SGML

SGML (Standard Generalized Markup Language) is a meta-language used to define markup languages. It provides a standardized way to structure and annotate documents using custom markup tags and serves as the foundation for HTML and XML.

Unlike specific markup languages (such as HTML), SGML does not dictate a specific set of tags but instead allows users to define their own document structure and elements.

How it works

A. Defining a Document Type (DTD - Document Type Definition)

SGML documents rely on a DTD (Document Type Definition) to specify the allowed tags, attributes, and document structure.

Example of an SGML DTD:

<!ELEMENT book (title, author, chapter+)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT chapter (#PCDATA)>

• <!ELEMENT> defines allowed elements in a document.
• #PCDATA represents parsed character data (text content).

B. Writing an SGML Document

A document based on the above DTD would look like:

<!DOCTYPE book SYSTEM "book.dtd">
<book>
    <title>Introduction to SGML</title>
    <author>John Doe</author>
    <chapter>Understanding SGML</chapter>
    <chapter>SGML vs XML</chapter>
</book>

• The <!DOCTYPE> declaration specifies the DTD reference.
• The <book> tag follows the structure defined in the DTD.