Sed with inplace editing changes group ownership of file

I have shell (php) script that gets in touch with target file this way: - inspects whether file and directory are writable with php 's is_writable() (I don't think that this is problem) - does in-place file edit with sed command: grep -q "$search" "$passwd_file" && { sed -i "s|$search|$replace|" "$passwd_file"; printf "Password changed!\n"; } || printf "Password not changed!\n" As a result I get (everything else correct but) file which was myuser:www-data to be myuser:myuser . Does sed changes file group ownership as it seems, and how do I avoid it, if possible?