diff --git a/h5p.classes.php b/h5p.classes.php
index a699dfb..0ed5cbe 100644
--- a/h5p.classes.php
+++ b/h5p.classes.php
@@ -1230,7 +1230,7 @@ class H5PContentValidator {
     }
     // Check if string is according to optional regexp in semantics
     if (isset($semantics->regexp)) {
-      $pattern = $semantics->regexp->pattern;
+      $pattern = '|' . $semantics->regexp->pattern . '|';
       $pattern .= isset($semantics->regexp->modifiers) ? $semantics->regexp->modifiers : '';
       if (preg_match($pattern, $text) === 0) {
         // Note: explicitly ignore return value FALSE, to avoid removing text