fix username lookup regex for mentions

app/assets/javascripts/discourse/components/markdown.js

@@ -142,13 +142,13 @@ Discourse.Markdown = {
     converter.hooks.chain("postConversion", function(text) {
       if (!text) return "";
 
-      // don't to mention voodoo in pres
+      // don't do @username mentions inside <pre> blocks
       text = text.replace(/<pre>([\s\S]*@[\s\S]*)<\/pre>/gi, function(wholeMatch, inner) {
         return "<pre>" + (inner.replace(/@/g, '&#64;')) + "</pre>";
       });
 
-      // Add @mentions of names
-      text = text.replace(/([\s\t>,:'|";\]])(@[A-Za-z0-9_-|\.]*[A-Za-z0-9_-|]+)(?=[\s\t<\!:|;',"\?\.])/g, function(x, pre, name) {
+      // add @username mentions, if valid; must be bounded on left and right by non-word characters
+      text = text.replace(/(\W)(@[A-Za-z0-9][A-Za-z0-9_]{2,14})(?=\W)/g, function(x, pre, name) {
         if (mentionLookup(name.substr(1))) {
           return pre + "<a href='" + Discourse.getURL("/users/") + (name.substr(1).toLowerCase()) + "' class='mention'>" + name + "</a>";
         } else {