diff --git a/cache/stores/memcache/addinstanceform.php b/cache/stores/memcache/addinstanceform.php
index 984966a8b56..eb7c999a1be 100644
--- a/cache/stores/memcache/addinstanceform.php
+++ b/cache/stores/memcache/addinstanceform.php
@@ -49,7 +49,8 @@ class cachestore_memcache_addinstance_form extends cachestore_addinstance_form {
         $form->addElement('text', 'prefix', get_string('prefix', 'cachestore_memcache'),
                 array('maxlength' => 5, 'size' => 5));
         $form->addHelpButton('prefix', 'prefix', 'cachestore_memcache');
-        $form->setType('prefix', PARAM_ALPHAEXT);
+        $form->setType('prefix', PARAM_TEXT); // We set to text but we have a rule to limit to alphanumext.
         $form->setDefault('prefix', 'mdl_');
+        $form->addRule('prefix', get_string('prefixinvalid', 'cachestore_memcache'), 'regex', '#^[a-zA-Z0-9\-_]+$#');
     }
 }
\ No newline at end of file
diff --git a/cache/stores/memcache/lang/en/cachestore_memcache.php b/cache/stores/memcache/lang/en/cachestore_memcache.php
index 5cba887de78..fb87d289836 100644
--- a/cache/stores/memcache/lang/en/cachestore_memcache.php
+++ b/cache/stores/memcache/lang/en/cachestore_memcache.php
@@ -31,6 +31,7 @@ $string['prefix'] = 'Key prefix';
 $string['prefix_help'] = 'This prefix is used for all key names on the memcache server.
 * If you only have one Moodle instance using this server, you can leave this value default.
 * Due to key length restrictions, a maximum of 5 characters is permitted.';
+$string['prefixinvalid'] = 'Invalid prefix. You can only use a-z A-Z 0-9-_.';
 $string['servers'] = 'Servers';
 $string['servers_help'] = 'This sets the servers that should be utilised by this memcache adapter.
 Servers should be defined one per line and consist of a server address and optionally a port and weight.
diff --git a/cache/stores/memcache/lib.php b/cache/stores/memcache/lib.php
index 882ab3775cb..2e104bd6385 100644
--- a/cache/stores/memcache/lib.php
+++ b/cache/stores/memcache/lib.php
@@ -404,7 +404,7 @@ class cachestore_memcache extends cache_store implements cache_is_configurable {
      * Generates an instance of the cache store that can be used for testing.
      *
      * @param cache_definition $definition
-     * @return false
+     * @return cachestore_memcache|false
      */
     public static function initialise_test_instance(cache_definition $definition) {
         if (!self::are_requirements_met()) {
diff --git a/cache/stores/memcached/addinstanceform.php b/cache/stores/memcached/addinstanceform.php
index 293b1988153..c833a18bbae 100644
--- a/cache/stores/memcached/addinstanceform.php
+++ b/cache/stores/memcached/addinstanceform.php
@@ -60,8 +60,9 @@ class cachestore_memcached_addinstance_form extends cachestore_addinstance_form
         $form->setType('serialiser', PARAM_INT);
 
         $form->addElement('text', 'prefix', get_string('prefix', 'cachestore_memcached'), array('size' => 16));
-        $form->setType('prefix', PARAM_ALPHANUM);
+        $form->setType('prefix', PARAM_TEXT); // We set to text but we have a rule to limit to alphanumext.
         $form->addHelpButton('prefix', 'prefix', 'cachestore_memcached');
+        $form->addRule('prefix', get_string('prefixinvalid', 'cachestore_memcached'), 'regex', '#^[a-zA-Z0-9\-_]+$#');
 
         $hashoptions = cachestore_memcached::config_get_hash_options();
         $form->addElement('select', 'hash', get_string('hash', 'cachestore_memcached'), $hashoptions);
diff --git a/cache/stores/memcached/lang/en/cachestore_memcached.php b/cache/stores/memcached/lang/en/cachestore_memcached.php
index 67c3cd22ec2..87713f6755d 100644
--- a/cache/stores/memcached/lang/en/cachestore_memcached.php
+++ b/cache/stores/memcached/lang/en/cachestore_memcached.php
@@ -42,6 +42,7 @@ $string['hash_murmur'] = 'Murmur';
 $string['pluginname'] = 'Memcached';
 $string['prefix'] = 'Prefix key';
 $string['prefix_help'] = 'This can be used to create a "domain" for your item keys allowing you to create multiple memcached stores on a single memcached installation. It cannot be longer than 16 characters in order to ensure key length issues are not encountered.';
+$string['prefixinvalid'] = 'Invalid prefix. You can only use a-z A-Z 0-9-_.';
 $string['serialiser_igbinary'] = 'The igbinary serializer.';
 $string['serialiser_json'] = 'The JSON serializer.';
 $string['serialiser_php'] = 'The default PHP serializer.';
diff --git a/cache/stores/memcached/lib.php b/cache/stores/memcached/lib.php
index 4f96fda5f14..a74dc83e053 100644
--- a/cache/stores/memcached/lib.php
+++ b/cache/stores/memcached/lib.php
@@ -444,7 +444,7 @@ class cachestore_memcached extends cache_store implements cache_is_configurable
      * Generates an instance of the cache store that can be used for testing.
      *
      * @param cache_definition $definition
-     * @return false
+     * @return cachestore_memcached|false
      */
     public static function initialise_test_instance(cache_definition $definition) {