php7.4-mbstring : Depends: libonig4 (>= 5.9.5) but it is not installable code example

Example: como instalar ext-mbstring en ubuntu 18.04

deb http://archive.ubuntu.com/ubuntu bionic main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-security main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-updates main multiverse restricted universe