regex (?!..) problem

[MRAB]

Wolfgang Rohdewald wrote:
> On Monday 05 October 2009, MRAB wrote:
>> You're currently looking for one that's not followed by another;
>>  the solution is to check first whether there are two:
>>
>>  >>> re.match(r'(?!.*?C1.*?C1)(.*?C1)','C1b1b1b1 b3b3b3b3
>>  C1C2C3').groups()
>>
>> Traceback (most recent call last):
>>    File "<pyshell#3>", line 1, in <module>
>>      re.match(r'(?!.*?C1.*?C1)(.*?C1)','C1b1b1b1 b3b3b3b3
>>  C1C2C3').groups() AttributeError: 'NoneType' object has no
>>  attribute 'groups'
> 
> that is a nice solution!
> 
> now to make it more similar to my real world case
> where C1 is actually part of the string:
> 
> same regex but using a group for C1 does not work - why?
> 
>>>> re.match(r'(?!.*?(C1).*?\1)(.*?\1)','C1b1b1b1 b3b3b3b3 C2C2C3').groups()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: 'NoneType' object has no attribute 'groups'
> 
>>>> re.match(r'(?!.*?(?P<tile>C1).*?(?P=tile))(.*?(?P=tile))','C1B1B1B1 b3b3b3b3 C2C2C3').groups()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: 'NoneType' object has no attribute 'groups'
> 
"(?!.*?(C1).*?\1)" will succeed only if ".*?(C1).*?\1" has failed, in
which case the group (group 1) will be undefined (no capture).