Treating cyrillic symbols

[L reg]

So, I was trying to use grep with my UTF8 file, which has some cyrillic
letters.

My command was: grep -n -P "sы{0,1}" ~/dicts/missing_F.xml
> 4:<description>Authorsы: </description><ar><k>fasolówka</k><def> <pos>ж.
разг.</pos> фасо́левый суп</def></ar>


But for the command: grep -n -P "в{0,1}sы{0,1}" ~/dicts/missing_F.xml
it gves me completely nothing.

How can this be, when that line should match it?

ps. My locale check outputs:
> locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=