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XML validation is the process of checking a document written in XML (eXtensible Markup Language) to confirm that it is both well-formed and also "valid" in that it follows a defined structure. A well-formed document follows the basic syntactic rules of XML, which are the same for all XML documents.^[1] A valid document also respects the rules dictated by a particular DTD or XML schema.^[2] Automated tools – validators – can perform well-formedness tests and many other validation tests, but not those that require human judgement, such as correct application of a schema to a data set.

Well-formedness versus validity

An XML document must be well-formed before it can be validated. A well-formed document conforms to the syntax rules defined by the XML specification, including proper nesting of elements, matching start and end tags, and correct attribute syntax.^[1]

Validation is a separate process in which a well-formed document is evaluated against a schema language such as a Document Type Definition (DTD), XML Schema, RELAX NG, or Schematron to determine whether it conforms to the required document structure and constraints.^[2]

A document may therefore be well-formed but not valid if it satisfies the syntax requirements of XML while failing to meet the constraints defined by its associated schema.

Standards

Several schema languages and validation standards have been developed for XML.^[2]

• Document Type Definition (DTD), the original schema language inherited from SGML.
• XML Schema (XSD), the W3C recommendation for defining XML document structure and datatypes.
• RELAX NG, an ISO standard schema language for XML.
• Schematron, a rule-based validation language for expressing constraints that cannot easily be represented in grammar-based schema languages.
• OASIS CAM, a specification providing contextual validation of XML content and structure.

Tools

• xmllint is a command line XML tool that can perform XML validation. It can be found in UNIX / Linux environments.
• XML toolkit. The XML C parser and toolkit of Gnome – libxml includes xmllint.
• XML Validator Online – Validate XML data.
• XML Schema Validator – Validate XML files against an XML Schema.

References

Articles discussing XML validation

• DEVX (March 2009) – Taking XML Validation to the Next Level: Introducing CAM Archived 2012-03-16 at the Wayback Machine

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