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53 | ||
54 | <H1><A NAME="SECTION003200000000000000000"></A><A NAME="notation"></A><a id='l2h-1' xml:id='l2h-1'></a> | |
55 | <BR> | |
56 | 1.2 Notation | |
57 | </H1> | |
58 | ||
59 | <P> | |
60 | The descriptions of lexical analysis and syntax use a modified BNF | |
61 | grammar notation. This uses the following style of definition: | |
62 | ||
63 | <P> | |
64 | <dl><dd class="grammar"> | |
65 | <div class="productions"> | |
66 | <table> | |
67 | <tr> | |
68 | <td><a id='tok-name' xml:id='tok-name'>name</a></td> | |
69 | <td>::=</td> | |
70 | <td><a class='grammartoken' href="notation.html#tok-lc_letter">lc_letter</a> (<a class='grammartoken' href="notation.html#tok-lc_letter">lc_letter</a> | "_")*</td></tr> | |
71 | <tr> | |
72 | <td><a id='tok-lc_letter' xml:id='tok-lc_letter'>lc_letter</a></td> | |
73 | <td>::=</td> | |
74 | <td>"a"..."z"</td></tr> | |
75 | </table> | |
76 | </div> | |
77 | <a class="grammar-footer" | |
78 | href="grammar.txt" type="text/plain" | |
79 | >Download entire grammar as text.</a> | |
80 | </dd></dl> | |
81 | ||
82 | <P> | |
83 | The first line says that a <code>name</code> is an <code>lc_letter</code> followed by | |
84 | a sequence of zero or more <code>lc_letter</code>s and underscores. An | |
85 | <code>lc_letter</code> in turn is any of the single characters "<tt class="character">a</tt>" | |
86 | through "<tt class="character">z</tt>". (This rule is actually adhered to for the | |
87 | names defined in lexical and grammar rules in this document.) | |
88 | ||
89 | <P> | |
90 | Each rule begins with a name (which is the name defined by the rule) | |
91 | and <code>::=</code>. A vertical bar (<code>|</code>) is used to separate | |
92 | alternatives; it is the least binding operator in this notation. A | |
93 | star (<code>*</code>) means zero or more repetitions of the preceding item; | |
94 | likewise, a plus (<code>+</code>) means one or more repetitions, and a | |
95 | phrase enclosed in square brackets (<code>[ ]</code>) means zero or one | |
96 | occurrences (in other words, the enclosed phrase is optional). The | |
97 | <code>*</code> and <code>+</code> operators bind as tightly as possible; | |
98 | parentheses are used for grouping. Literal strings are enclosed in | |
99 | quotes. White space is only meaningful to separate tokens. | |
100 | Rules are normally contained on a single line; rules with many | |
101 | alternatives may be formatted alternatively with each line after the | |
102 | first beginning with a vertical bar. | |
103 | ||
104 | <P> | |
105 | In lexical definitions (as the example above), two more conventions | |
106 | are used: Two literal characters separated by three dots mean a choice | |
107 | of any single character in the given (inclusive) range of ASCII | |
108 | characters. A phrase between angular brackets (<code><...></code>) gives an | |
109 | informal description of the symbol defined; e.g., this could be used | |
110 | to describe the notion of `control character' if needed. | |
111 | ||
112 | <P> | |
113 | Even though the notation used is almost the same, there is a big | |
114 | difference between the meaning of lexical and syntactic definitions: | |
115 | a lexical definition operates on the individual characters of the | |
116 | input source, while a syntax definition operates on the stream of | |
117 | tokens generated by the lexical analysis. All uses of BNF in the next | |
118 | chapter (``Lexical Analysis'') are lexical definitions; uses in | |
119 | subsequent chapters are syntactic definitions. | |
120 | ||
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155 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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