diff --git a/acme-http01-webroot.lua b/acme-http01-webroot.lua
index f45445d..efd9176 100644
--- a/acme-http01-webroot.lua
+++ b/acme-http01-webroot.lua
@@ -84,8 +84,8 @@ end
 -- see https://github.com/letsencrypt/acme-spec/blob/master/draft-barnes-acme.md
 --
 function sanitizeToken(token)
-	_strip="[^%a%d%+%-%_=]"
-	token = token:gsub(_strip,'')
+	local pattern="^[%a%d%-_]+$"
+	token = token:match(pattern)
 	return token
 end
 
@@ -104,4 +104,4 @@ function getKeyAuth(token)
 end
 
 core.register_init(acme.startup)
-core.register_service("acme-http01", "http", acme.http01)
\ No newline at end of file
+core.register_service("acme-http01", "http", acme.http01)