diff --git a/src/JShrink/Minifier.php b/src/JShrink/Minifier.php
index 9ddf41c..d4be0cf 100644
--- a/src/JShrink/Minifier.php
+++ b/src/JShrink/Minifier.php
@@ -112,6 +112,8 @@ class Minifier
 
     protected static $keywords = ["delete", "do", "for", "in", "instanceof", "return", "typeof", "yield"];
 
+    protected $max_keyword_len;
+
     /**
      * Contains lock ids which are used to replace certain code patterns and
      * prevent them from being minified
@@ -189,6 +191,8 @@ protected function initialize($js, $options)
         $this->b = "\n";
         $this->last_char = "\n";
         $this->output = "";
+
+        $this->max_keyword_len = max(array_map('strlen', static::$keywords));
     }
 
     /**
@@ -658,7 +662,8 @@ protected static function isAlphaNumeric($char)
     protected function endsInKeyword() {
 
         # When this function is called A is not yet assigned to output.
-        $testOutput = $this->output . $this->a;
+        # Regular expression only needs to check final part of output for keyword.
+        $testOutput = substr($this->output . $this->a, -1 * ($this->max_keyword_len + 10));
 
         foreach(static::$keywords as $keyword) {
             if (preg_match('/[^\w]'.$keyword.'[ ]?$/i', $testOutput) === 1) {