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Commit	Line	Data
	1	-- indented-text --
	2
	3	See the end of this file for copyright information.
	4
	5	This file contains two sections:
	6
	7	1) An EBNF (Extended Backus-Naur Form) description of the format of
	8	the tags file created by etags.c and interpreted by etags.el;
	9	2) A discussion of tag names and implicit tag names.
	10
	11	====================== 1) EBNF tag file description =====================
	12
	13	Productions created from current behavior to aid extensions
	14	Francesco Potorti` <pot@gnu.org> 2002
	15	----------------
	16
	17	FF ::= #x0c /* tag section starter */
	18
	19	LF ::= #x0a /* line terminator */
	20
	21	DEL ::= #x7f /* pattern terminator */
	22
	23	SOH ::= #x01 /* name terminator */
	24
	25	regchar ::= [^#x0a#x0c#x7f] /* regular character */
	26
	27	regstring ::= { regchar } /* regular string */
	28
	29	unsint ::= [0-9] { [0-9] } /* non-negative integer */
	30
	31
	32
	33	tagfile ::= { tagsection } /* a tags file */
	34
	35	tagsection ::= FF LF ( includesec \| regularsec ) LF
	36
	37	includesec ::= filename ",include" [ LF fileprop ]
	38
	39	regularsec ::= filename "," [ unsint ] [ LF fileprop ] { LF tag }
	40
	41	filename ::= regchar regstring /* a file name */
	42
	43	fileprop ::= "(" regstring ")" /* an elisp alist */
	44
	45	tag ::= directtag \| patterntag
	46
	47	directtag ::= DEL realposition /* no pattern */
	48
	49	patterntag ::= pattern DEL [ tagname SOH ] position
	50
	51	pattern ::= regstring /* a tag pattern */
	52
	53	tagname ::= regchar regstring /* a tag name */
	54
	55	position ::= realposition \| "," /* charpos,linepos */
	56
	57	realposition ::= "," unsint \| unsint "," \| unsint "," unsint
	58
	59	==================== end of EBNF tag file description ====================
	60
	61
	62
	63	======================= 2) discussion of tag names =======================
	64
	65	- WHAT ARE TAG NAMES
	66	Tag lines in a tags file are usually made from the above defined pattern
	67	and by an optional tag name. The pattern is a string that is searched
	68	in the source file to find the tagged line.
	69
	70	- WHY TAG NAMES ARE GOOD
	71	When a user looks for a tag, Emacs first compares the tag with the tag
	72	names contained in the tags file. If no match is found, Emacs compares
	73	the tag with the patterns. The tag name is then the preferred way to
	74	look for tags in the tags file, because when the tag name is present
	75	Emacs can find a tag faster and more accurately. These tag names are
	76	part of tag lines in the tags file, so we call them "explicit".
	77
	78	- WHY IMPLICIT TAG NAMES ARE EVEN BETTER
	79	When a tag line has no name, but a name can be deduced from the pattern,
	80	we say that the tag line has an implicit tag name. Often tag names are
	81	redundant; this happens when the name of a tag is an easily guessable
	82	substring of the tag pattern. We define a set of rules to decide
	83	whether it is possible to deduce the tag name from the pattern, and make
	84	an unnamed tag in those cases. The name deduced from the pattern of an
	85	unnamed tag is the implicit name of that tag.
	86	When the user looks for a tag, and Emacs finds no explicit tag names
	87	that match it, Emacs then looks for an tag whose implicit tag name
	88	matches the request. etags.c uses implicit tag names when possible, in
	89	order to reduce the size of the tags file.
	90	An implicit tag name is deduced from the pattern by discarding the
	91	last character if it is one of ` \f\t\n\r()=,;', then taking all the
	92	rightmost consecutive characters in the pattern which are not one of
	93	those.
	94
	95	===================== end of discussion of tag names =====================
	96
	97	Copyright (C) 2002-2013 Free Software Foundation, Inc.
	98
	99	COPYING PERMISSIONS:
	100
	101	This document is free software: you can redistribute it and/or modify
	102	it under the terms of the GNU General Public License as published by
	103	the Free Software Foundation, either version 3 of the License, or
	104	(at your option) any later version.
	105
	106	This program is distributed in the hope that it will be useful,
	107	but WITHOUT ANY WARRANTY; without even the implied warranty of
	108	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	109	GNU General Public License for more details.
	110
	111	You should have received a copy of the GNU General Public License
	112	along with this program. If not, see <http://www.gnu.org/licenses/>.