{
   foreach ($_) {
      if (/\G (?= >>> )/xgc) {
         # Substitution supressed by following ">>".
         redo if /\G >>> [^\n]* \n (?= >> (?!>) ) /xcg;

         # Do substitution.
         s/\G >>>/PYTHON_PROMPT/xcg;

         # Go process next line.
         /\G [^\n]* \n /xcg;
         redo;
      }

      # Substitution prevented by preceeding ">>"
      /\G >> (?!>) [^\n]* \n >>> [^\n]* \n /xcg && redo;
           # ^^^^^ optional due to statement order.

      # Go process next line.
      m/\G [^\n]* \n /xcg && redo;
   }

   print;
   print("==========\n");
}