Reasoning about Xml Schema Languages Using Formal Language Theory

A mathematical framework using formal language theory to describe and compare XML schema languages is presented. Our framework uses the work in two related areas { regular tree languages CDG + 97] and ambiguity in regular expressions BEGO71, BKW98]. Using these work as well as the content in two classical references HU79, AU79], we present the following results: (1) a normal form representation for regular tree grammars, (2) a framework of marked regular expressions and model groups, and their ambiguities, (3) ve subclasses of regular tree grammars and their corresponding languages to describe XML content models: regular tree languages, TD(1) (top-down input scan with 1-vertical lookahead), single-type constraint languages, TDLL(1) (top-down and left-right input scan with 1-vertical and 1-horizontal looka-heads), and local tree languages, (4) the closure properties of the ve language classes under boolean set operations, (5) a classiication and comparison of a few XML schema proposals and type systems: DTD, XML-Schema, DSD, XDuce, RELAX, and (6) properties of the grammar classes under two common operations: XML document validity checking and type resolution (i.e., XML document interpretation).