maybe_placeholder now doesn't apply to anonymous terminals

lark/load_grammar.py

@@ -176,7 +176,12 @@ class EBNF_to_BNF(Transformer_InPlace):
 
     def expr(self, rule, op, *args):
         if op.value == '?':
-            return ST('expansions', [rule, _EMPTY])
+            if isinstance(rule, Terminal) and rule.filter_out and not (
+                    self.rule_options and self.rule_options.keep_all_tokens):
+                empty = ST('expansion', [])
+            else:
+                empty = _EMPTY
+            return ST('expansions', [rule, empty])
         elif op.value == '+':
             # a : b c+ d
             #   -->

lark/parse_tree_builder.py

@@ -151,7 +151,7 @@ class ParseTreeBuilder:
             wrapper_chain = filter(None, [
                 (expand_single_child and not rule.alias) and ExpandSingleChild,
                 maybe_create_child_filter(rule.expansion, keep_all_tokens, self.ambiguous),
-                self.maybe_placeholders and partial(AddMaybePlaceholder, options.empty_indices),
+                self.maybe_placeholders and partial(AddMaybePlaceholder, options.empty_indices if options else None),
                 self.propagate_positions and PropagatePositions,
             ])
 

tests/test_parser.py

@@ -1249,6 +1249,16 @@ def _make_parser_test(LEXER, PARSER):
             self.assertEqual(len(res.children), 3)
 
         def test_maybe_placeholders(self):
+            # Anonymous tokens shouldn't count
+            p = Lark("""start: "a"? "b"? "c"? """, maybe_placeholders=True)
+            self.assertEqual(p.parse("").children, [])
+
+            # Anonymous tokens shouldn't count, other constructs should
+            p = Lark("""start: A? "b"? _c?
+                        A: "a"
+                        _c: "c" """, maybe_placeholders=True)
+            self.assertEqual(p.parse("").children, [None, None])
+
             p = Lark("""!start: "a"? "b"? "c"? """, maybe_placeholders=True)
             self.assertEqual(p.parse("").children, [None, None, None])
             self.assertEqual(p.parse("a").children, ['a', None, None])