Brain teaser: how to use XML to describe a data format that containsille

[Roger L Costello <costello@mitre.org>]

Hi Folks,

This is kind of a neat problem.

The technology called DFDL (Data Format Description Language) is used to describe data formats, both text and binary data formats. DFDL builds on top of XML Schema-XML Schema "hosts" DFDL in a way similar to how XSLT "hosts" XPath.

Some data formats that we want to describe contain characters that are not allowed in XML, which means they are not allowed in XML Schema since XML Schema is XML. For example, we might want to use DFDL to describe a binary data format that contains null-terminated strings. The null symbol (hex 0) is not allowed in XML, so how do we describe the data format?

Scroll down to see the answer ............

















One way to resolve this problem is to use a special string-with printable characters-that denotes the forbidden character. For the NUL character, the special string is NUL. To identify it as a "special string" we precede it with a percent symbol and follow it with a semicolon:

%NUL;

Applications written to process DFDL schemas are expected to recognize that %NUL; denotes the NUL character and replace it with the actual NUL character.

Pretty neat, I think!

/Roger