XML - Validation

Validation (in XML) means checking whether an XML document follows the rules defined in a DTD (Document Type Definition) or an XSD (XML Schema Definition).

• Well-formed XML:

  • Basic requirement: the XML follows the syntax rules (tags are properly opened/closed, attributes quoted, etc.).

  • Example: <book><title>XML Guide</title></book> is well-formed.

• Valid XML:

  • Goes one step further: the document not only is well-formed, but it also conforms to a specific structure described in a DTD or XSD.

  • A DTD/XSD acts like a blueprint or contract for the XML.

  • Validation checks:

    • Correct element order and nesting

    • Allowed/required attributes

    • Data types of values (especially with XSD, which supports types like integer, date, etc.)

Example with DTD
DTD:

<!DOCTYPE book [
  <!ELEMENT book (title, author)>
  <!ELEMENT title (#PCDATA)>
  <!ELEMENT author (#PCDATA)>
]>

Valid XML:

<book>
  <title>XML Guide</title>
  <author>A. Smith</author>
</book>

Invalid XML (missing author):

<book>
  <title>XML Guide</title>
</book>

Example with XSD
XSD allows stricter rules (like data types):

<xs:element name="price" type="xs:decimal"/>

Valid XML: <price>19.99</price>
Invalid XML: <price>abc</price>

In short:

• Validation = ensuring XML follows the structure and rules set by a DTD or XSD.

• This makes XML data reliable, consistent, and easier to exchange between systems.