[Tutor] regular expression question

[Liam Clarke]

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!
> >
> 
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