diff --git a/PHPCI/Plugin/PhpParallelLint.php b/PHPCI/Plugin/PhpParallelLint.php
index 420e06ed9..6885cd968 100644
--- a/PHPCI/Plugin/PhpParallelLint.php
+++ b/PHPCI/Plugin/PhpParallelLint.php
@@ -87,7 +87,7 @@ public function __construct(Builder $phpci, Build $build, array $options = array
 
         if (isset($options['extensions'])) {
             // Only use if this is a comma delimited list
-            $pattern = '/^[a-z]*,\ *[a-z]*$/';
+            $pattern = '/^([a-z]+)(,\ *[a-z]*)*$/';
 
             if (preg_match($pattern, $options['extensions'])) {
                 $this->extensions = str_replace(' ', '', $options['extensions']);