diff --git a/src/Str.php b/src/Str.php
index d1f91ac..60fa6b5 100644
--- a/src/Str.php
+++ b/src/Str.php
@@ -428,7 +428,7 @@ public static function mb_substr_replace($string, $replacement, $start, $length
 
     public static function placeholders(string $string, array $replacements, bool $caseInsensitive = false): string
     {
-        $regex = '/{([A-Z0-9_]+)}/' . ($caseInsensitive ? 'i' : '');
+        $regex = '/{?([a-z0-9_]+)}?/' . ($caseInsensitive ? 'i' : '');
 
         return preg_replace_callback(
             $regex,