maybe incorrect regexp_replace behavior in v8.3.4 ?

Hi:

bi_hsx_a0_latest=# select regexp_replace('xxx','^xxx$','abc');
regexp_replace
----------------
abc
(1 row)
expected behavior because there's a match

bi_hsx_a0_latest=# select regexp_replace('xxx','^xxxy$','abc');
regexp_replace
----------------
xxx
(1 row)
expected because there is no match (the 'y' in 'xxxy')

bi_hsx_a0_latest=# select regexp_replace('xxx','^xxxy$',null);
regexp_replace
----------------

(1 row)
But why did it return null in this case? I would think no match would leave it 'xxx'.

Thanks in Advance for any help and/or explanation.