dns_duckdns.sh - correctly extract domain

$fulldomain could be just 'domain.duckdns.org' if provided with --domain-alias or '_acme-challenge.domain.duckdns.org' otherwise. In the latter case, '_acme-challenge' is thrown away. Correctly extract 'domain' in both cases.

dnsapi/dns_duckdns.sh

@@ -91,13 +91,12 @@ dns_duckdns_rm() {
 
 ####################  Private functions below ##################################
 
-#fulldomain=acme-challenge-domain.duckdns.org
-#returns
-# _duckdns_domain=acme-challenge-domain
+# fulldomain may be 'domain.duckdns.org' (if using --domain-alias) or '_acme-challenge.domain.duckdns.org'
+# either way, return 'domain'. (duckdns does not allow further subdomains and restricts domains to [a-z0-9-].)
 _duckdns_get_domain() {
 
   # We'll extract the domain/username from full domain
-  _duckdns_domain="$(printf "%s" "$fulldomain" | _lower_case | sed  's/^\([a-z0-9-]*\)\.duckdns\.org/\1/')"
+  _duckdns_domain="$(printf "%s" "$fulldomain" | _lower_case | _egrep_o '^(_acme-challenge\.)?[a-z0-9-]*\.duckdns\.org' | sed -E 's/^(_acme-challenge\.)?([a-z0-9-]*)\.duckdns\.org/\2/')"
 
   if [ -z "$_duckdns_domain" ]; then
     _err "Error extracting the domain."