(flex.info)M4 Dependency

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A.3 M4 Dependency
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The macro processor 'm4'(1) must be installed wherever flex is
installed.  'flex' invokes 'm4', found by searching the directories in
the 'PATH' environment variable.  Any code you place in section 1 or in
the actions will be sent through m4.  Please follow these rules to
protect your code from unwanted 'm4' processing.

   * Do not use symbols that begin with, 'm4_', such as, 'm4_define', or
     'm4_include', since those are reserved for 'm4' macro names.  If
     for some reason you need m4_ as a prefix, use a preprocessor
     #define to get your symbol past m4 unmangled.

   * Do not use the strings '[[' or ']]' anywhere in your code.  The
     former is not valid in C, except within comments and strings, but
     the latter is valid in code such as 'x[y[z]]'.  The solution is
     simple.  To get the literal string '"]]"', use '"]""]"'.  To get
     the array notation 'x[y[z]]', use 'x[y[z] ]'.  Flex will attempt to
     detect these sequences in user code, and escape them.  However,
     it's best to avoid this complexity where possible, by removing such
     sequences from your code.

   'm4' is only required at the time you run 'flex'.  The generated
scanner is ordinary C or C++, and does _not_ require 'm4'.

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

   (1) The use of m4 is subject to change in future revisions of flex.
It is not part of the public API of flex.  Do not depend on it.