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8 GNU ‘sed’'s Limitations and Non-limitations
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For those who want to write portable ‘sed’ scripts, be aware that some
implementations have been known to limit line lengths (for the pattern
and hold spaces) to be no more than 4000 bytes.  The POSIX standard
specifies that conforming ‘sed’ implementations shall support at least
8192 byte line lengths.  GNU ‘sed’ has no built-in limit on line length;
as long as it can ‘malloc()’ more (virtual) memory, you can feed or
construct lines as long as you like.

   However, recursion is used to handle subpatterns and indefinite
repetition.  This means that the available stack space may limit the
size of the buffer that can be processed by certain patterns.