Attributes may be required or optional as well and may contain any of several specific value types; for example, the only. Beyond merely specifying that elements or attributes must contain strings describing particular datatypes, such as numbers or dates, schemas allow more flexible quantification of subelement occurrences.
For example, the following W3C XML Schema might describe an XML document for purchases: Formal specifications of schema languages can be found at the above-mentioned URLs; this example is meant simply to illustrate the types of capabilities they have.
As a simple case, the following DTD is one that the prior example of nested subelements would conform to.
Examples of each of these forms look like: XML documents may be either "well-formed" or "valid." The first characterization simply indicates that a document obeys the proper syntactic rules for XML documents in general: All tags are either self-closed or followed by a matching endtag; reserved characters are escaped; tags are properly hierarchically nested; and so on.
Of course, particular documents can also fail to be well-formed?
Extensible Markup Language (XML) is a text format increasingly used for a wide variety of storage and transport requirements.
Parsing and processing XML is an important element of many text processing applications.
Beyond well-formedness, some XML documents are also valid.
Validity means that a document matches a further grammatical specification given in a Document Type Definition (DTD), or in an XML Schema.
The most popular style of XML Schema is the W3C XML Schema specification, found in formal detail at .
The grammatical specifications indicated by DTDs are strictly structural.
This book will not cover most of the API details of XML tools; an excellent book dedicated to that subject is: The XML format is sufficiently rich to represent any structured data, some forms more straightforwardly than others. But XML is probably used more often to represent than texts? In many of these cases, the fit is more awkward and requires extra verbosity.
A task that XML is quite natural at is in representing marked-up text? XML itself is more like a metalanguage than a language?
A bare example is: As with a Python dictionary, no order is defined for the list of key/value attributes of one tag.