diff --git a/plugins/degesch/prime.lua b/plugins/degesch/prime.lua
new file mode 100644
index 0000000..bffc0e1
--- /dev/null
+++ b/plugins/degesch/prime.lua
@@ -0,0 +1,50 @@
+--
+-- prime.lua: highlight prime numbers in messages
+--
+-- Copyright (c) 2020, Přemysl Eric Janouch <p@janouch.name>
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+-- WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+-- MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+-- SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+-- WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+-- OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+-- CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+--
+
+-- The prime test is actually very fast, so there is no DoS concern
+local is_prime = function (n)
+	if tonumber (n) < 2 then return false end
+	for i = 2, n ^ (1 / 2) do if (n % i) == 0 then return false end end
+	return true
+end
+
+local do_interlink = function (run)
+	-- Let's not cripple formatting, colours especially
+	-- TODO: we can just employ the same approach as in do_text and skip over
+	if run:find ('%c') then return run end
+
+	return run:gsub ("%f[%w_]%d+%f[^%w_]", function (number)
+		if is_prime (number) then return "\x1f" .. number .. "\x1f" end
+		return number
+	end)
+end
+
+local do_text = function (text)
+	local rebuilt, last = {""}, 1
+	for run, link, endpos in text:gmatch ('(.-)(%f[%g]https?://%g+)()') do
+		last = endpos
+		table.insert (rebuilt, do_interlink (run) .. link)
+	end
+	return table.concat (rebuilt, "") .. do_interlink (text:sub (last))
+end
+
+-- XXX: sadly it won't typically highlight primes in our own messages,
+-- unless IRCv3 echo-message is on
+degesch.hook_irc (function (hook, server, line)
+	local start, message = line:match ("^(.- PRIVMSG .- :)(.*)$")
+	return message and start .. do_text (message) or line
+end)