[Tutor] regular expression question

[Mike Hall]

First, thanks for the response. Using your re:

> my_re = re.compile(r'(dog)(cat)?')

...I seem to simply be matching the pattern "Dog".  Example:

 >>> str1 = "The dog chased the car"
 >>> str2 = "The dog cat parade was under way"
 >>> x1 = re.compile(r'(dog)(cat)?')
 >>> rep1 = x1.sub("REPLACE", str1)
 >>> rep2 = x2.sub("REPLACE", str2)
 >>> print rep1
The REPLACE chased the car
 >>> print rep2
The REPLACE cat parade was under way


...what I'm looking for is a match for the position in front of "Cat", 
should it exist.



On Mar 8, 2005, at 5:54 PM, Sean Perry wrote:

> Mike Hall wrote:
>> I'd like to get a match for a position in a string preceded by a 
>> specified word (let's call it "Dog"), unless that spot in the string 
>> (after "Dog") is directly followed by a specific word(let's say 
>> "Cat"), in which case I want my match to occur directly after "Cat", 
>> and not "Dog."
>> I can easily get the spot after "Dog," and I can also get it to 
>> ignore this spot if "Dog" is followed by "Cat." But what I'm having 
>> trouble with is how to match the spot after "Cat" if this word does 
>> indeed exist in the string.
>
> . >>> import re
> . >>> my_re = re.compile(r'(dog)(cat)?') # the ? means "find one or 
> zero of these, in other words cat is optional.
> . >>> m = my_re.search("This is a nice dog is it not?")
> . >>> dir(m)
> ['__copy__', '__deepcopy__', 'end', 'expand', 'group', 'groupdict', 
> 'groups', 'span', 'start']
> . >>> m.span()
> (15, 18)
> . >>> m = my_re.search("This is a nice dogcat is it not?")
> . >>> m.span()
> (15, 21)
>
> If m is None then no match was found. span returns the locations in 
> the string where the match occured. So in the dogcat sentence the last 
> char is 21.
>
> . >>> "This is a nice dogcat is it not?"[21:]
> ' is it not?'
>
> Hope that helps.
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