diff --git a/flutter/lib/common/widgets/custom_password.dart b/flutter/lib/common/widgets/custom_password.dart
index 1fa6e3645..dafc23b44 100644
--- a/flutter/lib/common/widgets/custom_password.dart
+++ b/flutter/lib/common/widgets/custom_password.dart
@@ -28,7 +28,11 @@ class LowercaseValidationRule extends ValidationRule {
 
   @override
   bool validate(String value) {
-    return value.contains(RegExp(r'[a-z]'));
+    return value.runes.any((int rune) {
+      var character = String.fromCharCode(rune);
+      return character.toLowerCase() == character &&
+          character.toUpperCase() != character;
+    });
   }
 }