Hi everyone, This is a copy of an issue I filed at your issue tracker, https://bitbucket.org/pypy/pypy/issues/2459/assertionerror-in-parse_ebnf- when-nesting, where I was told to report it to the mailing list. I was using the rlib.parsing module for parsing a simple language, but I ran into an AssertionError which I can't explain (from the documentation, at least). It seems to occur whenever a symbol like `+` or `*` is used inside `[]` brackets. Here is an example file: from rpython.rlib.parsing.ebnfparse import parse_ebnf

regexes, rules, ToAST = parse_ebnf(""" test: ["foo"+] "bar"; """)

When I run this code, both on the default branch and on the `release-pypy2.7-v5.6.0` branch, the following error occurs: test/test_parser.py:10: in <module>

""") pypy/rpython/rlib/parsing/ebnfparse.py:57: in parse_ebnf s.visit(visitor) pypy/rpython/rlib/parsing/tree.py:86: in visit return getattr(visitor, "visit_" + self.symbol)(self) pypy/rpython/rlib/parsing/ebnfparse.py:110: in visit_file return node.children[0].visit(self) pypy/rpython/rlib/parsing/tree.py:86: in visit return getattr(visitor, "visit_" + self.symbol)(self) pypy/rpython/rlib/parsing/ebnfparse.py:114: in visit_list child.visit(self) pypy/rpython/rlib/parsing/tree.py:86: in visit return getattr(visitor, "visit_" + self.symbol)(self) pypy/rpython/rlib/parsing/ebnfparse.py:131: in visit_production expansions = node.children[2].visit(self) pypy/rpython/rlib/parsing/tree.py:86: in visit return getattr(visitor, "visit_" + self.symbol)(self) pypy/rpython/rlib/parsing/ebnfparse.py:146: in visit_body expansion = child.visit(self) pypy/rpython/rlib/parsing/tree.py:86: in visit return getattr(visitor, "visit_" + self.symbol)(self) pypy/rpython/rlib/parsing/ebnfparse.py:153: in visit_expansion expansion = child.visit(self) pypy/rpython/rlib/parsing/tree.py:86: in visit return getattr(visitor, "visit_" + self.symbol)(self) pypy/rpython/rlib/parsing/ebnfparse.py:161: in visit_enclosed assert change == " " or change == newchange AssertionError

Commenting out the assertion that is failing doesn't seem to cause any problems and the resulting parser has the right behavior. My current workaround is to make a new terminal symbol that behaves like `"foo"+` in the example code, which appears to work but is somewhat less readable. Did I miss something or is this an actual bug in RPython and/or its documentation? Thanks in advance! Tim