diff --git a/src/Composer/Autoload/ClassMapGenerator.php b/src/Composer/Autoload/ClassMapGenerator.php
index c8bf43099..2faef3bc6 100644
--- a/src/Composer/Autoload/ClassMapGenerator.php
+++ b/src/Composer/Autoload/ClassMapGenerator.php
@@ -89,6 +89,9 @@ class ClassMapGenerator
     {
         $contents = file_get_contents($path);
         try {
+            if (!preg_match('{\b(?:class|interface|trait)\b}i', $contents)) {
+                return array();
+            }
             $tokens   = token_get_all($contents);
         } catch (\Exception $e) {
             throw new \RuntimeException('Could not scan for classes inside '.$path.": \n".$e->getMessage(), 0, $e);