Platinum Edition Using HTML 4, XML, and Java 1.2
(Publisher: Macmillan Computer Publishing)
Author(s): Eric Ladd
ISBN: 078971759x
Publication Date: 11/01/98

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CAUTION:  

No official method exists yet for resolving public identifiers in XML. An SGML method has been used for years, however, and because most of the major XML tools come from SGML developers, the same method has quietly been implemented in their XML tools without any real discussion about whether it was needed.

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SGML uses a public identifier resolution commonly known as the SGML Open Catalog (SOC), which uses a catalog file located in the same directory as the document (the application is free to change this location, of course). This file is usually called catalog.soc or, more frequently, just catalog.

There is little point in going into all the technical details because the catalog file is really an SGML facility. As far as we are concerned, the catalog file is basically an ASCII file consisting of lines that couple a public identifier (officially a “Formal System Identifier” (FSI)) with a system object identifier. A system object identifier is basically a file, but it could also be some other kind of identifier that the system is able to convert into something meaningful. A typical catalog file looks like the following:

-- catalog: SGML Open style entity catalog for HTML --
-- $Id: catalog,v 1.3 1995/09/21 23:30:23 connolly Exp $ --
-- Hacked by jjc --
-- Ways to refer to Level 2: most general to most specific --
PUBLIC“-//IETF//DTD HTML//EN”                      “html.dtd”
PUBLIC“-//IETF//DTD HTML 2.0//EN”                  “html.dtd”
PUBLIC“-//IETF//DTD HTML Level 2//EN”              “html.dtd”
PUBLIC“-//IETF//DTD HTML 2.0 Level 2//EN”          “html.dtd”

-- Ways to refer to Level 1: most general to most specific --
PUBLIC“-//IETF//DTD HTML Level 1//EN”              “html-1.dtd”
PUBLIC“-//IETF//DTD HTML 2.0 Level 1//EN”          “html-1.dtd”

-- Ways to refer to Strict Level 2: most general to most specific --
PUBLIC“-//IETF//DTD HTML Strict//EN”               “html-s.dtd”
PUBLIC“-//IETF//DTD HTML 2.0 Strict//EN”           “html-s.dtd”
PUBLIC“-//IETF//DTD HTML Strict Level 2//EN”       “html-s.dtd”
PUBLIC“-//IETF//DTD HTML 2.0 Strict Level 2//EN”   “html-s.dtd”

-- Ways to refer to Strict Level 1: most general to most specific --
PUBLIC“-//IETF//DTD HTML Strict Level 1//EN”       “html-1s.dtd”
PUBLIC“-//IETF//DTD HTML 2.0 Strict Level 1//EN”   “html-1s.dtd”

-- ISO latin 1 entity set for HTML --
PUBLIC“ISO 8879-1986//ENTITIES Added Latin 1//EN//HTML”  ISOlat1.sgm

Note that this example is a modified XML version of an SGML entities file; in XML the filename has to be enclosed in quotes, but in SGML it does not.

Nothing prevents you from creating this file by hand using a text editor, but a few free catalog management packages (also called entity management packages) are available on the Internet. Some software packages have their own built-in facility, often called an entity manager, for resolving entities.

Parameter Entities

Parameter entity references may only appear in a DTD. To keep them distinct from general entities (and to prevent them from being used in a document), parameter entities are declared and referenced with a percent sign (%):

<!ENTITY % “front | body | back” >

Parameter entities are extremely useful as shortcuts for parts of declarations that occur often in DTD. They are not, however, allowed to contain markup (complete declarations); they can only contain parts of declarations:

<!ENTITY often-used “(para | body | text)”>
<!ELEMENT chapter ((%common;)*, section+)>
<!ELEMENT section (%common;)>

When a parameter entity reference is resolved, one leading and one trailing space character is added to the replacement text to make sure that it contains an integral number of grammatical tokens.

Entity Resolution

The rules governing entity resolution (when they are interpreted and when they are ignored) can be quite complicated.

Table 15.1 shows what happens to entity references and character references. The leftmost column describes where the entity reference appears: