diff --git a/css-checks/src/test/resources/checks/less/unknownFunctions.less b/css-checks/src/test/resources/checks/less/unknownFunctions.less
index 6c034f72..786519d3 100644
--- a/css-checks/src/test/resources/checks/less/unknownFunctions.less
+++ b/css-checks/src/test/resources/checks/less/unknownFunctions.less
@@ -7,4 +7,5 @@
   filter: alpha(opacity=50);
   filter: color("#123");
   filter: desaturate("#123");
+  top: -(@timeline-h-size-step/2)-(@timeline-h-size-line/2);
 }
diff --git a/css-frontend/src/main/java/org/sonar/css/parser/LexicalGrammar.java b/css-frontend/src/main/java/org/sonar/css/parser/LexicalGrammar.java
index 0f6effd3..775d8e11 100644
--- a/css-frontend/src/main/java/org/sonar/css/parser/LexicalGrammar.java
+++ b/css-frontend/src/main/java/org/sonar/css/parser/LexicalGrammar.java
@@ -514,11 +514,12 @@ private static void macros(LexerlessGrammarBuilder b) {
     b.rule(_IDENT).is(
       b.firstOf(
         b.regexp("(?i)(progid:DXImageTransform\\.Microsoft\\.[a-z]+)"),
+        b.sequence("-", b.oneOrMore(_NMCHAR)),
         b.sequence(_NMSTART, b.zeroOrMore(_NMCHAR))))
       .skip();
 
     b.rule(_NMSTART).is(
-      b.firstOf(b.regexp("(?i)[_a-z]"), "-", "*", _NONASCII, _ESCAPE)).skip();
+      b.firstOf(b.regexp("(?i)[_a-z]"), "*", _NONASCII, _ESCAPE)).skip();
 
     b.rule(_NONASCII).is(b.regexp("[^\\x00-\\xED]")).skip();
 
diff --git a/css-frontend/src/test/java/org/sonar/css/parser/css/IdentifierTreeTest.java b/css-frontend/src/test/java/org/sonar/css/parser/css/IdentifierTreeTest.java
index c716ea6c..7eb2112c 100644
--- a/css-frontend/src/test/java/org/sonar/css/parser/css/IdentifierTreeTest.java
+++ b/css-frontend/src/test/java/org/sonar/css/parser/css/IdentifierTreeTest.java
@@ -63,6 +63,8 @@ public void notIdent() {
     checkNotParsed("123");
     checkNotParsed("123px");
     checkNotParsed("@{abc}");
+    checkNotParsed("-");
+    checkNotParsed("-(");
   }
 
   private IdentifierTree checkParsed(String toParse, String expectedIdent) {
diff --git a/its/ruling/projects/custom/less/unknownFunctions.less b/its/ruling/projects/custom/less/unknownFunctions.less
index 6c034f72..786519d3 100644
--- a/its/ruling/projects/custom/less/unknownFunctions.less
+++ b/its/ruling/projects/custom/less/unknownFunctions.less
@@ -7,4 +7,5 @@
   filter: alpha(opacity=50);
   filter: color("#123");
   filter: desaturate("#123");
+  top: -(@timeline-h-size-step/2)-(@timeline-h-size-line/2);
 }