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Sams Teach Yourself XML in 21 Days

Sep 16, 2005

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What Makes an XML Document Well-Formed?

The W3C, which is responsible for the term well-formedness, defines it this way in the XML 1.0 recommendation:

A textual object is a well-formed XML document if:

Taken as a whole, it matches the production labeled document.
It meets all the well-formedness constraints given in this specification (that is, the XML 1.0 specification, http://www.w3.org/TR/REC-xml).
Each of the parsed entities, which is referenced directly or indirectly within the document, is well-formed.

Because the major differences between XML 1.0 and XML 1.1 have to do with what characters are legal, you probably won't be surprised to learn that a well-formed XML 1.0 document is also a well-formed XML 1.1 document, as long as it avoids certain characters. From the XML 1.1 specification:

If a document is well-formed or valid XML 1.0, and provided it does not contain any characters in the range [#x7F-#x9F] other than as character escapes, it may be made well-formed or valid XML 1.1 respectively simply by changing the version number.

Let's get into three conditions that make an XML document well-formed, starting with the requirement that the document must match the production named document.

Matching the Production Labeled `document`

W3C calls the individual specifications within a working draft or recommendation productions. In this case, to be well-formed, a document must follow the document production, which means that the document itself must have three parts:

a prolog (which can be empty)
a root element (which can contain other elements)
a miscellaneous part (unlike the preceding two parts, this part is optional)

You've seen XML prologs yesterday; they can contain an XML declaration (such as <?xml version = "1.0"?>), as well as comments, processing instructions, and doctype declarations (that is, DTDs).

You've also seen root elements; the root element is the XML element that contains all the other elements in your document. Each well-formed XML document must have one, and only one, root element.

The optional miscellaneous part can be made up of XML comments, processing instructions, and whitespace, all items you saw yesterday.

In other words, this first requirement says that an XML document must be made up of the parts you saw yesterday. So far, so good.

Meeting the Well-Formedness Constraints

The next requirement is a little more difficult to track down, because it says that to be well-formed, XML documents must also satisfy the well-formedness constraints in the XML 1.0 specification. This means that your XML documents should adhere to the syntax rules specified in the XML 1.0 recommendation. You'll discuss those rules, which are sprinkled throughout the XML 1.0 specification, in a few pages.

Making Parsed Entity Must Be Well-Formed

The final requirement is that each parsed entity in a well-formed document must itself be well-formed. When an XML document is parsed by an XML processor, entity references (such as π) are replaced by the entities they stand for (such as in this case). The requirement that all parsed entities must be well-formed simply means that when you replace entity references with the entities they stand for, the result must be well-formed.

That's the W3C's definition of a well-formed document, but you still need more information. What are the well-formedness constraints given throughout the XML specification? You're going to go over these constraints today; to start, you'll create an XML document that you'll use as we discuss what it means for a document to be well-formed.

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