Evaluating code in a regex at runtime: (??{...})

Dear Monks,

perlre says this:

(??{ code })

This is a "postponed" regular subexpression. The code is evaluated at run time, at the moment this subexpression may match. **The result of evaluation is considered a regular expression and matched as if it were inserted instead of this construct.**

Then perlre says:

Note that this means that the contents of capture groups defined inside an eval'ed pattern are not available outside of the pattern...

...Thus,
('a' x 100)=~/(??{'(.)' x 100})/
[download]

will match, it will not set $1.

Huh? I checked, and in fact $1 is not set. So in my opinion, the last quoted line should say:

Despite the regex insertion, *confoundingly* the capture groups inside this construct are not available after the insertion.

In other words, I don't see how by inserting the resulting regex instead of this construct it follows that the capture groups in the regex won't be available, which perlre seems to think is self-evident.

Comment on Evaluating code in a regex at runtime: (??{...})
Download Code

Re: Evaluating code in a regex at runtime: (??{...})
by Kenosis (Deacon) on Jan 28, 2013 at 00:15 UTC

Deleted... Misunderstood the issue...

[reply]

Re: Evaluating code in a regex at runtime: (??{...})
by dave_the_m (Parson) on Jan 28, 2013 at 01:12 UTC

/(....)...(...)...(??{foo()})....(...)/
 ^        ^                     ^
 1        2                     3
[download]

Each time the pattern is run, foo() might return a string or qr// which has a different number of captures; and which might recursively also include /(??{...})/ and further nested captures. This would make it nearly impossible to guess how to refer to the final capture in the outer pattern, unless we just always call it $3.

Dave.

[reply]
[d/l]

Re: Evaluating code in a regex at runtime: (??{...})
by Athanasius (Vicar) on Jan 28, 2013 at 02:33 UTC

Note that the contents of the match are still accessible via a named capture group:

#! perl
use Modern::Perl;

('a' x 100) =~ /(?<match>(??{'(.)' x 100}))/;

say $+{match};
[download]

Output:

12:28 >perl 504_SoPW.pl
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

12:28 >
[download]

See Capture groups.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

[reply]
[d/l]
[select]

Re^2: Evaluating code in a regex at runtime: (??{...})

by LanX (Monsignor) on Jan 28, 2013 at 07:34 UTC

> Note that the contents of the match are still accessible via a named capture group:

> ... ('a' x 100) =~ /(?<match>(??{'(.)' x 100}))/;

ehm ... what you're showing also works with unnamed capture groups:

  DB<113> ('a' x 10) =~ /((??{'(.)' x 10}))/; print "<$1>"
 => 1
<aaaaaaaaaa>
[download]

The OP's point was about dynamically generated capture groups, but yours is outside the embedded Perl code.

Cheers Rolf

[reply]
[d/l]
[select]

Re^3: Evaluating code in a regex at runtime: (??{...})

by Athanasius (Vicar) on Jan 28, 2013 at 08:44 UTC

Yes, you’re right. So in dave_the_m’s example, all we need to do is add another set of parentheses:

/(....)...(...)...((??{foo()}))....(...)/
 ^        ^       ^                ^
 1        2       3                4
[download]

and the dynamically generated capture group is assigned to $3. For example:

use Modern::Perl;

my $str  = 'abcd$$$$$ef';
my @caps = $str =~ /(a)b(c)d((??{'(.)' x 5}))e(f)/;

say join(', ', @caps);
[download]

Output:

18:33 >perl 504_SoPW.pl
a, c, $$$$$, f

18:38 >
[download]

What we (apparently) can’t get is multiple ~~capture groups~~ matches assigned to different capture groups (whether named or unnamed) from the same dynamic match. Correct?

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

[reply]
[d/l]
[select]

Re^4: Evaluating code in a regex at runtime: (??{...})

by LanX (Monsignor) on Jan 28, 2013 at 09:09 UTC


laziness, impatience, and hubris
	PerlMonks

Evaluating code in a regex at runtime: (??{...})

The best material for plates (tableware) is: