re question

[Schüle Daniel]

Hello all,

I didn't found more appropriate news group for
this question, please let me know if there is ng with
regular expression as its main topic

I am trying to construct a case where a greedy and
non greedy operation produce different result.
I dont see the difference between 'a??b' and 'a?b'
As far I understand is that ? will first try to match a
(it's greedy) and only if it fails then it step back
and lets a unmatched. The other doesn't match a at first,
only if the pattern fails to match it steps back and match a.

But don't they do eventually the same thing?
Can someone provide an example where 2 patterns yield
different results.

Regards, Daniel

[Steve Holden]

Re: re question

Schüle Daniel wrote:

Hello all,
>
I didn't found more appropriate news group for
this question, please let me know if there is ng with
regular expression as its main topic
>
I am trying to construct a case where a greedy and
non greedy operation produce different result.
I dont see the difference between 'a??b' and 'a?b'
As far I understand is that ? will first try to match a
(it's greedy) and only if it fails then it step back
and lets a unmatched. The other doesn't match a at first,
only if the pattern fails to match it steps back and match a.
>
But don't they do eventually the same thing?
Can someone provide an example where 2 patterns yield
different results.
>

>>target = "<h1>Sample String</h1>"
>>greedy = re.compile("<.+ >")
>>nongreedy = re.compile("<.+ ?>")
>>m1 = greedy.match(ta rget)
>>m2 = nongreedy.match (target)
>>m1.group(0)

'<h1>Sample String</h1>'

>>m2.group(0)

'<h1>'

>>>

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden

[attn.steven.kuo@gmail.com]

Re: re question

Schüle Daniel wrote:

(snipped)

I am trying to construct a case where a greedy and
non greedy operation produce different result.
I dont see the difference between 'a??b' and 'a?b'
As far I understand is that ? will first try to match a
(it's greedy) and only if it fails then it step back
and lets a unmatched. The other doesn't match a at first,
only if the pattern fails to match it steps back and match a.
>
But don't they do eventually the same thing?
Can someone provide an example where 2 patterns yield
different results.
>

Perhaps this sheds some light
on the matter:

>>import re
>>string = "aaaba"

>>one = re.findall(r'a? b?', string)
>>two = re.findall(r'a? ?b?', string)

>>print one, two

Yields:

['a', 'a', 'ab', 'a', ''] ['', '', '', 'b', '', '']


--
Hope this helps,
Steven