diff --git a/xed.py b/xed.py
index bd50274..b8e9cec 100644
--- a/xed.py
+++ b/xed.py
@@ -13,7 +13,7 @@ parser = Lark(r'''
                 | bracketliteral "-" bracketliteral
        brackets : "[" range+ "]" -> either
 
-           char : /[^\\\[\]\{\}\(\)\|\^~]/
+           char : /[^\\\[\]\{\}\(\)\|\^~\?]/
                 | "\\" /\D/
                 | //
 
@@ -23,6 +23,7 @@ parser = Lark(r'''
           ?unit : parens | brackets | char
    ?concat_func : unit
                 | concat_func "{" DIGITS "}"                     -> concat_repeat
+                | concat_func "?"                                -> zero_or_one
                 | concat_func "~"                                -> reverse
                 | concat_func "~" NUMBER                         -> roll
                 | concat_func "~{" NUMBER ["," DIGITS] "}"       -> roll
@@ -67,6 +68,9 @@ class Expand(Transformer):
         result.append(chr(a))
         return result
 
+    def zero_or_one(self, args):
+        return self.either([[''], args[0]])
+
     def either(self, args):
         result = []
         for x in args: