BUG #6457: Regexp not processing word (with special characters on ends) correctly (UTF-8)

The following bug has been logged on the website:

Bug reference:      6457
Logged by:          Albert Cieszkowski
Email address:      albert.cieszkowski@cc.com.pl
PostgreSQL version: 9.0.6
Operating system:   CentOS 5.x
Description:=20=20=20=20=20=20=20=20

OS, base and client encoding UTF-8:

peimp=3D> select '=C5=9Awinouj=C5=9Bcie' ~* '\m=C5=9Awinouj=C5=9Bcie\M';
 ?column?
----------
 f
(1 row)

peimp=3D> select '=C5=9Awinouj=C5=9Bcie' ~* '\A=C5=9Awinouj=C5=9Bcie\Z';
 ?column?
----------
 t
(1 row)

but:

peimp=3D> select 'Mr=C3=B3z' ~* '\mmr=C3=B3Z\M';
 ?column?
----------
 t
(1 row)

peimp=3D> select 'Mr=C3=B3z' ~* '\Amr=C3=B3Z\Z';
 ?column?
----------
 t
(1 row)

I believe it is connected with bug #5766 and #3433.