/[emacs]/emacs/etc/ETAGS.EBNF
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revision 1.4 by pot, Thu Jun 13 10:57:55 2002 UTC revision 1.5 by pot, Fri Jun 21 12:05:52 2002 UTC
# Line 1  Line 1 
1  EBNF (Extended Backus Normal Form) description of the format of the tags  This file contains two sections:
2  file created by etags.c and interpreted by etags.el  
3    1) An EBNF (Extended Backus Normal Form) description of the format of
4       the tags file created by etags.c and interpreted by etags.el
5    2) A discussion of tag names and implicit tag names
6    
7    ======================= EBNF tag file description =======================
8    
9  Productions created from current behaviour to aid extensions  Productions created from current behaviour to aid extensions
10  Francesco Potorti` <pot@gnu.org> 2002  Francesco Potorti` <pot@gnu.org> 2002
11  ================================================================  ----------------
12    
13  FF ::= #x0c                                /* tag section starter */  FF ::= #x0c                                /* tag section starter */
14    
# Line 46  tagname ::= regchar regstring             /* a t Line 51  tagname ::= regchar regstring             /* a t
51  position ::= realposition | ","            /* charpos,linepos */  position ::= realposition | ","            /* charpos,linepos */
52    
53  realposition ::= "," unsint | unsint "," | unsint "," unsint  realposition ::= "," unsint | unsint "," | unsint "," unsint
54    
55    ==================== end of EBNF tag file description ====================
56    
57    
58    
59    ======================== discussion on tag names =========================
60    
61    - What are tag names
62    Tag lines in a tags file are usually made from the above defined pattern
63    and by an optional tag name.  The pattern is a string that is searched
64    in the source file to find the tagged line.
65    
66    - Why tag names are good
67    When a user looks for a tag, Emacs first compares the tag with the tag
68    names contained in the tags file.  If no match is found, Emacs compares
69    the tag with the patterns.  The tag name is then the preferred way to
70    look for tags in the tags file, because when the tag name is present
71    Emacs can find a tag faster and more accurately.  These tag names are
72    part of tag lines in the tags file, so we call them "explicit".
73    
74    - Why implicit tag names are even better
75    The purpose of implicit tag names is to reduce the number of tag names
76    in a tags file, thus reducing the size of the tags file.  When the user
77    looks for a tag, and Emacs founds no explicit tag names that match it,
78    Emacs then tries to match the tag with an implicit tag name.  Such a
79    match occurs when the tag matches a pattern, subject to the satisfaction
80    of all the following four rules:
81    
82     NONAM=" \f\t\n\r()=,;";
83      1. the tag does not contain any of the characters in NONAM;
84      2. the pattern contains the tag as either a rightmost, or rightmost
85         but one character, substring;
86      3. the character, if any, immediately before the tag in the pattern
87         must be a character in NONAM;
88      4. the character, if any, immediately after the tag in the pattern
89         must also be a character in NONAM.
90    
91    ===================== end of discussion on tag names =====================

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