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Microsoft Word Applied xml a toolkit for Programmers Wiley docBog'liq Ceponkus, Hoodbhoy - Applied XML - Toolkit for ProgrammersXML Documents in Depth
Now that we’ve had a brief introduction to the overall structure of an XML system, let’s
start breaking things down into greater detail.
XML documents are logical, hierarchically structured information chunks. Recall our
analogy of XML documents as blueprints from
Chapter 1
. XML documents are not
solutions in themselves. Remember, unlike HTML documents whose specific purpose is
the display of information, XML documents have the agenda of presenting information so
that it can be further processed. Therefore, an XML document includes a lot of markup to
describe the content (raw information) it contains.
XML Concept: Well Formed
In
Chapter 1
, we briefly touched upon the concept of well formed XML documents. Well
formed XML documents are those XML documents that are syntactically correct. XML,
unlike HTML, is extremely picky about syntax. Fortunately, it’s not that hard to write
syntactically correct XML. The following are general guidelines for writing well formed
XML documents.
Case Sensitivity
Unlike HTML, XML is case sensitive. You must be very careful when writing markup.
Spelling mistakes can sometimes take a long time to detect, especially if you are using a
plain text editor for creating your XML document.
Closing Tags
Another thing that the XML specification is rather picky about is that every open tag must
be closed. So, though it’s quite acceptable to begin an HTML document with an
tag and to not bother to close it, an XML parser will set off all kinds of red flags for not
closing a tag.
No Overlapping Tags
It is easy to confuse overlapping tags with nesting tags.
Nesting tags
is a way of
developing a logical hierarchy of information similar to the BODMAS (brackets of division,
multiplication, addition, subtraction—wouldn’t our teachers be proud of us for
remembering?) rules of eighth grade math. Nesting tags in XML documents is fine and is
strongly encouraged from a design perspective. However, you cannot overlap tags.
Here’s an example of an XML clip that contains nested tags:
Contents of third tag
There is a hierarchy that is implicitly defined by the above structure.
Here’s an example of an unacceptable (that is, not well formed) XML snippet with
overlapping tags.
Contents of third tag
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The term
overlapping
means that a tag that is not a child of the first starts before the
other ends. Do you see how in the above case that an overlap occurs between
and
? In the XML world, this is a definite no-no. For those
of you who are experienced in HTML, this may seem a little inconvenient; after all, most
HTML parsers don’t care when you overlap (they just act weird when you overlap tags). If
you think about it for a minute from the parser’s perspective, however, things could get
very confusing with overlapping tags. If you intend to write your XML own documents, we
strongly encourage you to watch out for overlapping tags.
Several tools out in the market take care of fussing around with the markup syntax. Tools
like Vervet Logic’s XML Pro and Microsoft’s XML Notepad allow you to visually create
your structures and fill in content. The advantage of these programs is obvious; you don’t
have to worry about well formed nonconformities. However, we must forewarn you that
should you use any of these programs (or ones similar to these), you may end up losing
several degrees of flexibility in your documents (for example, XML Notepad doesn’t let
you work with processing instructions very easily). For an updated list of tools for editing
XML documents, check out this book’s companion Web site.
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