diff --git a/lark/common.py b/lark/common.py
index 50bea90..d7a5e71 100644
--- a/lark/common.py
+++ b/lark/common.py
@@ -52,29 +52,29 @@ class ParserConf:
 
 
 class Pattern(object):
-    def __init__(self, value, flags=None):
+    def __init__(self, value, flags=()):
         self.value = value
-        self.flags = flags
+        self.flags = frozenset(flags)
 
     def __repr__(self):
         return repr(self.to_regexp())
 
     # Pattern Hashing assumes all subclasses have a different priority!
     def __hash__(self):
-        return hash((type(self), self.value))
+        return hash((type(self), self.value, self.flags))
     def __eq__(self, other):
-        return type(self) == type(other) and self.value == other.value
+        return type(self) == type(other) and self.value == other.value and self.flags == other.flags
 
     if Py36:
         # Python 3.6 changed syntax for flags in regular expression
         def _get_flags(self, value):
-            for f in self.flags or ():
+            for f in self.flags:
                 value = ('(?%s:%s)' % (f, value))
             return value
 
     else:
         def _get_flags(self, value):
-            for f in self.flags or ():
+            for f in self.flags:
                 value = ('(?%s)' % f) + value
             return value
 
diff --git a/lark/load_grammar.py b/lark/load_grammar.py
index 76caaaa..19543f0 100644
--- a/lark/load_grammar.py
+++ b/lark/load_grammar.py
@@ -332,7 +332,7 @@ def _literal_to_pattern(literal):
         s = s.replace('\\\\', '\\')
 
     return { 'STRING': PatternStr,
-             'REGEXP': PatternRE }[literal.type](s, flags or None)
+             'REGEXP': PatternRE }[literal.type](s, flags)
 
 
 class PrepareLiterals(InlineTransformer):
@@ -368,7 +368,7 @@ class TokenTreeToPattern(Transformer):
     def expansions(self, exps):
         if len(exps) == 1:
             return exps[0]
-        assert all(i.flags is None for i in exps)
+        assert all(not i.flags for i in exps)
         return PatternRE('(?:%s)' % ('|'.join(i.to_regexp() for i in exps)))
 
     def expr(self, args):
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 0b3a9f1..9ef7ab5 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -750,13 +750,13 @@ def _make_parser_test(LEXER, PARSER):
             tree = l.parse('aA')
             self.assertEqual(tree.children, ['a', 'A'])
 
-            g = """!start: "a"i "a"
-                """
-            self.assertRaises(GrammarError, _Lark, g)
+            # g = """!start: "a"i "a"
+            #     """
+            # self.assertRaises(GrammarError, _Lark, g)
 
-            g = """!start: /a/i /a/
-                """
-            self.assertRaises(GrammarError, _Lark, g)
+            # g = """!start: /a/i /a/
+            #     """
+            # self.assertRaises(GrammarError, _Lark, g)
 
             g = """start: NAME "," "a"
                    NAME: /[a-z_]/i /[a-z0-9_]/i*