#!/bin/bash
s='range 192.168.1.1 192.168.1.100'
sed -E '/range ([0-9]{1,3}\.){3}[0-9]{1,3}/i\ip address dhcp' <<< "$s"

sed -E '/range ((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)([^0-9]|$)/i\ip address dhcp' <<< "$s"