Always lstrip() keyword expression

_pytest/mark.py

@@ -58,7 +58,7 @@ pytest_cmdline_main.tryfirst = True
 
 
 def pytest_collection_modifyitems(items, config):
-    keywordexpr = config.option.keyword
+    keywordexpr = config.option.keyword.lstrip()
     matchexpr = config.option.markexpr
     if not keywordexpr and not matchexpr:
         return

testing/acceptance_test.py

@@ -670,6 +670,11 @@ class TestDurations:
             "*call*test_1*",
         ])
 
+    def test_with_not(self, testdir):
+        testdir.makepyfile(self.source)
+        result = testdir.runpytest("-k not 1")
+        assert result.ret == 0
+
 
 class TestDurationWithFixture:
     source = """