$Date: 2005/03/25 21:54:04 $$Revision: 1.1 $Parsing DocBook DocumentsparsingDocBook documentsDocBook DTDparsing documents
A key feature of &SGML; and &XML; markup is that you
validate it. The DocBook &DTD; is a precise description
of valid nesting, the order of elements, and their content. All
DocBook documents must conform to this description or they are not
DocBook documents (by definition).
parsersvalidatingvalidationparsers
A validating parser is a program that can read
the &DTD; and a particular document and determine whether the exact
nesting and order of elements in the document is valid according to
the &DTD;.
text editorsunstructured, parsing and
If you are not using a structured editor that can enforce the markup
as you type, validation with an external parser is a particularly
important step in the document creation process. You cannot expect
to get rational results from subsequent processing (such as document
publishing) if your documents are not valid.
SP nsgmls parsernsgmls parser
The most popular free &SGML; parser is SP by James Clark,
available at http://www.jclark.com/.
nsgmls parserparsersfreeXMLparsers, freexp parser
SP includes nsgmls, a fast command-line
parser. In the world of free validating &XML; parsers,
James
Clark's xp is a popular choice.
parsersnon-validating
Not all &XML; parsers are validating, and although a
non-validating parser may have many uses, it cannot ensure that your
documents are valid according to the &DTD;.Validating Your Documents
The exact way in which the parser is executed varies according to
the parser in use, naturally. For information about your particular
parser, consult the documentation that came with it.
Using nsgmlsnsgmls parserSGML documents, validating
The nsgmls command from SP is a
validating &SGML; parser. The options used in the example below
suppress the normal output (), except for error messages,
print the version number (), and specify the catalog file
that should be used to map public identifiers to system
identifiers. Printing the version number guarantees that you always
get some output, so that you know the command ran
successfully:
[n:\dbtdg] nsgmls -sv -c \share\sgml\catalog test.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
error messagesnsgmls parseroptionserror messages (parser), directing to file
Because no error messages were printed, we know our document is valid.
If you're working with a document that you discover has many errors,
the option offers a handy way to direct the errors to a file
so they don't all scroll off your screen.
nsgmls parserXML documents, validatingdeclarationsnsgmls parserxml.dcl declaration (nsgmls parser)XMLnsgmls parser
If you want to validate an &XML; document with SP,
you must make sure that SP uses the correct
declaration. An &XML; declaration called xml.dcl is
included with SP.
The easiest way to make sure that SP uses xml.dcl is to
include the declaration explicitly on the command line when you run
nsgmls (or Jade, or other SP tools):
[n:\dbtdg] nsgmls -sv -c \share\sgml\catalog m:\jade\xml.dcl test.xml
m:\jade\nsgmls.exe:I: SP version "1.3.2"
Using xpxp parserTime program (xp parser)
The xp distribution includes several sample programs.
One of these programs, Time, performs a validating parse
of the document and prints the amount of time required to parse the
&DTD; and the document. This program makes an excellent validity
checker:
java com.jclark.xml.apps.Time examples\simple.xml
6.639
The result states that it took 6.639 seconds to parse the &DTD; and
the document. This indicates that the document is valid. If the
document is invalid, additional error messages are displayed.
Understanding Parse Errorserrorsparsingparsingerrorserror messagesparsers
Every parser produces slightly different error messages, but most
indicate exactly (at least technically)
It is often the case that you can correct an error in the document in
several ways. The parser suggests one possible fix, but this is not
always the right fix. For example, the parser may suggest that you
can correct out of context data by adding another element, when in
fact it's “obvious” to human eyes that the problem is a
missing end tag.
what is wrong and where the error occurred. With a little
experience, this information is all you'll need to quickly identify
what's wrong.
JadeSP parser, using
In the rest of this section, we'll look at a number of common errors
and the messages they produce in SP. We've
chosen SP for the rest of these examples because
that is the same parser used by Jade, which we'll be discussing
further in .
&DTD; Cannot Be Foundcannot be found errorsDTDscannot be found errorspublic identifierserrors
The telltale sign that SP could not find the &DTD;, or some module of
the &DTD;, is the error message: "cannot generate system identifier
for public text …". Generally, the errors that occur after
this are spurious; if SP couldn't find some part of the &DTD;,
it's likely to think that everything is wrong.
Careful examination of the following document will show that we've
introduced a simple typographic error into the public identifier (the
word DocBook is misspelled with a lowercase b):
SP responds dramatically to this error:
hermes:/documents/books/tdg/examples/errs$ nsgmls -sv -c cat1 /usr/lib/sgml/declaration/xml.dcl nodtd.sgm
nsgmls:I: SP version "1.3.4"
nsgmls:nodtd.sgm:2:76:E: could not resolve host "www.oasis-open.org" (try again later)
nsgmls:nodtd.sgm:2:76:E: DTD did not contain element declaration for document type name
nsgmls:nodtd.sgm:3:8:E: element "chapter" undefined
nsgmls:nodtd.sgm:3:15:E: element "title" undefined
nsgmls:nodtd.sgm:4:5:E: element "para" undefined
nsgmls:nodtd.sgm:10:5:E: element "para" undefined
nsgmls:nodtd.sgm:11:15:E: there is no attribute "role"
nsgmls:nodtd.sgm:11:21:E: element "emphasis" undefined
nsgmls:nodtd.sgm:12:9:E: element "emphasis" undefined
nsgmls:nodtd.sgm:12:24:E: element "emphasis" undefined
nsgmls:nodtd.sgm:13:18:E: element "superscript" undefined
nsgmls:nodtd.sgm:14:16:E: element "subscript" undefined
nsgmls:nodtd.sgm:16:5:E: element "para" undefined
catalog fileserrors
Other things to look for, if you haven't misspelled the public
identifier, are typos in the catalog or failure to specify a catalog
that resolves the public identifier that can't be found.
ISO Entity Set Missingentity setsmissing
A missing entity set is another example of either a misspelled public
identifier, or a missing catalog or catalog entry.
In this case, there's nothing wrong with the document, but the catalog
that's been specified is missing the public identifiers for the
ISO entity sets:
[n:\dbtdg]nsgmls -sv -c examples\errs\cat2 examples\simple.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:53:65:W: cannot generate system identifier for public text "ISO 8879:1986//ENTITIES Added Math Symbols:Arrow Relations//EN"
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:54:8:E: reference to entity "ISOamsa" for which no system identifier could be generated
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:52:0: entity was defined here
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:60:66:W: cannot generate system identifier for public text "ISO 8879:1986//ENTITIES Added Math Symbols:Binary Operators//EN"
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:61:8:E: reference to entity "ISOamsb" for which no system identifier could be generated
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:59:0: entity was defined here
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:67:60:W: cannot generate system identifier for public text "ISO 8879:1986//ENTITIES Added Math Symbols:Delimiters//EN"
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:68:8:E: reference to entity "ISOamsc" for which no system identifier could be generated
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:66:0: entity was defined here
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:74:67:W: cannot generate system identifier for public text "ISO 8879:1986//ENTITIES Added Math Symbols:Negated Relations//EN"
...
entity setslocating
The ISO entity sets are required by the DocBook
&DTD;, but they are not distributed with it. That's because they
aren't maintained by OASIS.
If you need to locate the entity sets, consult http://www.oasis-open.org/cover/topics.html#entities.
Character Data Not Allowed Hereerror messagescharacter data out of contextcharacterserrors (out of context)start tagserrors
Out of context character data is frequently caused by a missing start
tag, but sometimes it's just the result of typing in the wrong place!
[n:\dbtdg] nsgmls -sv -c \share\sgml\catalog examples\errs\badpcdata.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\badpcdata.sgm:9:0:E: character data is not allowed here
Chapter elementerrors, character dataChapters aren't allowed to contain character data
directly. Here, a wrapper element, such as Para, is
missing around the sentence between the first two paragraphs.
Misspelled Start Tagstart tagserrorsmisspelling
If you spell it wrong, the parser gets confused.
[n:\documents\books\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\misspe
ll.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\misspell.sgm:9:5:E: element "PAAR" undefined
m:\jade\nsgmls.exe:examples\errs\misspell.sgm:14:6:E: end tag for element "PARA" which is not open
m:\jade\nsgmls.exe:examples\errs\misspell.sgm:21:9:E: end tag for "PAAR" omitted, but OMITTAG NO was specified
m:\jade\nsgmls.exe:examples\errs\misspell.sgm:9:0: start tag was here
Luckily, these are pretty easy to spot, unless you accidentally spell
the name of another element. In that case, your error might appear to
be out of context.
Misspelled End Tagend tagserrors, misspelling
Spelling the end tag wrong is just as confusing.
[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\misspell2.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:2:35:E: end tag for element "TITEL" which is not open
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:3:5:E: document type does not allow element "PARA" here; missing one of "FOOTNOTE", "MSGTEXT" start-tag
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:9:5:E: document type does not allow element "PARA" here; missing one of "FOOTNOTE", "MSGTEXT" start-tag
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:15:5:E: document type does not allow element "PARA" here; missing one of "FOOTNOTE", "MSGTEXT" start-tag
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:21:9:E: end tag for "TITLE" omitted, but OMITTAG NO was specified
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:2:9: start tag was here
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:21:9:E: end tag for "CHAPTER" which is not finished
These are pretty easy to spot as well, but look at how confused the
parser became. From the parser's point of view, failure to close the
open Title element means that all the following elements
appear out of context.
Out of Context Start Tagstart tagserrorsout of contexttagscontext errors
Sometimes the problem isn't spelling, but placing a tag in the wrong
context. When this happens, the parser tries to figure out what it
can add to your document to make it valid. Then it proceeds as if it
had seen what was added in order to recover from the error seen, which
can cause future errors.
[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\badstarttag.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\badstarttag.sgm:9:12:E: document type does not allow element "TITLE" here; missing one of "CALLOUTLIST", "SEGMENTEDLIST", "VARIABLELIST", "CAUTION", "IMPORTANT", "NOTE", "TIP", "WARNING", "BLOCKQUOTE", "EQUATION", "EXAMPLE", "FIGURE", "TABLE" start-tag
titlesformal elementserrorsparagraphstitles
In this example, we probably wanted a FormalPara, so that
we could have a title on the paragraph. But note that the parser
didn't suggest this alternative. The parser only tries to add
additional elements, rather than rename elements that it's already
seen.
Missing End Tagend tagsmissing, errors
Leaving out an end tag is a lot like an out of context start tag. In
fact, they're really the same error. The problem is never caused by
the missing end tag per se, rather it's caused by the fact that
something following it is now out of context.
[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\noendtag.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\noendtag.sgm:14:5:E: document type does not allow element "PARA" here; missing one of "FOOTNOTE", "MSGTEXT", "CAUTION", "IMPORTANT", "NOTE", "TIP", "WARNING", "BLOCKQUOTE", "INFORMALEXAMPLE" start-tag
m:\jade\nsgmls.exe:examples\errs\noendtag.sgm:20:9:E: end tag for "PARA" omitted, but OMITTAG NO was specified
m:\jade\nsgmls.exe:examples\errs\noendtag.sgm:9:0: start tag was here
In this case, the parser figured out that the best thing it could do
is end the paragraph.
Bad Entity Referenceentitieserrors, spelling
If you spell an entity name wrong, the parser will catch it.
[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\badent.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\badent.sgm:10:26:E: general entity "xyzzy" not defined and no default entity
More often than not, you'll see this when you misspell a character
entity name. For example, this happens when you type
&ldqou; instead of
“.
Invalid 8-Bit Charactercharacterscharacter setsUnicode character set (SGML errors)8-bit characters, errors (SGML)errors8-bit characters (SGML)
In &XML;, the entire range of Unicode characters is available to you,
but in &SGML;, the declaration indicates what characters are valid.
The distributed DocBook declaration doesn't allow a bunch of fairly
common 8-bit characters.
[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\badchar.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\badchar.sgm:11:0:E: non SGML character number 147
m:\jade\nsgmls.exe:examples\errs\badchar.sgm:11:5:E: non SGML character number 148
declarationscharacter sets, errorsentitiescharacterserrors
In this example, the Windows code page values for
curly left and right quotes have been used, but they aren't in the
declared character set. Fix this by converting them to character
entities.
interchange partners
You can also fix them by changing the declaration, but if you do that,
make sure all your interchange partners are aware of, and have a copy of,
the modified declaration. See .
Considering Other Schema LanguagesHistorically, &DTD;s were the only way to describe the valid
stricture of SGML and XML documents, but that is no longer the case.
At the time of this writing (January, 2001), DocBook is experimentally
available in three other schema languages:XML SchemaThe schema language being defined by the
W3C as the successor to &DTD;s
for describing the structure of &XML;. XML Schema are likely to become
a W3C Recommendation in 2001.
RELAXRELAX, the Regular Language description for XML) is a less
complex alternative to XML Schemas. The RELAX Core module is defined by
ISO in ISO/IEC DTR 22250-1, Document Description and
Processing Languages -- Regular Language Description for XML (RELAX)
-- Part 1: RELAX Core, 2000. The RELAX Namespaces module
is currently under development.
TREXTREX, Tree Regular Expressions for XML, is another less complex
alternative to XML Schemas. It is concise, powerful, and datatype
neutral.Parsing and ValidationBefore we look closer at these new schema languages, there's one
significant difference between &DTD;s and all of them that we should
get out of the way: &XML; parsers (which may understand &DTD;s) build an
&XML; information set out of a stream of characters, all of these other
schema languages begin with an information set and perform validation
on it.What I mean by that is that an &XML; parser reads a stream of
bytes:
"<" "?" "x" "m" "l" " " "v" "e" ...
"<" "!" "D" "O" "C" "T" "Y" "P" "E" " " "b" "o" "o" "k" ...
"<" "b" "o" "o" "k" " " "i" "d" "=" "'" "f" "o" "o" "'" ">"
...
"<" "/" "b" "o" "o" "k" ">"
interprets them as a stream of characters (which may change the
interpretation of some sequences of bytes) and constructs some
representation of the &XML; document. This representation is the set
of all the &XML; information items encountered: the information
set of the document. The W3CXML Core Working Group
is in the process of defining what an
&XML; Information Set
contains.The other schema languages are defined not in terms of the sequence
of characters in the file but in terms of the information set of the
&XML; document. They have to work this way because the
&XML; Recommendation
says what an &XML; document is and they all want to work on top of
&XML;.So what, you might ask? Well, it turns out that this has at least
one very significant implication: there's no way for these languages to
provide support for entity declarations.An entity, like &ora;
as a shortcut for O'Reilly & Associates or
é as a mnemonic for
é, is a feature of the character stream seen by
the &XML; parser, it doesn't exist in the information set of valid
&XML; documents. More importantly, this means that even if the schema
language had a syntax for declaring entities, it wouldn't help the &XML;
parser that needs to know the definitions long before the schema language
processor comes into play.There are a couple of other &XML; features that are impacted,
though not necessarily as significantly: notations and default
attribute values. One use for notations is on external entity
declarations, and as we've already seen, the schema language is too
late to be useful for anything entity related. Default attribute values
are also problematic since you would like them to be in the information
set produced by the parser so that the schema language sees them.A Coarse Comparison of Three &XML; Schema LanguagesFIXME: write a short synopsis of how these languages compare.