Re: [xml-dev] Why the double escape for lt ? (that is <!ENTITY lt "&#38

["G. Ken Holman" <gkholman@CraneSoftwrights.com>]

At 2016-11-04 17:19 -0700, David John Burrowes wrote:
> Your comment was useful, and helped pull my mind 
> out of the rut it was in. Still, it doesn’t 
> really seem to offer a definitive reason why 
> internal and external entities are processed differently.
Because the replacement text of an internal 
entity is determined after parsing that entity 
for any references to parameter entities that 
need to be expanded as part of the replacement 
text.  Since the content is being parsed for the 
parameter entities, the parsing process resolves 
the character entities (so as not to 
inadvertently reference an undesired parameter 
entity).  The end result of that parsing step becomes the replacement text.

The replacement text of an external entity is 
manifest in what is in the file and so does not 
need to be parsed for references to parameter 
entities.  Without the need to parse the content, 
the content can be used as is as the replacement text.

The replacement text is then parsed in the 
context of where it is placed in the stream by the entity reference.

In my XML work for my training material I compose 
numerous entities using the values of multiple 
parameter entities.  It was a practice I followed 
in the SGML days.  It ain't pretty but it does 
what I need it to do.  Here is an excerpt:

<!ENTITY % imgdir "images/">
<!ENTITY % b "bmp"><!--bitmap extension "gif" or "bmp"-->
<!ENTITY % v "wmf"><!--vector extension "cgm" or "wmf"-->
<!ENTITY % areas    "<!ENTITY areas    SYSTEM 
'%imgdir;areas.%v;'     NDATA %v;>">%areas;
<!ENTITY % axes     "<!ENTITY axes     SYSTEM 
'%imgdir;axes.%v;'      NDATA %v;>">%axes;
<!ENTITY % book1    "<!ENTITY book1    SYSTEM 
'%imgdir;book1.%b;'     NDATA %b;>">%book1;
<!ENTITY % bookalt  "<!ENTITY bookalt  SYSTEM 
'%imgdir;bookalt.%b;'   NDATA %b;>">%bookalt;

So ... in the general entity replacement for 
"&areas;" I have the sequence "%imgdir;" which 
gets expanded because it is a parameter entity 
reference.  But what if I wanted the string 
"%imgdir;" instead of the parameter entity 
reference?  I need to escape the "%", so I need 
to use &#x25; or &#37; in order to encode the "%" 
to be a simple "%" and not the reference.  So, 
internal entity replacement text processing needs 
to do numeric character reference processing 
which is part and parcel of entity processing.

Therefore, if you want an entity reference 
injected into your stream, one found in an 
external entity is coded as you would think, but 
one found in an internal entity has to be the 
result of entity reference processing, thus 
requiring the double escaping.  You have to 
compose the replacement string you then want processed.

There is a reason.  A bit arcane, but it wasn't done frivolously.

I hope this helps.

. . . . . . . Ken


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