diff --git a/liberty/LibertyLex.ll b/liberty/LibertyLex.ll
index a69d736a..4001db06 100644
--- a/liberty/LibertyLex.ll
+++ b/liberty/LibertyLex.ll
@@ -60,7 +60,7 @@ BUS_LEFT [\[<]
 BUS_RIGHT [\]>]
 BUS_SUB {BUS_LEFT}{DIGIT}+{BUS_RIGHT}
 BUS_RANGE {BUS_LEFT}{DIGIT}+:{DIGIT}+{BUS_RIGHT}
-PIN_NAME ({ALPHA}|_)({ALPHA}|{DIGIT}|_)*
+PIN_NAME ({ALPHA}|_)({ALPHA}|{DIGIT}|_)*!?
 BUS_NAME {PIN_NAME}({BUS_SUB}|{BUS_RANGE})
 BUS_NAME2 {PIN_NAME}{BUS_SUB}({BUS_SUB}|{BUS_RANGE})
 MIXED_NAME {BUS_NAME}_{PIN_NAME}