How to include [] in python regex code? - python-2.7

Question

I am using python 2.7.8 to write a small python code that reads a rule in a form A ==> B by using regex and return it in a form of 'A, B'.
This is my code:
import re
def fixp1(s):
pattern = re.compile("(?P<g1>([A-Z0-9a-z]|\?)*):(?P<g2>([A-Z0-9a-z]|\?)*)")
return eval(pattern.sub("('\g<g1>', '\g<g2>')", s))
x = "[ABCD:NP, [PQR:?TAG1]] ==> [XXX:?P]"
def readrule(r):
r.split("==>")
return [fixp1(r[0].strip()), fixp1(r[1].strip())]
When I test this code:
>>> readrule(x)
I got the following error message:
readrule(y)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File ".../patterns.py", line 12, in readrule
return [fixp1(r[0].strip()), fixp1(r[1].strip())]
File ".../patterns.py", line 5, in fixp1
return eval(pattern.sub("('\g<g1>', '\g<g2>')", s))
File "<string>", line 1
[
^
SyntaxError: unexpected EOF while parsing
>>>
I think this problem happened because I couldn't add '[' and ']' in here
([A-Z0-9a-z]|\?)
If that's right, how to do it? if not; where is my mistake?

Answer 1

Remove the eval command, the RegEx.sub returns a string which is your match with the replacements applied, you cannot evaluate the string. This yields the SyntaxError you are seeing.
If you want to include [] in your patterns, you need to escape them with \:
pattern = re.compile(r'[\[\]0-9]+')
would match strings like '[1234]'.