>> text
>>> text

##</code><code>##

>> text
PYTHON_PROMPT text

##</code><code>##

foreach (split("==========\n", <<'__EOI__'))
>>> i = q
==========
YES
>>> i = q
==========
>>> i = q
YES
==========
>>> i = q
>>
==========
>>
>>> i = q
==========
>>> i = q
>> No!
==========
>> No!
>>> i = q
==========
YES
>>> i = q
YES
>>> i = q
==========
>>> i = q
>> No!
YES
>>> i = q      <--- FAILS!!
__EOI__

##</code><code>##

{
   s/
      \G
      ( # $1
         (?:
            # Substitution prevented by preceeding ">>"
            >> (?!>) [^\n]* \n >>> [^\n]* \n
         |
            (?! >> ) [^\n]* \n
         )*
      )

      >>>

      ( # $2
         [^\n]* \n
      )

      # Substitution supressed by following ">>".
      (?! >> (?!>))

   /${1}PYTHON_PROMPT${2}/xg;

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

##</code><code>##

foreach (split("==========\n", <<'__EOI__'))
>>> i = q
==========
YES
>>> i = q
==========
>>> i = q
YES
==========
>>> i = q
>>
==========
>>
>>> i = q
==========
>>> i = q
>> No!
==========
>> No!
>>> i = q
==========
YES
>>> i = q
YES
>>> i = q
==========
>>> i = q
>> No!
YES
>>> i = q
__EOI__

##</code><code>##

{
   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");
}