

  (3) If <i>callback</i> is not NULL, it must point to a function that is
      called with <i>data</i> as an argument at the start of matching, in
      order to set up a JIT stack. If the return from the callback
      function is NULL, the internal 32K stack is used; otherwise the
      return value must be a valid JIT stack, the result of calling
      <b>pcre_jit_stack_alloc()</b>.
</pre>
A callback function is obeyed whenever JIT code is about to be run; it is not
obeyed when <b>pcre_exec()</b> is c