While doctools is similar to LaTeX docidx is only superficially so. Input written in this format consists of a series of markup commands, which may be separated by whitespace. Other text is not allowed. The best comparison would be to imagine a LaTeX document with all regular text removed.

The format used to mark commands is different from LaTeX however. All text between matching pairs of [ and ] is a command, possibly with arguments. Note that both brackets have to be on the same line for a command to be recognized.

The overall syntax of a keyword index is best captured in a formal context-free grammar. Our notation for the grammar is EBNF. Strings will stand for markup commands, however their arguments (if they have any) are not part of the grammar. Our grammar contains lexical elements as well.

First we specify the whitespace at the lexical level, which also includes comments.

    COMMENT  ::= "comment"
    WHITE    ::= { '\n' | '\t' | ' ' | '\r' | COMMENT }

Then we define rules for all the keywords. Here we introduce our knowledge that all commands can be separated by whitespace, and also that the inclusion of other files may happen essentially everywhere, like the definition of document variables. The content of any included file has to fit into the including file according to the location in the grammar the inclusion is at.

    BEGIN    ::= "index_begin" WHITE DEFUN
    END      ::= "index_end"   WHITE
    KEY      ::= "key"         WHITE DEFUN
    MANPAGE  ::= "manpage"     WHITE DEFUN
    URL      ::= "url"         WHITE DEFUN

INCLUDE ::= "include" WHITE VSET ::= "vset" WHITE

DEFUN ::= { INCLUDE | VSET }

At last we can specify the whole index.

    INDEX    ::= DEFUN BEGIN CONTENT END

CONTENT ::= KEYWORD { KEYWORD } KEYWORD ::= KEY REF { REF } REF ::= MANPAGE | URL

Here we specify the commands used in the grammar. Some commands specified here were not used in the grammar at all. The usage of these commands is confined to the arguments of other commands.

comment text

This command declares that the argument text is a comment.

include filename

This command loads the contents of the file filename for processing at its own place.

vset varname value

This form of the command sets the document variable varname to the specified value. It does not generate output. It is this form the grammar is refering to.

vset varname

This form of the command returns the value associated with the document variable varname. This form is not used by the grammar and restricted to usage in the arguments of other commands.

lb

This command adds a left bracket to the output. Its usage is restricted to the arguments of other commands.

rb

This command adds a right bracket to the output. Its usage is restricted to the arguments of other commands.

index_begin text title

This is the command to start a keyword index. The text argument provides a label for the whole group of documents the index refers to. Often this is the name of the package (or extension) the documents belong to. The title argument provides the overall title text for the index.

index_end

This is the command to close a keyword index.

key text

This command adds the keyword text to the index.

manpage file text

This command adds an element to the index which refers to a document. The document is specified through the symbolic name file. The text argument is used to label the reference. The symbolic names are used to preserve the convertibility of this format to any output format. The actual name of the file will be inserted by the chosen formatting engine when converting the input. This will be based on a mapping from symbolic to actual names given to the engine.

url url label

This is the second command to add an element to the index. To refer to a document it is not using a symbolic name however, but a (possibly format-specific) url describing the exact location of the document indexed here.