Fix bugs while using python2 re lib

[prikevs]

Firstly, thanks a lot for providing such a handy tool to do ZG <=> Unicode conversions!

While using this library under python2.7, I found a bug related to the re lib of python.

Reproduce:

• translate ပြင်သစ်ဘာသာကီးဘုတ် from unicode to zg with Rabbit.uni2zg(u"ပြင်သစ်ဘာသာကီးဘုတ်")

Error:

Traceback (most recent call last):
  File "convert.py", line 5, in <module>
    print(Rabbit.uni2zg(u'ပြင်သစ်ဘာသာကီးဘုတ်'))
  File "/usr/local/google/home/jiebu/Documents/rabbit/rabbit.py", line 10, in uni2zg
    text = re.sub(rule["from"], rule["to"], text)
  File "/usr/lib/python2.7/re.py", line 155, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/usr/lib/python2.7/re.py", line 291, in filter
    return sre_parse.expand_template(template, match)
  File "/usr/lib/python2.7/sre_parse.py", line 831, in expand_template
    raise error, "unmatched group"
sre_constants.error: unmatched group

Solution:

• referred to http://gerardjp.com/solved-python-regex-raising-exception-unmatched-group/

This happens because the second group represented with “g<2>” in the replacement string returns a “None” instead of an empty string. That is (seems) the bug.

• Change all (xxxx)? to (|xxxx) e.g. ([\u1000-\u1021])? -> (|[\u1000-\u1021]) please see the link above for more details.