An XML schema is the structural layout of an XML document, expressed in terms of constraints and contents of the document. Constraints are expressed using a combination of the following:
- Grammatical rules governing the order of elements
- Data types governing an element and content attribute
- Boolean predicates that the content has to satisfy
- Specialized rules including uniqueness and referential integrity constraints
XML schemas are expressed using Document Type Definition (DTD) language, which is native to the XML specification but with a fairly limited capability. An XML document can be associated with a schema language, either by markup in the XML document or through some external means.
The process of checking whether an XML document adheres to a schema is called validation. XML documents are valid if they satisfy schema requirements with which they are associated, including constraints such as:
The process of checking whether an XML document adheres to a schema is called validation. XML documents are valid if they satisfy schema requirements with which they are associated, including constraints such as:
- Structure as specified by a regular expression syntax
- Requirements for interpretation of character data
- Elements and attributes to be included along with their allowed structure
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