[Tutor] regular expression question

[Mike Hall]

> but yeah, it
> seems you're expecting it to examine the string as a whole.

I guess I was, good point.



On Mar 9, 2005, at 12:28 PM, Liam Clarke wrote:

> Actually, you should get that anyway...
>
> """
> |
>     Alternation, or the ``or'' operator. If A and B are regular
> expressions, A|B will match any string that matches either "A" or "B".
> | has very low precedence in order to make it work reasonably when
> you're alternating multi-character strings. Crow|Servo will match
> either "Crow" or "Servo", not "Cro", a "w" or an "S", and "ervo".
> """
>
> So, for each letter in that string, it's checking to see if any letter
> matches 'A' or 'B' ...
> the engine steps through one character at a time.
> sorta like -
>
> for letter in s:
>      if letter == 'A':
>         #Do some string stuff
>      elif letter == 'B':
>         #do some string stuff
>
>
> i.e.
>
> k = ['A','B', 'C', 'B']
>
> for i in range(len(k)):
>     if k[i] == 'A' or k[i]=='B':
>        k[i]==13
>
> print k
>
> [13, 13, 'C', 13]
>
> You can limit substitutions using an optional argument, but yeah, it
> seems you're expecting it to examine the string as a whole.
>
>
> Check out the example here -
> http://www.amk.ca/python/howto/regex/ 
> regex.html#SECTION000320000000000000000
>
> Also
>
> http://www.regular-expressions.info/alternation.html
>
> Regards,
>
> Liam Clarke
>
>
> On Thu, 10 Mar 2005 09:09:13 +1300, Liam Clarke <cyresse at gmail.com>  
> wrote:
>> Hi Mike,
>>
>> Do you get the same results for a search pattern of 'A|B'?
>>
>>
>> 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!
>>>>
>>>
>>> _______________________________________________
>>> Tutor maillist  -  Tutor at python.org
>>> http://mail.python.org/mailman/listinfo/tutor
>>>
>>
>> --
>> 'There is only one basic human right, and that is to do as you damn  
>> well please.
>> And with it comes the only basic human duty, to take the consequences.
>>
>
>
> -- 
> 'There is only one basic human right, and that is to do as you damn  
> well please.
> And with it comes the only basic human duty, to take the consequences.
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor
>