#!/bin/bash
s='https://s...content-available-to-author-only...n.tld/[...]
https://s...content-available-to-author-only...n.tld/test/[...]
https://s...content-available-to-author-only...n.tld/[...]
https://s...content-available-to-author-only...n.tld/files/[...]'
grep -oP 'https?://\K[\w-]+(?=\.stage\.domain\.tld)' <<< "$s"
IyEvYmluL2Jhc2gKcz0naHR0cHM6Ly9zLi4uY29udGVudC1hdmFpbGFibGUtdG8tYXV0aG9yLW9ubHkuLi5uLnRsZC9bLi4uXQpodHRwczovL3MuLi5jb250ZW50LWF2YWlsYWJsZS10by1hdXRob3Itb25seS4uLm4udGxkL3Rlc3QvWy4uLl0KaHR0cHM6Ly9zLi4uY29udGVudC1hdmFpbGFibGUtdG8tYXV0aG9yLW9ubHkuLi5uLnRsZC9bLi4uXQpodHRwczovL3MuLi5jb250ZW50LWF2YWlsYWJsZS10by1hdXRob3Itb25seS4uLm4udGxkL2ZpbGVzL1suLi5dJwpncmVwIC1vUCAnaHR0cHM/Oi8vXEtbXHctXSsoPz1cLnN0YWdlXC5kb21haW5cLnRsZCknIDw8PCAiJHMi