Added possibility for terminals with different flags to be joined in python3.6+

lark/load_grammar.py

@@ -5,7 +5,7 @@ import sys
 from copy import copy, deepcopy
 from io import open
 
-from .utils import bfs, eval_escaping
+from .utils import bfs, eval_escaping, Py36
 from .lexer import Token, TerminalDef, PatternStr, PatternRE
 
 from .parse_tree_builder import ParseTreeBuilder
@@ -441,9 +441,12 @@ class TerminalTreeToPattern(Transformer):
         assert items
         if len(items) == 1:
             return items[0]
-        if len({i.flags for i in items}) > 1:
-            raise GrammarError("Lark doesn't support joining terminals with conflicting flags!")
-        return PatternRE(''.join(i.to_regexp() for i in items), items[0].flags if items else ())
+        if not Py36:
+            if len({i.flags for i in items}) > 1:
+                raise GrammarError("Lark doesn't support joining terminals with conflicting flags in python <3.6!")
+            return PatternRE(''.join(i.to_regexp() for i in items), items[0].flags if items else ())
+        else:
+            return PatternRE(''.join(i.to_regexp() for i in items), ())
 
     def expansions(self, exps):
         if len(exps) == 1:

tests/test_parser.py

@@ -7,6 +7,9 @@ import logging
 import os
 import sys
 from copy import copy, deepcopy
+
+from lark.utils import Py36
+
 try:
     from cStringIO import StringIO as cStringIO
 except ImportError:
@@ -1062,6 +1065,19 @@ def _make_parser_test(LEXER, PARSER):
             g = _Lark(g)
             self.assertEqual( g.parse('"hello"').children, ['"hello"'])
             self.assertEqual( g.parse("'hello'").children, ["'hello'"])
+        
+        @unittest.skipIf(not Py36, "Required re syntax only exists in python3.6+")
+        def test_join_regex_flags(self):
+            g = r"""
+                start: A
+                A: B C
+                B: /./s
+                C: /./
+            """
+            g = _Lark(g)
+            self.assertEqual(g.parse("  ").children,["  "])
+            self.assertEqual(g.parse("\n ").children,["\n "])
+            self.assertRaises(UnexpectedCharacters, g.parse, "\n\n")
 
 
         def test_lexer_token_limit(self):