XML Syntax Rules

In this section, we explain the various syntactical rules of XML. Documents that follow these rules are called well-formed, but not necessarily valid, as we'll see. If your document breaks any of these rules, it will be rejected by most, if not all, XML parsers.

Well-Formedness

The minimal requirement for an XML document is that it be well-formed, meaning that it adheres to a small number of syntax rules, ⁶ which are summarized in Table 3-1 and explained in the following sections. However, a document can abide by all these rules and still be invalid. To be valid, a document must both be well-formed and adhere to the constraints imposed by a DTD or XML Schema.

Table 3.1 XML Syntax Rules (Well-Formedness Constraints)
• The document must have a consistent, well-defined structure.
• All attribute values must be quoted (single or double quotes).
• White space in content, including line breaks, is significant.
• All start tags must have corresponding end tags (exception: empty elements).
• The root element must contain all others, which must nest properly by start/end tag pairing.
• Elements must not overlap; they may be nested, however. (This is also technically true for HTML. Browsers ignore overlapping in HTML, but not in XML.)
• Each element except the root element must have exactly one parent element that contains it.
• Element and attribute names are case-sensitive: Price and PRICE are different elements.
• Keywords such as DOCTYPE and ENTITY must always appear in uppercase; similarly for other DTD keywords such as ELEMENT and ATTLIST.
• Tags without content are called empty elements and must end in "/>".

Legal XML Name Characters

An XML Name (sometimes called simply a Name ) is a token that

• begins with a letter, underscore, or colon (but not other punctuation)
• continues with letters, digits, hyphens, underscores, colons, or full stops [periods], known as name characters.

Names beginning with the string "xml", or any string which would match ((`X'|`x')(`M'|`m')(`L'|`l')), are reserved.

Element and attribute names must be valid XML Names. (Attribute values need not be.) An NMTOKEN (name token) is any mixture of name characters (letters, digits, hyphens, underscores, colons, and periods).

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Note:
The Namespaces in XML Recommendation assigns a meaning to names that contain colon characters. Therefore, authors should not use the colon in XML names except for namespace purposes (e.g., xsl:template).

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Listing 3-2 illustrates a number of legal XML Names, followed by three that should be avoided but may or may not be identified as illegal, depending on the XML parser you use, and four that are definitely illegal. (This is file name-tests.xml on the CD; you can try this with your favorite parser, or with one of the ones provided on the CD.)

Listing 3-2 Legal, Illegal, and Questionable XML Names

<?xml version = "1.0" standalone = "yes" encoding = "UTF-8"?>
<Test>
<!-- legal -->
    <price />
    <Price />
    <pRice />
    <_price />
    <subtotal07 />
    <discounted-price />
    <discounted_price />
    <discounted.price />
    <discountedPrice />
    <DiscountedPrice />
    <DISCOUNTEDprice />
    <kbs:DiscountedPrice />
    <xlink:role />
    <xsl:apply-templates />
<!-- discouraged -->
    <xml-price />
    <xml:price />
    <discounted:price />
<!-- illegal -->
    <7price />
    <-price />
    <.price />
    <discounted price />
</Test>

From the legal examples, we see that any mixture of uppercase and lowercase is fine, as are numbers, and the punctuation characters that were in the definition.

Since the last three examples in the first group use a colon, they are assumed to be elements in the namespaces identified by the prefixes "kbs", "xlink", and "xsl". Of these, the last two refer to W3C-specified namespaces; xlink:role is an attribute defined by the XLink specification and xsl:apply-templates is an element defined by the XSLT specification. The "kbs" prefix refers to a hypothetical namespace, which I could have declared (but didn't), since namespaces do not come only from the W3C. (See chapter 5 for a thorough discussion of namespaces.)

The three debatable examples are xml-price, xml:price, and discounted: price. The first two use the reserved letters "xml"; you shouldn't use them, but most parsers won't reject them. The discounted:price example uses a colon, which is frowned upon if "discounted" is not meant to be a prefix associated with a declared namespace.

The four illegal cases are much more clear. The first three, 7price, -price, and .price, are illegal because the initial character is not a letter, underscore, or colon. The fourth example is illegal because a space character cannot occur in an XML Name. Most parsers will think this is supposed to be the element named discounted and the attribute named price, minus a required equal sign and value.

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6. See the well-formedness discussion in the XML 1.0 Recommendation, http://www.w3.org/TRREC-xml#sec-well-formed/ .

Markup, Character Data, and Parsing
XML Family of Specifications: A Practical Guide
Elements and Attributes Are Case-Sensitive