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1 -*- indented-text -*-
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2
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3 This file contains two sections:
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4
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5 1) An EBNF (Extended Backus-Naur Form) description of the format of
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6 the tags file created by etags.c and interpreted by etags.el;
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7 2) A discussion of tag names and implicit tag names.
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8
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9 ====================== 1) EBNF tag file description =====================
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10
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11 Productions created from current behaviour to aid extensions
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12 Francesco Potorti` <pot@gnu.org> 2002
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13 ----------------
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14
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15 FF ::= #x0c /* tag section starter */
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16
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17 LF ::= #x0a /* line terminator */
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18
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19 DEL ::= #x7f /* pattern terminator */
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20
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21 SOH ::= #x01 /* name terminator */
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22
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23 regchar ::= [^#x0a#x0c#x7f] /* regular character */
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24
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25 regstring ::= { regchar } /* regular string */
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26
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27 unsint ::= [0-9] { [0-9] } /* non-negative integer */
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28
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29
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30
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31 tagfile ::= { tagsection } /* a tags file */
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32
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33 tagsection ::= FF LF ( includesec | regularsec ) LF
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34
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35 includesec ::= filename ",include" [ LF fileprop ]
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36
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37 regularsec ::= filename "," [ unsint ] [ LF fileprop ] { LF tag }
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38
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39 filename ::= regchar regstring /* a file name */
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40
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41 fileprop ::= "(" regstring ")" /* an elisp alist */
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42
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43 tag ::= directtag | patterntag
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44
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45 directtag ::= DEL realposition /* no pattern */
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46
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47 patterntag ::= pattern DEL [ tagname SOH ] position
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48
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49 pattern ::= regstring /* a tag pattern */
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50
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51 tagname ::= regchar regstring /* a tag name */
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52
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53 position ::= realposition | "," /* charpos,linepos */
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54
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55 realposition ::= "," unsint | unsint "," | unsint "," unsint
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56
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57 ==================== end of EBNF tag file description ====================
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58
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59
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60
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61 ======================= 2) discussion of tag names =======================
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62
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63 - WHAT ARE TAG NAMES
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64 Tag lines in a tags file are usually made from the above defined pattern
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65 and by an optional tag name. The pattern is a string that is searched
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66 in the source file to find the tagged line.
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67
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68 - WHY TAG NAMES ARE GOOD
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69 When a user looks for a tag, Emacs first compares the tag with the tag
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70 names contained in the tags file. If no match is found, Emacs compares
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71 the tag with the patterns. The tag name is then the preferred way to
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72 look for tags in the tags file, because when the tag name is present
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73 Emacs can find a tag faster and more accurately. These tag names are
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74 part of tag lines in the tags file, so we call them "explicit".
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75
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76 - WHY IMPLICIT TAG NAMES ARE EVEN BETTER
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77 When a tag line has no name, but a name can be deduced from the pattern,
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78 we say that the tag line has an implicit tag name. Often tag names are
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79 redundant; this happens when the name of a tag is an easily guessable
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80 substring of the tag pattern. We define a set of rules to decide
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81 whether it is possible to deduce the tag name from the pattern, and make
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82 an unnamed tag in those cases. The name deduced from the pattern of an
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83 unnamed tag is the implicit name of that tag.
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84 When the user looks for a tag, and Emacs finds no explicit tag names
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85 that match it, Emacs then looks for an tag whose implicit tag name
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86 matches the request. etags.c uses implicit tag names when possible, in
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87 order to reduce the size of the tags file.
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88 An implicit tag name is deduced from the pattern by discarding the
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89 last character if it is one of ` \f\t\n\r()=,;', then taking all the
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90 rightmost consecutive characters in the pattern which are not one of
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91 those.
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92
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93 ===================== end of discussion of tag names =====================
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