regexp_replace( , , , NULL ) returns null?

Is there any particular reason for this:

decibel(at)decina(dot)local=# SELECT regexp_replace( 'a', 'a', 'b', null ) IS NULL;
?column?
----------
t
(1 row)

ISTM it’d be a lot better if it treated NULL flags the same as ‘’...
--
Jim Nasby, Lead Data Architect (512) 569-9461