diff --git a/src/SpdxLicenses.php b/src/SpdxLicenses.php
index 9f9b322b..7b94b24e 100644
--- a/src/SpdxLicenses.php
+++ b/src/SpdxLicenses.php
@@ -309,10 +309,10 @@ private function isValidLicenseString($license)
     (?<idstring>[\pL\pN.-]{1,})
 
     # license-id: taken from list
-    (?<licenseid>${licenses})
+    (?<licenseid>{$licenses})
 
     # license-exception-id: taken from list
-    (?<licenseexceptionid>${exceptions})
+    (?<licenseexceptionid>{$exceptions})
 
     # license-ref: [DocumentRef-1*(idstring):]LicenseRef-1*(idstring)
     (?<licenseref>(?:DocumentRef-(?&idstring):)?LicenseRef-(?&idstring))