Fix regex bug

Dotenv.php

@@ -167,7 +167,7 @@ final class Dotenv {
    * @param string $key
    */
   private function findEnvironmentVariables(string &$item, string &$key): void {
-    $pattern = '@\{\$env:(.*)\}@i';
+    $pattern = '@\{\$env:([\w_]*)\}@i';
     preg_match_all($pattern, $item, $item_matches);
     preg_match_all($pattern, $key, $key_matches);