--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_muc_restrict_nick/mod_muc_restrict_nick.lua	Tue Jun 15 12:35:17 2021 +0100
@@ -0,0 +1,13 @@
+local jid = require "util.jid";
+local st = require "util.stanza";
+
+local nick_pattern = module:get_option_string("muc_restrict_nick_pattern", "^%w+$");
+
+module:hook("muc-occupant-pre-join", function (event)
+	local nick = jid.resource(event.occupant.nick);
+	if not nick:match(nick_pattern) then
+		local reply = st.error_reply(event.stanza, "modify", "policy-violation", "Unacceptable nickname, please try another");
+		module:send(reply);
+		return true;
+	end
+end);