sed 's/.* //;s/\(.*\)\..*/\1/' <<<"
/home/user/my file 1_single_extension_12345.txt
/home/user/my file 10_empty_extension_43961.
/home/user/my log 11_double_extension_432.log.gz
/home/user/my 100_no_extension_12333333
/home/user/NOTmine doc 101_ye_200.doc
/root/root's file 110_azzz_45445123112.dat
/home/us er/myfile_WHITESPACE_ONLY_IN_PATH_1.wtf1
/home/user/myfile_NO_WHITESPACE_2.wtf2
"

_string='/home/user/my file 1хххххххх.txt'; 
echo ${_string:19:-4}; 
unset -v _string;