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.