The XML FAQ

Seán McGrath writes:

To convert SGML DTDs to XML:

1. No equivalent of the SGML Declaration. So keywords, character set etc are essentially fixed;

2. Tag minimisation is not allowed, so <!ELEMENT x - O (A,B)> becomes <!ELEMENT X (A,B)> and <!ELEMENT x - O EMPTY> becomes <!ELEMENT X EMPTY> ;

3. #PCDATA must only occur at the extreme left (ie first) in an OR model, eg <!ELEMENT x - - (A|B|#PCDATA|C)> (in SGML) becomes <!ELEMENT x (#PCDATA|A|B|C)*> , and <!ELEMENT x (A,#PCDATA)> is illegal;

4. No CDATA, RCDATA elements [declared content];

5. Some SGML attribute types are not allowed in XML eg NUTOKEN;

6. Some SGML attribute defaults are not allowed in XML eg CONREF;

7. Comments cannot be inline to declarations like <!ELEMENT x - - (A,B) -- an SGML comment in a declaration --> ;

8. A whole bunch of SGML optional features are not present in XML: all forms of tag minimisation (OMITTAG, DATATAG, SHORTREF, etc); Link Process Definitions; Multiple DTDs per document; and many more: see http://www.w3.org/TR/NOTE-sgml-xml-971215 for the list of bits of SGML that were removed for XML;

9. And [nearly] last but not least, no CONCUR!

10. There are some important differences between the internal and external subset portion of a DTD in XML: Marked Sections can only occur in the external subset; and Parameter Entities must be used to replace entire declarations in the internal subset portion of a DTD, eg the following is invalid XML:

     
    <!DOCTYPE x [ 
    <!ENTITY % modelx "(A|B)*"> 
    <!ELEMENT x %modelx;> 
    ]> 
    <x></x>
    		

For more information, see XML by Example: Building E-Commerce Applications (McGrath, 1998).