Corrected regex for subdomains

A fix to handle subdomains of a duckdns domain.  I.e.  subdomain.mydomain.duckdns.org
Handles n number of subdomains

dnsapi/dns_duckdns.sh

@@ -96,7 +96,7 @@ dns_duckdns_rm() {
 _duckdns_get_domain() {
 
   # We'll extract the domain/username from full domain
-  _duckdns_domain="$(printf "%s" "$fulldomain" | _lower_case | _egrep_o '^(_acme-challenge\.)?[a-z0-9-]*\.duckdns\.org' | sed 's/^\(_acme-challenge\.\)\{0,1\}\([a-z0-9-]*\)\.duckdns\.org/\2/')"
+  _duckdns_domain="$(printf "%s" "$fulldomain" | _lower_case | _egrep_o '^(_acme-challenge\.)?([a-z0-9-]+\.)+duckdns\.org' | sed -n 's/^\([^.]\{1,\}\.\)*\([a-z0-9-]\{1,\}\)\.duckdns\.org$/\2/p;')"
 
   if [ -z "$_duckdns_domain" ]; then
     _err "Error extracting the domain."