#!/bin/bash
s="product: 13.6.0.35_0"
grep -ow '13\.[0-9]*' <<< "$s" | head -1

sed -En 's/.* ([0-9]+\.[0-9]+).*/\1/p' <<< "$s"
awk -F'[[:space:].]' '{print $2"."$3}' <<< "$s"