| Re: Another XML Question [message #43053 is a reply to message #43052] |
Wed, 23 March 2005 16:37   |
Michael Wallace
Messages: 409
Registered: December 2003
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Senior Member |
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> Oh, of course. Now what the hell is a DTD again?
Document Type Definition.
It all begins with SGML, Standard Generalized Markup Language, which is
what other markup languages (XML, HTML, XHTML, etc) are based on. SGML
defines the basics of markup and sets a few rules, but not many. XML
was created by the W3C to be a simplified version of SGML suitable for
sharing data over the Internet. There is nothing that limits XML to
Internet-only work -- it just happened this was the original audience,
but it has now grown into a number of different worlds. Anyway, XML
simplified SGML by adding some extra structure, but didn't limit the
possible tags someone could define.
Even though XML is simpler, it could still be really ugly to work with
because the tags used are arbitrary. It'd be difficult to try to work
with a file without some prior knowledge about acceptable tags and
attributes, as our friend David has discovered. To get around the
problems of arbitrary definitions, a DTD is used. A DTD simply defines
all of the acceptable tags, nesting rules, acceptable attributes for
given tags and so on. Having such a definition allows data to be
interpreted and transfered much easier as you don't have to guess at
what's a valid tag or attribute and don't have to guess about what comes
next. There are plenty of XML parsers which only need a valid DTD and
they can read any XML file corresponding to that DTD. It's much easier
than rolling your own each time.
And if you ever here Schema or XSD mentioned along with XML, that's a
reference to the successor of DTD, but DTDs are still in high use, so
it's worthwhile to know how they work. Plus, it's easier to learn DTDs
first and then pick up XSD. Anyway, I saw another message in this
thread with an example of a DTD, so I won't bother with throwing up
another one. Also, one of the best resources I've found for getting
started on stuff like this is the W3C's www.w3schools.com site.
There's a lot of good info on XML in all it's variations.
-Mike
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