The following bug has been logged on the website:
Bug reference: 6457
Logged by: Albert Cieszkowski
Email address: albert(dot)cieszkowski(at)cc(dot)com(dot)pl
PostgreSQL version: 9.0.6
Operating system: CentOS 5.x
Description:
OS, base and client encoding UTF-8:
peimp=> select 'Świnoujście' ~* '\mŚwinoujście\M';
?column?
----------
f
(1 row)
peimp=> select 'Świnoujście' ~* '\AŚwinoujście\Z';
?column?
----------
t
(1 row)
but:
peimp=> select 'Mróz' ~* '\mmróZ\M';
?column?
----------
t
(1 row)
peimp=> select 'Mróz' ~* '\AmróZ\Z';
?column?
----------
t
(1 row)
I believe it is connected with bug #5766 and #3433.