-*- mode: indented-text; coding: utf-8 -*-

See the end of this file for copyright information.

This file contains two sections:

1) An EBNF (Extended Backus-Naur 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.

====================== 1) EBNF tag file description =====================

Productions created from current behavior to aid extensions
Francesco Potortì <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 ====================



======================= 2) discussion of 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
When a tag line has no name, but a name can be deduced from the pattern,
we say that the tag line has an implicit tag name.  Often tag names are
redundant; this happens when the name of a tag is an easily guessable
substring of the tag pattern.  We define a set of rules to decide
whether it is possible to deduce the tag name from the pattern, and make
an unnamed tag in those cases.  The name deduced from the pattern of an
unnamed tag is the implicit name of that tag.
  When the user looks for a tag, and Emacs finds no explicit tag names
that match it, Emacs then looks for a tag whose implicit tag name
matches the request.  etags.c uses implicit tag names when possible, in
order to reduce the size of the tags file.
  An implicit tag name is deduced from the pattern by discarding the
last character if it is one of ' \f\t\n\r()=,;', then taking all the
rightmost consecutive characters in the pattern which are not one of
those.

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

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