Source

File: wp-includes/Requests/Cookie.php

public function domain_matches($string) {
		if (!isset($this->attributes['domain'])) {
			// Cookies created manually; cookies created by Requests will set
			// the domain to the requested domain
			return true;
		}

		$domain_string = $this->attributes['domain'];
		if ($domain_string === $string) {
			// The domain string and the string are identical.
			return true;
		}

		// If the cookie is marked as host-only and we don't have an exact
		// match, reject the cookie
		if ($this->flags['host-only'] === true) {
			return false;
		}

		if (strlen($string) <= strlen($domain_string)) {
			// For obvious reasons, the string cannot be a suffix if the domain
			// is shorter than the domain string
			return false;
		}

		if (substr($string, -1 * strlen($domain_string)) !== $domain_string) {
			// The domain string should be a suffix of the string.
			return false;
		}

		$prefix = substr($string, 0, strlen($string) - strlen($domain_string));
		if (substr($prefix, -1) !== '.') {
			// The last character of the string that is not included in the
			// domain string should be a %x2E (".") character.
			return false;
		}

		// The string should be a host name (i.e., not an IP address).
		return !preg_match('#^(.+\.)\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$#', $string);
	}