Only word characters, numbers, . (dot) and _

E.g. arrays (Hash::flatten'ed) to be used in substitutions.

This allows for the standard . notation of arrays to be used within substitions like
attendee.fistname and event.name from e.g. Hash::flatten($entity->toArray());

src/View/StringTemplate.php

@@ -178,7 +178,7 @@ protected function _compileTemplates(array $templates = [])
                 $this->_compiled[$name] = [null, null];
             }
 
-            preg_match_all('#\{\{(\w+)\}\}#', $template, $matches);
+            preg_match_all('#\{\{([\w\d\._]+)\}\}#', $template, $matches);
             $this->_compiled[$name] = [
                 str_replace($matches[0], '%s', $template),
                 $matches[1]