diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 754e9ae3..cc32aa53 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -33,6 +33,8 @@ jobs:
       fail-fast: false
       matrix:
         php-versions:
+          - '5.4'
+          - '5.5'
           - '5.6'
           - '7.0'
           - '7.1'
diff --git a/Highlight/RegEx.php b/Highlight/RegEx.php
index d61ab033..4fba2036 100644
--- a/Highlight/RegEx.php
+++ b/Highlight/RegEx.php
@@ -76,13 +76,13 @@ public function exec($str)
     {
         $index = null;
         $results = array();
-        preg_match_all($this->source, $str, $results, PREG_SET_ORDER | PREG_OFFSET_CAPTURE, $this->lastIndex);
+        preg_match($this->source, $str, $results, PREG_OFFSET_CAPTURE, $this->lastIndex);
 
         if ($results === null || count($results) === 0) {
             return null;
         }
 
-        foreach ($results[0] as &$result) {
+        foreach ($results as &$result) {
             if ($result[1] !== -1) {
                 // Only save the index if it hasn't been set yet
                 if ($index === null) {
@@ -95,7 +95,8 @@ public function exec($str)
             }
         }
 
-        $results = $results[0];
+        unset($result);
+
         $this->lastIndex += mb_strlen($results[0]) + ($index - $this->lastIndex);
 
         $matches = new RegExMatch($results);