Validate against zero-width terminals in XEarley (Issue #63)

lark/parser_frontends.py

@@ -122,9 +122,13 @@ class XEarley:
         for t in lexer_conf.tokens:
             regexp = t.pattern.to_regexp()
             try:
-                assert get_regexp_width(regexp)
+                width = get_regexp_width(regexp)[0]
             except ValueError:
                 raise ValueError("Bad regexp in token %s: %s" % (t.name, regexp))
+            else:
+                if width == 0:
+                    raise ValueError("Dynamic Earley doesn't allow zero-width regexps")
+
             self.regexps[t.name] = re.compile(regexp)
 
     def parse(self, text):

tests/test_parser.py

@@ -159,7 +159,7 @@ def _make_full_earley_test(LEXER):
             # Fails an Earley implementation without special handling for empty rules,
             # or re-processing of already completed rules.
             g = Lark(r"""start: B
-                         B: ("ab"|/[^b]/)*
+                         B: ("ab"|/[^b]/)+
                       """, lexer=LEXER)
 
             self.assertEqual( g.parse('abc').children[0], 'abc')