diff --git a/static/js/filter.js b/static/js/filter.js
index 5955c37364..5ec8cf4f11 100644
--- a/static/js/filter.js
+++ b/static/js/filter.js
@@ -1,5 +1,18 @@
 var Filter = (function () {
 
+function mit_edu_stream_name_match(message, operand) {
+    // MIT users expect narrowing to "social" to also show messages to /^(un)*social(.d)*$/
+    // (unsocial, ununsocial, social.d, etc)
+    // TODO: hoist the regex compiling out of the closure
+    var m = /^(?:un)*(.+?)(?:\.d)*$/i.exec(operand);
+    var base_stream_name = operand;
+    if (m !== null && m[1] !== undefined) {
+        base_stream_name = m[1];
+    }
+    var related_regexp = new RegExp(/^(un)*/.source + util.escape_regexp(base_stream_name) + /(\.d)*$/.source, 'i');
+    return related_regexp.test(message.stream);
+}
+
 function message_in_home(message) {
     if (message.type === "private") {
         return true;
@@ -230,16 +243,7 @@ Filter.prototype = {
 
                     operand = operand.toLowerCase();
                     if (page_params.domain === "mit.edu") {
-                        // MIT users expect narrowing to "social" to also show messages to /^(un)*social(.d)*$/
-                        // (unsocial, ununsocial, social.d, etc)
-                        // TODO: hoist the regex compiling out of the closure
-                        m = /^(?:un)*(.+?)(?:\.d)*$/i.exec(operand);
-                        var base_stream_name = operand;
-                        if (m !== null && m[1] !== undefined) {
-                            base_stream_name = m[1];
-                        }
-                        related_regexp = new RegExp(/^(un)*/.source + util.escape_regexp(base_stream_name) + /(\.d)*$/.source, 'i');
-                        if (! related_regexp.test(message.stream)) {
+                        if (!mit_edu_stream_name_match(message, operand)) {
                             return false;
                         }
                     } else if (message.stream.toLowerCase() !== operand) {