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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<title>Simple Examples (Lexical Analysis With Flex, for Flex 2.6.4)</title>

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<body lang="en">
<a name="Simple-Examples"></a>
<div class="header">
<p>
Next: <a href="Format.html#Format" accesskey="n" rel="next">Format</a>, Previous: <a href="Introduction.html#Introduction" accesskey="p" rel="prev">Introduction</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Some-Simple-Examples"></a>
<h2 class="chapter">4 Some Simple Examples</h2>

<p>First some simple examples to get the flavor of how one uses
<code>flex</code>.
</p>
<a name="index-username-expansion"></a>
<p>The following <code>flex</code> input specifies a scanner which, when it
encounters the string &lsquo;<samp>username</samp>&rsquo; will replace it with the user&rsquo;s
login name:
</p>
<div class="example">
<pre class="verbatim">    %%
    username    printf( &quot;%s&quot;, getlogin() );
</pre></div>

<a name="index-default-rule"></a>
<a name="index-rules_002c-default"></a>
<p>By default, any text not matched by a <code>flex</code> scanner is copied to
the output, so the net effect of this scanner is to copy its input file
to its output with each occurrence of &lsquo;<samp>username</samp>&rsquo; expanded.  In this
input, there is just one rule.  &lsquo;<samp>username</samp>&rsquo; is the <em>pattern</em> and
the &lsquo;<samp>printf</samp>&rsquo; is the <em>action</em>.  The &lsquo;<samp>%%</samp>&rsquo; symbol marks the
beginning of the rules.
</p>
<p>Here&rsquo;s another simple example:
</p>
<a name="index-counting-characters-and-lines"></a>
<div class="example">
<pre class="verbatim">            int num_lines = 0, num_chars = 0;

    %%
    \n      ++num_lines; ++num_chars;
    .       ++num_chars;

    %%

    int main()
            {
            yylex();
            printf( &quot;# of lines = %d, # of chars = %d\n&quot;,
                    num_lines, num_chars );
            }
</pre></div>

<p>This scanner counts the number of characters and the number of lines in
its input. It produces no output other than the final report on the
character and line counts.  The first line declares two globals,
<code>num_lines</code> and <code>num_chars</code>, which are accessible both inside
<code>yylex()</code> and in the <code>main()</code> routine declared after the
second &lsquo;<samp>%%</samp>&rsquo;.  There are two rules, one which matches a newline
(&lsquo;<samp>\n</samp>&rsquo;) and increments both the line count and the character count,
and one which matches any character other than a newline (indicated by
the &lsquo;<samp>.</samp>&rsquo; regular expression).
</p>
<p>A somewhat more complicated example:
</p>
<a name="index-Pascal_002dlike-language"></a>
<div class="example">
<pre class="verbatim">    /* scanner for a toy Pascal-like language */

    %{
    /* need this for the call to atof() below */
    #include &lt;math.h&gt;
    %}

    DIGIT    [0-9]
    ID       [a-z][a-z0-9]*

    %%

    {DIGIT}+    {
                printf( &quot;An integer: %s (%d)\n&quot;, yytext,
                        atoi( yytext ) );
                }

    {DIGIT}+&quot;.&quot;{DIGIT}*        {
                printf( &quot;A float: %s (%g)\n&quot;, yytext,
                        atof( yytext ) );
                }

    if|then|begin|end|procedure|function        {
                printf( &quot;A keyword: %s\n&quot;, yytext );
                }

    {ID}        printf( &quot;An identifier: %s\n&quot;, yytext );

    &quot;+&quot;|&quot;-&quot;|&quot;*&quot;|&quot;/&quot;   printf( &quot;An operator: %s\n&quot;, yytext );

    &quot;{&quot;[^{}\n]*&quot;}&quot;     /* eat up one-line comments */

    [ \t\n]+          /* eat up whitespace */

    .           printf( &quot;Unrecognized character: %s\n&quot;, yytext );

    %%

    int main( int argc, char **argv )
        {
        ++argv, --argc;  /* skip over program name */
        if ( argc &gt; 0 )
                yyin = fopen( argv[0], &quot;r&quot; );
        else
                yyin = stdin;

        yylex();
        }
</pre></div>

<p>This is the beginnings of a simple scanner for a language like Pascal.
It identifies different types of <em>tokens</em> and reports on what it has
seen.
</p>
<p>The details of this example will be explained in the following
sections.
</p>
<hr>
<div class="header">
<p>
Next: <a href="Format.html#Format" accesskey="n" rel="next">Format</a>, Previous: <a href="Introduction.html#Introduction" accesskey="p" rel="prev">Introduction</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
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