This file contains two sections:

1) An EBNF (Extended Backus Normal Form) description of the format of
   the tags file created by etags.c and interpreted by etags.el 
2) A discussion of tag names and implicit tag names

======================= EBNF tag file description =======================

Productions created from current behaviour to aid extensions
Francesco Potorti` <pot@gnu.org> 2002
----------------

FF ::= #x0c				   /* tag section starter */

LF ::= #x0a				   /* line terminator */

DEL ::= #x7f				   /* pattern terminator */

SOH ::= #x01				   /* name terminator */

regchar ::= [^#x0a#x0c#x7f]		   /* regular character */

regstring ::= { regchar }		   /* regular string */

unsint ::= [0-9] { [0-9] }		   /* non-negative integer */



tagfile ::= { tagsection }		   /* a tags file */

tagsection ::= FF LF ( includesec | regularsec ) LF

includesec ::= filename ",include" [ LF fileprop ]

regularsec ::= filename "," [ unsint ] [ LF fileprop ] { LF tag }

filename ::= regchar regstring		   /* a file name */

fileprop ::= "(" regstring ")"		   /* an elisp alist */

tag ::= directtag | patterntag

directtag ::= DEL realposition		   /* no pattern */

patterntag ::= pattern DEL [ tagname SOH ] position

pattern ::= regstring			   /* a tag pattern */

tagname ::= regchar regstring		   /* a tag name */

position ::= realposition | "," 	   /* charpos,linepos */

realposition ::= "," unsint | unsint "," | unsint "," unsint

==================== end of EBNF tag file description ====================



======================== discussion on tag names =========================

- What are tag names
Tag lines in a tags file are usually made from the above defined pattern
and by an optional tag name.  The pattern is a string that is searched
in the source file to find the tagged line.

- Why tag names are good
When a user looks for a tag, Emacs first compares the tag with the tag
names contained in the tags file.  If no match is found, Emacs compares
the tag with the patterns.  The tag name is then the preferred way to
look for tags in the tags file, because when the tag name is present
Emacs can find a tag faster and more accurately.  These tag names are
part of tag lines in the tags file, so we call them "explicit".

- Why implicit tag names are even better
The purpose of implicit tag names is to reduce the number of tag names
in a tags file, thus reducing the size of the tags file.  When the user
looks for a tag, and Emacs founds no explicit tag names that match it,
Emacs then tries to match the tag with an implicit tag name.  Such a
match occurs when the tag matches a pattern, subject to the satisfaction
of all the following four rules:

 NONAM=" \f\t\n\r()=,;";
  1. the tag does not contain any of the characters in NONAM;
  2. the pattern contains the tag as either a rightmost, or rightmost
     but one character, substring;
  3. the character, if any, immediately before the tag in the pattern
     must be a character in NONAM;
  4. the character, if any, immediately after the tag in the pattern
     must also be a character in NONAM.

===================== end of discussion on tag names =====================