<?php

function encontrarValor($domain)
{
	if(preg_match('/(http(s)?:\/\/)?(www\.)?(.+)/', $domain, $matches)){
		return explode('/',$matches[4])[0];
	}
}
echo encontrarValor('https://w...content-available-to-author-only...m.br/url-123');
echo "\n";
echo encontrarValor('https://s...content-available-to-author-only...m.br/url-123');