built-in regex matches wrong character

[mamatb]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-unknown-linux-gnu' 
-DCONF_VENDOR='unknown' -DLOCALEDIR='/usr/local/share/locale' -DPACKAGE='bash' 
-DSHELL -DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib   -g -O2 
-Wno-parentheses -Wno-format-security
uname output: Linux mamatb-laptop 4.4.0-98-generic #121-Ubuntu SMP Tue Oct 10 
14:24:03 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-unknown-linux-gnu

Bash Version: 4.4
Patch Level: 0
Release Status: release

Description:
        It seems like bash built-in regex matches some symbols that shouldn't. 
The following commands shows this:
                [[ 'º' =~ [o-p] ]] && [[ ! 'º' =~ o ]] && [[ ! 'º' =~ p ]] 
&& echo 'º between o and p but none of them'
                [[ 'ª' =~ [a-b] ]] && [[ ! 'ª' =~ a ]] && [[ ! 'ª' =~ b ]] 
&& echo 'ª between a and b but none of them'

Repeat-By:
        Actually found out this while developing a bigger bash script, but it 
can be reproduced with the previous lines. Would you reply me at 
amatbaeza@gmail.com to know if this was in fact a bug? Thanks.