diff --git a/grammar.bnf b/grammar.bnf
index 2905420f4..8edbfab7a 100644
--- a/grammar.bnf
+++ b/grammar.bnf
@@ -36,5 +36,5 @@ word        ::=
 file        ::=
 id          ::= digit+ ;
 digit       ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
-substitution ::= "/" word+ "/" word* "/" ;
+substitution ::= "/" word+ "/" word* "/" "g"? ;