PCRE: Use backreference in pattern repetition bracket
PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 10:42:20 von Honza Mach
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Hi everybody,
I was wondering, if it is possible to use backreferences in the pattern
repetition bracket operator.
Consider the following string:
my $string =3D "5 abcdefghijklmn";
The number five at the beginning of the string means, that I want to
extract first five characters from the latter part of the string. I
tried the following code, but it doesn`t work:
$string =3D~ s/(\d+)\s+(.{\g1})//;
print "extracted: $1 $2\n";
The desired output would be:
extracted: 5 abcde
It seems, that it is not possible to use backreferences within the
bracket operator (or am I doing something wrong?).
Is there other solution to my problem.
Than you in advance for your time
Regards
Honza Mach
Cesnet
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--=-V4JZtDxx7p7iE1Oc+jBw--
Re: PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 10:56:45 von Shlomi Fish
×Hi Honza,
On Thu, 25 Aug 2011 10:42:20 +0200
Honza Mach wrote:
> Hi everybody,
>=20
> I was wondering, if it is possible to use backreferences in the pattern
> repetition bracket operator.
>=20
> Consider the following string:
>=20
> my $string =3D "5 abcdefghijklmn";
>=20
> The number five at the beginning of the string means, that I want to
> extract first five characters from the latter part of the string. I
> tried the following code, but it doesn`t work:
>=20
> $string =3D~ s/(\d+)\s+(.{\g1})//;
> print "extracted: $1 $2\n";
>=20
> The desired output would be:
>=20
> extracted: 5 abcde
>=20
> It seems, that it is not possible to use backreferences within the
> bracket operator (or am I doing something wrong?).
>=20
I don't think it's possible, because the perl regex engine is not that smar=
t.
> Is there other solution to my problem.
>=20
You can use incremental parsing using \G and /g. See:
http://perl-begin.org/uses/text-parsing/
Regards,
Shlomi Fish
--=20
------------------------------------------------------------ -----
Shlomi Fish http://www.shlomifish.org/
Freecell Solver - http://fc-solve.berlios.de/
Knuth is not God! Typing â=9CGodâ=9D into Google and pressing =E2=
Iâ=99m Feeling Luckyâ=9D
will not lead you to his homepage.
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Re: PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 11:13:37 von Honza Mach
--=-hvjggRTzZhfaoKmTiRl9
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Hi Schlomi,
thank you so much for such a quick response.
On Thu, 2011-08-25 at 11:56 +0300, Shlomi Fish wrote:
> ×Hi Honza,
>=20
> On Thu, 25 Aug 2011 10:42:20 +0200
> Honza Mach wrote:
>=20
> > Hi everybody,
> >=20
> > I was wondering, if it is possible to use backreferences in the pattern
> > repetition bracket operator.
> >=20
> > Consider the following string:
> >=20
> > my $string =3D "5 abcdefghijklmn";
> >=20
> > The number five at the beginning of the string means, that I want to
> > extract first five characters from the latter part of the string. I
> > tried the following code, but it doesn`t work:
> >=20
> > $string =3D~ s/(\d+)\s+(.{\g1})//;
> > print "extracted: $1 $2\n";
> >=20
> > The desired output would be:
> >=20
> > extracted: 5 abcde
> >=20
> > It seems, that it is not possible to use backreferences within the
> > bracket operator (or am I doing something wrong?).
> >=20
>=20
> I don't think it's possible, because the perl regex engine is not that sm=
art.
That`s a pity, but I will walk around it somehow.
> > Is there other solution to my problem.
> >=20
>=20
> You can use incremental parsing using \G and /g. See:
>=20
> http://perl-begin.org/uses/text-parsing/
Thank you for the tip, I will check it out.
> Regards,
>=20
> Shlomi Fish
>=20
Regards
Honza Mach
> --=20
> ------------------------------------------------------------ -----
> Shlomi Fish http://www.shlomifish.org/
> Freecell Solver - http://fc-solve.berlios.de/
>=20
> Knuth is not God! Typing â=9CGodâ=9D into Google and pressing =
â=9CIâ=99m Feeling Luckyâ=9D
> will not lead you to his homepage.
>=20
> Please reply to list if it's a mailing list post - http://shlom.in/reply =
..
>=20
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--=-hvjggRTzZhfaoKmTiRl9--
Re: PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 11:35:10 von Paul Johnson
> > On Thu, 25 Aug 2011 10:42:20 +0200
> > Honza Mach wrote:
> >
> > > Hi everybody,
> > >
> > > I was wondering, if it is possible to use backreferences in the pattern
> > > repetition bracket operator.
> > >
> > > Consider the following string:
> > >
> > > my $string = "5 abcdefghijklmn";
> > >
> > > The number five at the beginning of the string means, that I want to
> > > extract first five characters from the latter part of the string. I
> > > tried the following code, but it doesn`t work:
> > >
> > > $string =~ s/(\d+)\s+(.{\g1})//;
> > > print "extracted: $1 $2\n";
> > >
> > > The desired output would be:
> > >
> > > extracted: 5 abcde
Something like this will work:
say "extracted: ", /^(\d+\s+.{$len})/ if ($len) = /^(\d+)/
Sometimes two passes are are better than one.
--
Paul Johnson - paul@pjcj.net
http://www.pjcj.net
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Re: PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 12:02:37 von Honza Mach
--=-Gw8KP/zW/WZf2sctlsAx
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Hi,
On Thu, 2011-08-25 at 11:35 +0200, Paul Johnson wrote:
> > > On Thu, 25 Aug 2011 10:42:20 +0200
> > > Honza Mach wrote:
> > >=20
> > > > Hi everybody,
> > > >=20
> > > > I was wondering, if it is possible to use backreferences in the pat=
tern
> > > > repetition bracket operator.
> > > >=20
> > > > Consider the following string:
> > > >=20
> > > > my $string =3D "5 abcdefghijklmn";
> > > >=20
> > > > The number five at the beginning of the string means, that I want t=
o
> > > > extract first five characters from the latter part of the string. I
> > > > tried the following code, but it doesn`t work:
> > > >=20
> > > > $string =3D~ s/(\d+)\s+(.{\g1})//;
> > > > print "extracted: $1 $2\n";
> > > >=20
> > > > The desired output would be:
> > > >=20
> > > > extracted: 5 abcde
>=20
> Something like this will work:
>=20
> say "extracted: ", /^(\d+\s+.{$len})/ if ($len) =3D /^(\d+)/
> Sometimes two passes are are better than one.
>=20
Thank you for your advice, however I wanted to do it in one pass because
of the following reasons:
1. I wanted to compile the pattern because of the performance
(I guess it is not possible to compile the pattern, when there are =20
some substitutions of the variables from outside of the RE engine,=20
or am I wrong?)
2. Perhaps I should have mentioned, that the application is parsing the=20
continuos string stream coming from a socket, there are many of=20
these "commands" coming one after each other and there will not=20
always be the complete command present in the buffer, next chunk=20
may come later. I was actually trying to use it this way:
my $string =3D "5 abcde 2 fg 3 hij 6 klm";
while ($string =3D~ s/\s*(\d+)\s+(.{\1})//)
{
print "extracted: $1 $2\n";
}=20
This way the while loop elegantly fails, when the pattern does not=20
match and when the next data arrives, the check is made again.
I was just trying to find the most elegant solution, but if it is not
possible, I will create some work around, that is not the problem.
But thank you all for your time and advice.
Regards
Honza Mach
--=-Gw8KP/zW/WZf2sctlsAx
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--=-Gw8KP/zW/WZf2sctlsAx--
Re: PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 12:54:00 von Paul Johnson
On Thu, Aug 25, 2011 at 12:02:37PM +0200, Honza Mach wrote:
> Hi,
>
> On Thu, 2011-08-25 at 11:35 +0200, Paul Johnson wrote:
> >
> > say "extracted: ", /^(\d+\s+.{$len})/ if ($len) = /^(\d+)/
> > Sometimes two passes are are better than one.
> >
>
> Thank you for your advice, however I wanted to do it in one pass because
> of the following reasons:
>
> 1. I wanted to compile the pattern because of the performance
Ah. Then you have already benchmarked a solution, found it to be too
slow, profiled your code and discovered that this section of the code
was the bottleneck in such a way that compiling the regualar expression
would solve the problem?
Why on earth are you worried about performance? Especially when your
data is coming from a socket. I suggest not worrying about performance
until you need to.
Please don't think I'm geting at you personally here. Perhaps you have
already done this. But in general there is far too much talk about
speed on this list. Make your code correct. Make it nice. Until that
point performance cannot be a concern.
> (I guess it is not possible to compile the pattern, when there are
> some substitutions of the variables from outside of the RE engine,
> or am I wrong?)
See below.
> 2. Perhaps I should have mentioned, that the application is parsing the
> continuos string stream coming from a socket, there are many of
> these "commands" coming one after each other and there will not
> always be the complete command present in the buffer, next chunk
> may come later. I was actually trying to use it this way:
>
> my $string = "5 abcde 2 fg 3 hij 6 klm";
> while ($string =~ s/\s*(\d+)\s+(.{\1})//)
> {
> print "extracted: $1 $2\n";
> }
>
> This way the while loop elegantly fails, when the pattern does not
> match and when the next data arrives, the check is made again.
OK. So try this:
say "extracted: $1 $2" while s/\s*(\d+)(?{$len = $^N})\s+((??{"." x $len}))//
That uses features marked as experimental. It would probably be a good
idea to read perlre to understand them. You can even compile that if
you want to but I'm not sure what difference, if any, you will notice in
doing so.
> I was just trying to find the most elegant solution, but if it is not
> possible, I will create some work around, that is not the problem.
Some might class this as an elegant solution. Others might take the
opposite position.
Good luck,
--
Paul Johnson - paul@pjcj.net
http://www.pjcj.net
--
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Re: PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 13:05:54 von Honza Mach
--=-fUGoA3Hbr88fcH02+5Ln
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Hi,
On Thu, 2011-08-25 at 12:54 +0200, Paul Johnson wrote:
> On Thu, Aug 25, 2011 at 12:02:37PM +0200, Honza Mach wrote:
> > Hi,
> >=20
> > On Thu, 2011-08-25 at 11:35 +0200, Paul Johnson wrote:
> > >=20
> > > say "extracted: ", /^(\d+\s+.{$len})/ if ($len) =3D /^(\d+)/
> > > Sometimes two passes are are better than one.
> > >=20
> >=20
> > Thank you for your advice, however I wanted to do it in one pass becaus=
e
> > of the following reasons:
> >=20
> > 1. I wanted to compile the pattern because of the performance
>=20
> Ah. Then you have already benchmarked a solution, found it to be too
> slow, profiled your code and discovered that this section of the code
> was the bottleneck in such a way that compiling the regualar expression
> would solve the problem?
>=20
> Why on earth are you worried about performance? Especially when your
> data is coming from a socket. I suggest not worrying about performance
> until you need to.
>=20
> Please don't think I'm geting at you personally here. Perhaps you have
> already done this. But in general there is far too much talk about
> speed on this list. Make your code correct. Make it nice. Until that
> point performance cannot be a concern.
Point taken, you are right. I haven`t done any benchmarking so far, but
I wanted it to be as effective as possible, because the system I am
bulding needs to be capable of handling as many events per second as
possible and it seems I got caught in the over-optimizing trap every
developer should avoid ;)
> > (I guess it is not possible to compile the pattern, when there are =
=20
> > some substitutions of the variables from outside of the RE engine,=
=20
> > or am I wrong?)
>=20
> See below.
>=20
> > 2. Perhaps I should have mentioned, that the application is parsing the=
=20
> > continuos string stream coming from a socket, there are many of=20
> > these "commands" coming one after each other and there will not=20
> > always be the complete command present in the buffer, next chunk=20
> > may come later. I was actually trying to use it this way:
> >=20
> > my $string =3D "5 abcde 2 fg 3 hij 6 klm";
> > while ($string =3D~ s/\s*(\d+)\s+(.{\1})//)
> > {
> > print "extracted: $1 $2\n";
> > }=20
> >=20
> > This way the while loop elegantly fails, when the pattern does not=
=20
> > match and when the next data arrives, the check is made again.
>=20
> OK. So try this:
>=20
> say "extracted: $1 $2" while s/\s*(\d+)(?{$len =3D $^N})\s+((??{"." x $=
len}))//
>=20
> That uses features marked as experimental. It would probably be a good
> idea to read perlre to understand them. You can even compile that if
> you want to but I'm not sure what difference, if any, you will notice in
> doing so.
Thank you for the idea, I`ll try that.
> > I was just trying to find the most elegant solution, but if it is not
> > possible, I will create some work around, that is not the problem.
>=20
> Some might class this as an elegant solution. Others might take the
> opposite position.
>=20
> Good luck,
Thanks for the advices
Honza Mach
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--=-fUGoA3Hbr88fcH02+5Ln--
Re: PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 16:01:00 von jwkrahn
Honza Mach wrote:
> Hi everybody,
Hello,
> I was wondering, if it is possible to use backreferences in the pattern
> repetition bracket operator.
>
> Consider the following string:
>
> my $string = "5 abcdefghijklmn";
>
> The number five at the beginning of the string means, that I want to
> extract first five characters from the latter part of the string. I
> tried the following code, but it doesn`t work:
>
> $string =~ s/(\d+)\s+(.{\g1})//;
> print "extracted: $1 $2\n";
>
> The desired output would be:
>
> extracted: 5 abcde
>
> It seems, that it is not possible to use backreferences within the
> bracket operator (or am I doing something wrong?).
>
> Is there other solution to my problem.
If it always involves a single digit followed by a single space you
could use unpack()
$ perl -le'
my $string = "5 abcdefghijklmn";
my ( $num, $extract ) = unpack "A2 X2 A2/a*", $string;
print "$num $extract";
'
5 abcde
John
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Re: PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 19:12:57 von merlyn
>>>>> "Honza" == Honza Mach writes:
Honza> Hi everybody,
In the subject, you mention "PCRE". Are you actually using PCRE, and
not Perl? If so, this mailing list isn't for you, because PCRE is
misnamed: it isn't Perl, and it's not even Perl Compatible. This mailing
list will likely give you Perl answers that have nothing to do with
PCRE.
--
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Smalltalk/Perl/Unix consulting, Technical writing, Comedy, etc. etc.
See http://methodsandmessages.posterous.com/ for Smalltalk discussion
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Re: PCRE: Use backreference in pattern repetition bracket
am 25.08.2011 21:20:06 von derykus
On Aug 25, 7:01=A0am, jwkr...@shaw.ca ("John W. Krahn") wrote:
> Honza Mach wrote:
> > Hi everybody,
>
> Hello,
>
>
>
>
>
>
>
>
>
> > I was wondering, if it is possible to use backreferences in the pattern
> > repetition bracket operator.
>
> > Consider the following string:
>
> > my $string =3D "5 abcdefghijklmn";
>
> > The number five at the beginning of the string means, that I want to
> > extract first five characters from the latter part of the string. I
> > tried the following code, but it doesn`t work:
>
> > $string =3D~ s/(\d+)\s+(.{\g1})//;
> > print "extracted: $1 $2\n";
>
> > The desired output would be:
>
> > extracted: 5 abcde
>
> > It seems, that it is not possible to use backreferences within the
> > bracket operator (or am I doing something wrong?).
>
> > Is there other solution to my problem.
>
> If it always involves a single digit followed by a single space you
> could use unpack()
>
> $ perl -le'
> my $string =3D "5 abcdefghijklmn";
> my ( $num, $extract ) =3D unpack "A2 X2 A2/a*", $string;
> print "$num $extract";
> '
> 5 abcde
>
Or, eliminating the single digit requirement
with split/substr:
( $num, $ss ) =3D split " ", $string;
print "$num ", substr $ss, 0 , $num;
--
Charles DeRykus
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