(flex.info)Definitions Section

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5.1 Format of the Definitions Section
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The "definitions section" contains declarations of simple "name"
definitions to simplify the scanner specification, and declarations of
"start conditions", which are explained in a later section.

   Name definitions have the form:

         name definition

   The 'name' is a word beginning with a letter or an underscore ('_')
followed by zero or more letters, digits, '_', or '-' (dash).  The
definition is taken to begin at the first non-whitespace character
following the name and continuing to the end of the line.  The
definition can subsequently be referred to using '{name}', which will
expand to '(definition)'.  For example,

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

   Defines 'DIGIT' to be a regular expression which matches a single
digit, and 'ID' to be a regular expression which matches a letter
followed by zero-or-more letters-or-digits.  A subsequent reference to

         {DIGIT}+"."{DIGIT}*

   is identical to

         ([0-9])+"."([0-9])*

   and matches one-or-more digits followed by a '.' followed by
zero-or-more digits.

   An unindented comment (i.e., a line beginning with '/*') is copied
verbatim to the output up to the next '*/'.

   Any _indented_ text or text enclosed in '%{' and '%}' is also copied
verbatim to the output (with the %{ and %} symbols removed).  The %{ and
%} symbols must appear unindented on lines by themselves.

   A '%top' block is similar to a '%{' ...  '%}' block, except that the
code in a '%top' block is relocated to the _top_ of the generated file,
before any flex definitions (1).  The '%top' block is useful when you
want certain preprocessor macros to be defined or certain files to be
included before the generated code.  The single characters, '{' and '}'
are used to delimit the '%top' block, as show in the example below:

         %top{
             /* This code goes at the "top" of the generated file. */
             #include <stdint.h>
             #include <inttypes.h>
         }

   Multiple '%top' blocks are allowed, and their order is preserved.

   ---------- Footnotes ----------

   (1) Actually, 'yyIN_HEADER' is defined before the '%top' block.