diff --git a/src/sprintf.js b/src/sprintf.js
index 5a8462c..1337f10 100644
--- a/src/sprintf.js
+++ b/src/sprintf.js
@@ -17,7 +17,7 @@
         placeholder: /^\x25(?:([1-9]\d*)\$|\(([^\)]+)\))?(\+)?(0|'[^$])?(-)?(\d+)?(?:\.(\d+))?([b-gijostTuvxX])/,
         key: /^([a-z_][a-z_\d]*)/i,
         key_access: /^\.([a-z_][a-z_\d]*)/i,
-        index_access: /^\[(\d+)\]/,
+        index_access: /^\[([A-Z0-9a-z]+)\]/,
         sign: /^[\+\-]/
     }