Constraint: string length must be 32 chars

Hello,

I'm trying to create a table, where md5 strings will serve as primary keys.
So I'd like to add a constraing that the key length should be 32 chars long
(and contain [a-fA-F0-9] only):

create table gps (
id varchar(32) primary key CONSTRAINT char_length(id)==32,
stamp timestamp DEFAULT current_timestamp,
pos point);

But it fails:

ERROR: syntax error at or near "("
LINE 2: id varchar(32) primary key CONSTRAINT char_length(id)==32,
^
Does anybody please know what's wrong here?

Thank you
Alex