diff --git a/src/Monicelli.lpp b/src/Monicelli.lpp
index 3b46f1f..e29a6bf 100644
--- a/src/Monicelli.lpp
+++ b/src/Monicelli.lpp
@@ -40,7 +40,7 @@ bool in(const char *sub, const std::string &str) {
 %option yyclass="Scanner"
 
 DIGIT [0-9]
-HEXDIGIT [0-9a-zA-Z]
+HEXDIGIT [0-9a-fA-F]
 CHAR  [a-zA-Z_]
 
 %x shift