Re: Regex lookahead example not as stated in Camel 4th
thanks everyone for pointing out what I hadn't seen in the docs. On Wed, Jun 19, 2013 at 4:00 PM, Yitzchak Scott-Thoennes wrote: > On Wed, Jun 19, 2013 at 7:28 AM, Bob MacCallum > wrote: > > so, forgetting the typo for a moment, why doesn't it output > > 012 > > 012 > > 012 > > 012 > > ... > > ? > > > > From my reading of the 5.12 perlre docs, there is no mention of moving > the > > cursor along by one position after each match. > > Maybe that is a more general thing I wasn't aware of. > > It's not as simple as moving by one after each match; when it goes to > match it won't allow a 0-length match at the same position as the > previous 0-length match. > > > http://perldoc.perl.org/5.12.5/perlre.html#Repeated-Patterns-Matching-a-Zero-length-Substring >
Re: Regex lookahead example not as stated in Camel 4th
On Wed, Jun 19, 2013 at 7:28 AM, Bob MacCallum wrote: > so, forgetting the typo for a moment, why doesn't it output > 012 > 012 > 012 > 012 > ... > ? > > From my reading of the 5.12 perlre docs, there is no mention of moving the > cursor along by one position after each match. > Maybe that is a more general thing I wasn't aware of. It's not as simple as moving by one after each match; when it goes to match it won't allow a 0-length match at the same position as the previous 0-length match. http://perldoc.perl.org/5.12.5/perlre.html#Repeated-Patterns-Matching-a-Zero-length-Substring
Re: Regex lookahead example not as stated in Camel 4th
Bob MacCallum wrote: > From my reading of the 5.12 perlre docs, there is no mention of moving the > cursor along by one position after each match. > Maybe that is a more general thing I wasn't aware of. Yes, though only when the match was zero-length. This is done on the grounds that an infinite loop would be a relatively unhelpful outcome in such situations. The documentation for this feature in 5.12 is here: https://metacpan.org/module/LBROCARD/perl-5.12.4/pod/perlre.pod#Repeated-Patterns-Matching-a-Zero-length-Substring but I suggest reading the latest version, as it's been expanded a little (without any change to the underlying feature): https://metacpan.org/module/RJBS/perl-5.18.0/pod/perlre.pod#Repeated-Patterns-Matching-a-Zero-length-Substring -- Aaron Crane ** http://aaroncrane.co.uk/
Re: Regex lookahead example not as stated in Camel 4th
On 2013-06-19 Bob MacCallum wrote: > so, forgetting the typo for a moment, why doesn't it output > 012 > 012 > 012 > 012 > ... > ? > > From my reading of the 5.12 perlre docs, there is no mention of > moving the cursor along by one position after each match. Maybe > that is a more general thing I wasn't aware of. L Summary: since C would be an infinite loop, Perl breaks it for you, advancing the C by one after each (0-len) match. -- Dakkar - GPG public key fingerprint = A071 E618 DD2C 5901 9574 6FE2 40EA 9883 7519 3F88 key id = 0x75193F88 Beware of Programmers who carry screwdrivers. -- Leonard Brandwein signature.asc Description: PGP signature
Re: Regex lookahead example not as stated in Camel 4th
I was thinking you'd have to use something like this
perl -wE 'say for "0123456789" =~ /(\d(?=(\d{2})))/g'
0
12
1
23
2
34
3
45
4
56
5
67
6
78
7
89
obviously with some post-processing required.
On Wed, Jun 19, 2013 at 3:28 PM, Bob MacCallum wrote:
> so, forgetting the typo for a moment, why doesn't it output
> 012
> 012
> 012
> 012
> ...
> ?
>
> From my reading of the 5.12 perlre docs, there is no mention of moving the
> cursor along by one position after each match.
> Maybe that is a more general thing I wasn't aware of.
>
>
>
>
>
>
> On Wed, Jun 19, 2013 at 3:02 PM, gvim wrote:
>
>> On 19/06/13 14:52, Abigail wrote:
>>
>>
>>> That's not a lookahead assertion. This is:
>>>
>>>$ perl -wE 'say for "0123456789" =~ /(?=(\d{3}))/g'
>>>012
>>>123
>>>234
>>>345
>>>456
>>>567
>>>678
>>>789
>>>$
>>>
>>>
>>>
>> So there's a typo on p.248
>>
>> gvim
>>
>
>
Re: Regex lookahead example not as stated in Camel 4th
so, forgetting the typo for a moment, why doesn't it output
012
012
012
012
...
?
>From my reading of the 5.12 perlre docs, there is no mention of moving the
cursor along by one position after each match.
Maybe that is a more general thing I wasn't aware of.
On Wed, Jun 19, 2013 at 3:02 PM, gvim wrote:
> On 19/06/13 14:52, Abigail wrote:
>
>
>> That's not a lookahead assertion. This is:
>>
>>$ perl -wE 'say for "0123456789" =~ /(?=(\d{3}))/g'
>>012
>>123
>>234
>>345
>>456
>>567
>>678
>>789
>>$
>>
>>
>>
> So there's a typo on p.248
>
> gvim
>
Re: Regex lookahead example not as stated in Camel 4th
On Wed, Jun 19, 2013 at 03:02:32PM +0100, gvim wrote: > On 19/06/13 14:52, Abigail wrote: > > > That's not a lookahead assertion. > > So there's a typo on p.248 Yes, and it's listed in the errata on the publisher's Web site: http://oreilly.com/catalog/errata.csp?isbn=9780596004927 Tom
Re: Regex lookahead example not as stated in Camel 4th
Quoting gvim :
On 19/06/13 14:52, Abigail wrote:
That's not a lookahead assertion. This is:
$ perl -wE 'say for "0123456789" =~ /(?=(\d{3}))/g'
012
123
234
345
456
567
678
789
$
So there's a typo on p.248
Patches welcome - http://oreilly.com/catalog/errata.csp?isbn=9780596004927
Dave...
Re: Regex lookahead example not as stated in Camel 4th
On 19/06/13 14:52, Abigail wrote:
That's not a lookahead assertion. This is:
$ perl -wE 'say for "0123456789" =~ /(?=(\d{3}))/g'
012
123
234
345
456
567
678
789
$
So there's a typo on p.248
gvim
Re: Regex lookahead example not as stated in Camel 4th
I'd say typo in the book. A "look-ahead assertion" looks like C<< (?=
) >> (note C<=> instead of C<:>). C<< (?: ) >> is just a
non-capturing group.
$ perl -E 'say for "0123456789" =~ /(?=(\d{3}))/g'
012
123
234
345
456
567
678
789
--
Dakkar -
GPG public key fingerprint = A071 E618 DD2C 5901 9574
6FE2 40EA 9883 7519 3F88
key id = 0x75193F88
Storms beget storms. Rage begets rage.
Revenge begets revenge. Wars beget wars.
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Description: PGP signature
Re: Regex lookahead example not as stated in Camel 4th
On Wed, Jun 19, 2013 at 02:36:14PM +0100, gvim wrote:
> The 4th edition of the Camel states:
>
> *
> "0123456789" =~ /(\d{3})/g
>
> returns only three strings: 012, 345, and 678. By wrapping the capture
> group with a lookahead assertion:
>
> "0123456789" =~ /(?:(\d{3}))/g
>
> you now retrieve all of 012, 123, 234, 345, 456, 567, 678, and 789.
>
> **
>
> Howevery, my test with perl 5.14.2 on Ubuntu 13.04 does not concur:
>
> $ perl -E 'say for "0123456789" =~ /(\d{3})/g'
> 012
> 345
> 678
>
> $ perl -E 'say for "0123456789" =~ /(?:(\d{3}))/g'
> 012
> 345
> 678
That's not a lookahead assertion. This is:
$ perl -wE 'say for "0123456789" =~ /(?=(\d{3}))/g'
012
123
234
345
456
567
678
789
$
Regards,
Abigail
Regex lookahead example not as stated in Camel 4th
The 4th edition of the Camel states:
*
"0123456789" =~ /(\d{3})/g
returns only three strings: 012, 345, and 678. By wrapping the capture
group with a lookahead assertion:
"0123456789" =~ /(?:(\d{3}))/g
you now retrieve all of 012, 123, 234, 345, 456, 567, 678, and 789.
**
Howevery, my test with perl 5.14.2 on Ubuntu 13.04 does not concur:
$ perl -E 'say for "0123456789" =~ /(\d{3})/g'
012
345
678
$ perl -E 'say for "0123456789" =~ /(?:(\d{3}))/g'
012
345
678
gvim
