from_lark fails for grammars with complex regexps

[erezsh]

I tried to generate an example of Python code, by loading Lark's builtin Python3 grammar, but I think it failed to generate one of the regexps.

import hypothesis.extra.lark

h = hypothesis.extra.lark.from_lark(Lark.open('lark/examples/python3.lark', start='file_input')) 

h.example()

Results in the exception:

  File ".local/lib/python3.6/site-packages/hypothesis/searchstrategy/regex.py", line 185, in add_char
    if self._ignorecase and re.match(c, c.swapcase(), re.IGNORECASE) is not None:
  File "/usr/lib64/python3.6/re.py", line 172, in match
    return _compile(pattern, flags).match(string)
  File "/usr/lib64/python3.6/re.py", line 301, in _compile
    p = sre_compile.compile(pattern, flags)
  File "/usr/lib64/python3.6/sre_compile.py", line 562, in compile
    p = sre_parse.parse(p, flags)
  File "/usr/lib64/python3.6/sre_parse.py", line 855, in parse
    p = _parse_sub(source, pattern, flags & SRE_FLAG_VERBOSE, 0)
  File "/usr/lib64/python3.6/sre_parse.py", line 416, in _parse_sub
    not nested and not items))
  File "/usr/lib64/python3.6/sre_parse.py", line 616, in _parse
    source.tell() - here + len(this))
sre_constants.error: nothing to repeat at position 0
nothing to repeat at position 0

Btw, this isn't high-priority. I just found it while playing around, and thought you might want to know.

[Zac-HD]

Turns out that in a few places we're treating character literals as regular expressions, and of course there are some single characters that are not valid regex - here's the minimal example:

from_regex(re.compile('\\\\', flags=re.IGNORECASE)).example()

Thanks for reporting this - we definitely want to know about any bugs you find and will get a fix out soon.