[Tutor] regular expression question

[Mike Hall]

This will match the position in front of "dog":

(?<=dog)

This will match the position in front of "cat":

(?<=cat)

This will not match in front of "dog" if "dog" is followed by "cat":

(?<=dog)\b (?!cat)

Now my question is how to get this:

(?<=cat)

...but ONLY if "cat" is following "dog." If "dog" does not have "cat"  
following it, then I simply want this:

(?<=dog)



...if that makes sense :) thanks.




On Mar 8, 2005, at 6:05 PM, Danny Yoo wrote:

>
>
> On Tue, 8 Mar 2005, 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."
>
> Hi Mike,
>
> You may want to look at "lookahead" assertions.  These are patterns of  
> the
> form '(?=...)' or '(?!...).  The documentation mentions them here:
>
>    http://www.python.org/doc/lib/re-syntax.html
>
> and AMK's excellent "Regular Expression HOWTO" covers how one might use
> them:
>
> http://www.amk.ca/python/howto/regex/ 
> regex.html#SECTION000540000000000000000
>