Well formed XML and DTD example

Well-formed XML document

A well-formed XML document follows the basic syntax rules of XML, so any XML parser can read it. Rules:

1. There must be exactly one root element.
2. Every start tag must have a matching end tag (or be self-closed, <br/>).
3. Elements must be properly nested (<a><b></b></a>, not <a><b></a></b>).
4. Tag names are case-sensitive (<Name> is not </name>).
5. Attribute values must be quoted, and an attribute cannot appear twice in an element.
6. Special characters must be escaped (<, >, &, ", ').
7. Names must start with a letter or underscore and contain no spaces.

Well-formed does not mean valid; valid means it also follows a DTD or schema.

DTD (Document Type Definition)

A DTD defines the allowed structure of an XML document: element names, their order and nesting, number of occurrences, and attributes. It can be internal (in the document) or external (a separate .dtd file).

Declarations: <!ELEMENT ...>, <!ATTLIST ...>, <!ENTITY ...>. Content symbols: , sequence, | choice, + one or more, * zero or more, ? optional, #PCDATA text.

Example

<?xml version="1.0"?>
<!DOCTYPE book [
  <!ELEMENT book (title, author+, price)>
  <!ATTLIST book isbn CDATA #REQUIRED>
  <!ELEMENT title  (#PCDATA)>
  <!ELEMENT author (#PCDATA)>
  <!ELEMENT price  (#PCDATA)>
]>
<book isbn="978-0-13-1">
  <title>Web Technology</title>
  <author>A. Sharma</author>
  <author>B. Rai</author>
  <price>650</price>
</book>

Here book must contain one title, one or more author elements and one price, in that order, and must carry the required attribute isbn. Anything else makes the document invalid.