a paragraph may contain none, some, or all three of these element choices, in any order, so it’s a very versatile expression for keeping our options open! How would it be different if we wrote the following? (name | location | date)+ (Hint: look up at the Grouping section to review this.)

Data Types (such as unique identifiers, numbers, or standard date entries):

We may want to constrain the contents of an element or attribute to specialized kinds of data. For example, if we’re coding a list of people who show up in a big pile of documents, and who might show up under different names, we might want to designate a core list of person elements that contain a unique identifier in an attribute value, sort of an id-card in XML to help us keep track of them. It’s traditional in XML to designate an @xml:id to hold a unique identifier that cannot be used anywhere else, and for this there’s a data type called xsd:ID. Notice two things about this example: 1) Relax NG apparently doesn't permit colons inside labels, so our label is simply "xmlid" to escape the problem, and 2) We’ve written this so the identifying info can all be held inside an empty (self-closing) element!)

person = element person { xmlid, type, gender, empty }
xmlid = attribute xml:id { xsd:ID }
type = attribute type { text }
gender = attribute gender { "m" | "f" }

If we want to indicate that an element or an attribute may only contain a number (or integer value), use the xsd:int data type:

year = element year {xsd:int}

Here are examples of how we constrain a date element to contain attributes, either @when to indicate one specific date, OR a pair of attributes, @from and @to, used together to indicate a span of time, like this <date from="1976-07-05" to="1980-08-15"/>. We’re using standard ISO format for dates, which is represented like this: yyyy-mm-dd. However, we might imagine that in a given document, we might not know every part of a date. Maybe we just have a year, or a year and month. There are different data type values for each option, which we give below, since this is something we frequently need to indicate in our projects. Notice how we’ve written these out as options. The attribute value must be written in one of the three different data type formats:

date = element date {(when | (from, to)), text}
when = attribute when {xsd:date | xsd:gYearMonth | xsd:gYear}
from = attribute from {xsd:date | xsd:gYearMonth | xsd:gYear}
to = attribute to {xsd:date | xsd:gYearMonth | xsd:gYear}

Now, that example works just fine, but it's repetitive. We can set a pattern in Relax-NG if we create a complicated content model and need to use it on multiple elements or attributes. We show you how below.

Writing Patterns (so you don't have to repeat yourself):

In the previous example we used the same content model repeatedly for three different attributes, and we really don't have to do that. If you've modelled something complicated in your curly braces { } and want to re-use it on multiple elements or attributes, we recommend setting up a pattern, which just involves setting a label = ( a pattern ). Note we use parentheses for this (not curly braces). Here's how to set and refer to a pattern:

datePattern = (xsd:date | xsd:gYearMonth| xsd:gYear)
date = element date {(when |(from, to)), empty}
when = attribute when {datePattern}
from = attribute from {datePattern}
to = attribute to {datePattern}

Escaping a reserved word so we can use it as a label:

If you need to use the word, text, as label in a rule, you’ll have to “escape” it by using a backslash in front, like this: \text. (Or, you can use a different word entirely for the label.) The same goes for the other reserve words in Relax NG, including the word, list. Here’s a link to the complete list of “keywords” (or reserved words) in Relax NG.

Note that when you escape a reserved word, you only escape it in the label position, remembering that we use labels at the beginning of a rule and refer to them inside the { } curly braces. Note: You are not permitted to escape the word when it is an element or attribute name: those names must be literal (and the backslash escape character is not needed or allowed there).

Here are some example rules involving us escaping a reserved word. Either approach (version 1 or version 2) is fine; they result in the same thing. Notice where we use the escape character, and where we don't!

Version 1 (with the reserved word, list):

chapter = element chapter {mixed {(paragraph | \list)*}}
\list = element list {item+}
item = element item {mixed {\list*}}

Version 2 (with the same reserved word, list):

chapter = element chapter {mixed {(paragraph | listing)*}}
listing = element list {item+}
item = element item {mixed {listing*}}

The Big Picture: A Sample XML File, and Its Schema Rule

We borrow this sample code, with appreciation, from Obdurodon, to show you a sample, fairly short XML file, and its Relax NG Schema:

<play>
<heading>
<title>The Importance of Being Ernest</title>
<year>1895</year>
</heading>
<body>
<quote speaker="Algernon">I don’t know that I am much interested in your family life, Lane.</quote>
<quote speaker="Lane">No, sir; it is not a very interesting subject. I never think of it myself.</quote>
</body>
</play>

Here’s a complete Relax NG schema for this sample XML file. In order for the schema to be complete and for it to function as a valid schema for the XML, it must account for every component of the XML document, all of its elements, attributes and text. Our friends at Obdurodon give an example of the schema below missing one line, and indicate how this will throw off the entire schema validation. Every element and attribute must be defined for your schema to work and be valid.

start = play
play = element play {heading, body}
heading = element heading {title, year}
title = element title {text}
year = element year {xsd:int}
body = element body {quote*}
quote = element quote {speaker, text}
speaker = attribute speaker {text}

Using <oXygen/> to Write a Relax NG Schema

To start writing a Relax NG Schema in Compact Syntax, go to File and New, or click on the icon like a sheet of paper in the upper left corner. In the menu, look under “New” or “Recently Used” for “RELAX NG Schema - Compact,” and select it and click the “create” button. Just delete all the boilerplate lines at the top of the file and start with a completely blank file. Begin by typing start = and designate your root element as we discussed above. Keep writing until you’ve completely defined rules for the full content of your XML document. (Notice that as you write, <oXygen/> generates errors until you’ve defined each new label you enter.) Save your work somewhere convenient. (We usually save our schema files in the same folder as our XML.)

How to Associate a Schema and Run a Validation Check in <oXygen/>

In order to run a validation check on your XML file, you need to associate your Relax NG schema with the XML. To do this, in <oXygen/> open the XML file you want to validate. Go to “Document” on the top menu bar, and on the drop-down menu, select “Schema,” and then “Associate Schema.” Then browse for the Relax NG Schema you wrote, select it and click “OK.” When you successfully associate your schema, <oXygen/> inserts a line of code at the top of your document, called a “schema line.”, which looks like this:

<?xml-model href="ExampleRelaxNG.rnc" type="application/relax-ng-compact-syntax"?>

If your document fails its schema validation, the square in the upper left corner turns red (just as it does for a formedness error), only you’ll see a different error message for failing a validity check. To be sure you’re running a validity check (or if you’ve just edited your schema file and want to re-run it), select the icon that looks like a red check-mark on a piece of paper, and oXygen will run a fresh validation check. (The keyboard shortcut in Windows for running a validation check is Ctrl+Shift+v, and on a Mac the shortcut is Cmd+Shift+v.) One more thing! When you've associated a schema once with your XML file, you don't need to repeat the association! Once you have your schema line at the top of the XML file, you can simply update your Relax NG file, save it, and simply re-run the validation check in <oXygen/>.