The XML specification defines a standard way to add markup to documents.
(This is the information set which a structure-controlled conforming SGML application should act upon.) If more than one system identifier is specified, then the corresponding entities will be concatenated to form the document entity.For our purposes, the word "document" refers not only to traditional documents, like this one, but also to the myriad of other XML "data formats". SGML has been the standard, vendor-independent way to maintain repositories of structured documentation for more than a decade, but it is not well suited to serving documents over the web (for a number of technical reasons beyond the scope of this article). As I have answered in HTML-and-regex questions here so many times before, the use of regex will not allow you to consume HTML.This introduction to XML presents the Extensible Markup Language at a reasonably technical level for anyone interested in learning more about structured documents.In addition to covering the XML 1.0 Specification, this article outlines related XML specifications, which are evolving.