Fix ambiguous syntax parsing

src/Lexer.php

@@ -506,8 +506,9 @@ class Lexer implements \Twig_LexerInterface
             }
 
             // an operator that begins with a character must have a space before
+            // or a parenthesis
             if (ctype_alpha($operator[0])) {
-                $r = '(?<=\s)'.$r;
+                $r = '(?<=[\s(])'.$r;
             }
 
             // an operator with a space can be any amount of whitespaces

tests/Fixtures/expressions/not.test

@@ -0,0 +1,10 @@
+--TEST--
+not
+--TEMPLATE--
+{{ (not false) ? 'OK' : 'KO' }}
+{{ not false ? 'OK' : 'KO' }}
+--DATA--
+return []
+--EXPECT--
+OK
+OK