ETAGS.EBNF 3.37 KB
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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 =======================
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Productions created from current behaviour to aid extensions
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Francesco Potorti` <pot@gnu.org> 2002
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----------------
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FF ::= #x0c				   /* tag section starter */
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LF ::= #x0a				   /* line terminator */
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DEL ::= #x7f				   /* pattern terminator */
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SOH ::= #x01				   /* name terminator */
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regchar ::= [^#x0a#x0c#x7f]		   /* regular character */
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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 */

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fileprop ::= "(" regstring ")"		   /* an elisp alist */
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tag ::= directtag | patterntag

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directtag ::= DEL realposition		   /* no pattern */
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patterntag ::= pattern DEL [ tagname SOH ] position
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pattern ::= regstring			   /* a tag pattern */

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

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position ::= realposition | "," 	   /* charpos,linepos */
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realposition ::= "," unsint | unsint "," | unsint "," unsint
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==================== 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
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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.  The use of
implicit tag names reduces the size of the tags file.  When the user
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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 =====================