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<H1><A NAME="SECTION003200000000000000000"></A><A NAME="notation"></A><a id='l2h-1' xml:id='l2h-1'></a>
<BR>
1.2 Notation
</H1>

<P>
The descriptions of lexical analysis and syntax use a modified BNF
grammar notation.  This uses the following style of definition:

<P>
<dl><dd class="grammar">
<div class="productions">
<table>
<tr>
    <td><a id='tok-name' xml:id='tok-name'>name</a></td>
    <td>::=</td>
    <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>
  <tr>
    <td><a id='tok-lc_letter' xml:id='tok-lc_letter'>lc_letter</a></td>
    <td>::=</td>
    <td>"a"..."z"</td></tr>
</table>
</div>
<a class="grammar-footer"
  href="grammar.txt" type="text/plain"
  >Download entire grammar as text.</a>
</dd></dl>

<P>
The first line says that a <code>name</code> is an <code>lc_letter</code> followed by
a sequence of zero or more <code>lc_letter</code>s and underscores.  An
<code>lc_letter</code> in turn is any of the single characters "<tt class="character">a</tt>"
through "<tt class="character">z</tt>".  (This rule is actually adhered to for the
names defined in lexical and grammar rules in this document.)

<P>
Each rule begins with a name (which is the name defined by the rule)
and <code>::=</code>.  A vertical bar (<code>|</code>) is used to separate
alternatives; it is the least binding operator in this notation.  A
star (<code>*</code>) means zero or more repetitions of the preceding item;
likewise, a plus (<code>+</code>) means one or more repetitions, and a
phrase enclosed in square brackets (<code>[ ]</code>) means zero or one
occurrences (in other words, the enclosed phrase is optional).  The
<code>*</code> and <code>+</code> operators bind as tightly as possible;
parentheses are used for grouping.  Literal strings are enclosed in
quotes.  White space is only meaningful to separate tokens.
Rules are normally contained on a single line; rules with many
alternatives may be formatted alternatively with each line after the
first beginning with a vertical bar.

<P>
In lexical definitions (as the example above), two more conventions
are used: Two literal characters separated by three dots mean a choice
of any single character in the given (inclusive) range of ASCII
characters.  A phrase between angular brackets (<code>&lt;...&gt;</code>) gives an
informal description of the symbol defined; e.g., this could be used
to describe the notion of `control character' if needed.

<P>
Even though the notation used is almost the same, there is a big
difference between the meaning of lexical and syntactic definitions:
a lexical definition operates on the individual characters of the
input source, while a syntax definition operates on the stream of
tokens generated by the lexical analysis.  All uses of BNF in the next
chapter (``Lexical Analysis'') are lexical definitions; uses in
subsequent chapters are syntactic definitions.

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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