title: With the Flex -d command line flag, why am I getting --(end of buffer or a NUL)?
body: <p>I am using the -d flag when I run Flex, so that the scanner will generate debug messages. It works fine but the output contains some odd (unexpected) things. Below I show the output for a lexer that tokenizes names, numbers and newlines. Notice the first line of the output:</p>
  <pre><code>--(end of buffer or a NUL)
  </code></pre>
  <p>Huh? What is that?</p>
  <p>Then there are a couple more towards the end. What are they all about?</p>
  <pre><code>--(end of buffer or a NUL)
  --accepting rule at line 5 (&quot;John&quot;)
  --accepting rule at line 7 (&quot;
  &quot;)
  --accepting rule at line 6 (&quot;24&quot;)
  --accepting rule at line 7 (&quot;
  &quot;)
  --accepting rule at line 5 (&quot;Sally&quot;)
  --accepting rule at line 7 (&quot;
  &quot;)
  --accepting rule at line 6 (&quot;30&quot;)
  --accepting rule at line 7 (&quot;
  &quot;)
  --accepting rule at line 5 (&quot;Bill&quot;)
  --accepting rule at line 7 (&quot;
  &quot;)
  --(end of buffer or a NUL)
  --accepting rule at line 6 (&quot;36&quot;)
  --(end of buffer or a NUL)
  --EOF (start condition 0)
  </code></pre>