using replace function

Hello list

I want to replace a string (not a substring) in a field but making sure
that the string in the full field.

In the example below since 'blue' is not the full text in 'blue shark'
there should not have a replacement.

How to say "replace only with 'blue' if it is the full string in the field".

With REPLACE I get:

SELECT REPLACE('blue shark','blue','blue fish');
     replace
-----------------
 blue fish shark
(1 row)

but I would like to get:

SELECT REPLACE('blue shark','blue','blue fish');
  replace
------------
 blue shark
(1 row)

Thanks a lot

Antonio Olinto