Re: [Tutor] regular expression question

[Mike Hall]

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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