[Tutor] regular expression question

[Mike Hall]

But I only want to ignore "B" if "A" is a match. If "A" is not a match, 
I'd like it to advance on to "B".


On Mar 9, 2005, at 12:07 PM, Marcos Mendonça wrote:

> Hi
>
> Not and regexp expert. But it seems to me that if you want to ignora
> "B" then it should be
> (A) | (^B)
>
> Hope it helps!
>
>
> On Wed, 9 Mar 2005 11:11:57 -0800, Mike Hall
> <michael.hall at critterpixstudios.com> wrote:
>> I'm having some strange results using the "or" operator.  In every 
>> test
>> I do I'm matching both sides of the "|" metacharacter, not one or the
>> other as all documentation says it should be (the parser supposedly
>> scans left to right, using the first match it finds and ignoring the
>> rest). It should only go beyond the "|" if there was no match found
>> before it, no?
>>
>> Correct me if I'm wrong, but your regex is saying "match dog, unless
>> it's followed by cat. if it is followed by cat there is no match on
>> this side of the "|" at which point we advance past it and look at the
>> alternative expression which says to match in front of cat."
>>
>> However, if I run a .sub using your regex on a string contain both dog
>> and cat, both will be replaced.
>>
>> A simple example will show what I mean:
>>
>>>>> import re
>>>>> x = re.compile(r"(A) | (B)")
>>>>> s = "X R A Y B E"
>>>>> r = x.sub("13", s)
>>>>> print r
>> X R 13Y13 E
>>
>> ...so unless I'm understanding it wrong, "B" is supposed to be ignored
>> if "A" is matched, yet I get both matched.  I get the same result if I
>> put "A" and "B" within the same group.
>>
>>
>> On Mar 8, 2005, at 6:47 PM, Danny Yoo wrote:
>>
>>>
>>>
>>>>
>>>> Regular expressions are a little evil at times; here's what I think
>>>> you're
>>>> thinking of:
>>>>
>>>> ###
>>>>>>> import re
>>>>>>> pattern = re.compile(r"""dog(?!cat)
>>>> ...                    | (?<=dogcat)""", re.VERBOSE)
>>>>>>> pattern.match('dogman').start()
>>>> 0
>>>>>>> pattern.search('dogcatcher').start()
>>>
>>>
>>>
>>> Hi Mike,
>>>
>>> Gaaah, bad copy-and-paste.  The example with 'dogcatcher' actually 
>>> does
>>> come up with a result:
>>>
>>> ###
>>>>>> pattern.search('dogcatcher').start()
>>> 6
>>> ###
>>>
>>> Sorry about that!
>>>
>>
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>