When you ran your code, you probably got this error:

Eval-group not allowed at runtime, use re 'eval' in regex m/some((?{pr
+int "Hey!";})/ at - line 6.
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use eval 're';
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$regex = qr/(?{print "Hey!";})/x;
$string = "some word";

$string =~ /some($regex)/;
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     qr/STRING/imosx
             This operator quotes (and possibly compiles) its
             STRING as a regular expression.  STRING is
             interpolated the same way as PATTERN in
             "m/PATTERN/".  If "'" is used as the delimiter, no
             interpolation is done.  Returns a Perl value which
             may be used instead of the corresponding
             "/STRING/imosx" expression.

             For example,

                 $rex = qr/my.STRING/is;
                 s/$rex/foo/;

             is equivalent to

                 s/my.STRING/foo/is;

             The result may be used as a subpattern in a match:

                 $re = qr/$pattern/;
                 $string =~ /foo${re}bar/;   # can be interpolated in 
+other patterns
                 $string =~ $re;             # or used standalone
                 $string =~ /$re/;           # or this way

             Since Perl may compile the pattern at the moment of
             execution of qr() operator, using qr() may have
             speed advantages in some situations, notably if the
             result of qr() is used standalone:

                 sub match {
                     my $patterns = shift;
                     my @compiled = map qr/$_/i, @$patterns;
                     grep {
                         my $success = 0;
                         foreach my $pat (@compiled) {
                             $success = 1, last if /$pat/;
                         }
                         $success;
                     } @_;
                 }

             Precompilation of the pattern into an internal
             representation at the moment of qr() avoids a need
             to recompile the pattern every time a match "/$pat/"
             is attempted.  (Perl has many other internal
             optimizations, but none would be triggered in the
             above example if we did not use qr() operator.)
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# Under Construction
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It just needs some repair for syntax problems:

#!/usr/bin/perl

$regex = qr/(?{print "Hey!";}).../;
$string = "some word";

$string =~ s/some($regex)/$1 x 5/e;
print $string,$/;
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Update: The 'quote regex' operator, qr//, precompiles the regex before storing it. That improves performance in the same way as m//o and s///o, but the result is stored rather than scoped as /o is. Precompilation gains nothing if the regex is only used once, but if the regex is to be stored in a variable anyway, it does no harm that I know of.

After Compline,
Zaxo

I added qr// because it's a good thing to do,

Could you explain why you think it's a good thing to do?

Abigail

I'm not convinced that so called "compiled regular expressions" are significantly faster. Here's a benchmark, matching IP addresses. One using the 'variable' approach, and one using compiled regular expressions:

#!/usr/bin/perl

use strict;
use warnings 'all';
 
use Benchmark;
 
use vars qw /$ip_v $ip_re @data/;

my $quad_v   = q  '(?:25[0-5]|2[0-4]\d|1\d\d|\d\d?)';
my $quad_re  = qr '(?:25[0-5]|2[0-4]\d|1\d\d|\d\d?)';
 
my $sep_v    = q  '\.';
my $sep_re   = qr '\.';
 
   $ip_v     = qq "$quad_v$sep_v$quad_v$sep_v$quad_v$sep_v$quad_v";   
+          
   $ip_re    = qr "$quad_re$sep_re$quad_re$sep_re$quad_re$sep_re$quad_
+re";
 
   @data = map {join "." => map {int rand 1000} 1 .. 4} 1 .. 1_000; 
 
timethese -5 => {
    var  =>  'for (@data) {/$ip_v/}',
    re   =>  'for (@data) {/$ip_re/}',
};
__END__
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Running this results in:

Benchmark: running re, var for at least 5 CPU seconds...
        re:  5 wallclock secs ( 5.25 usr +  0.00 sys =  5.25 CPU) @ 46.10/s (n=242)
       var:  5 wallclock secs ( 5.16 usr +  0.00 sys =  5.16 CPU) @ 45.93/s (n=237)

Not what I call a significant win for compiled regular expressions. Perhaps you have examples where the gain is large - I've yet to encounter them.

Abigail

Perhaps you have examples where the gain is large - I've yet to encounter them.

jryan's benchmark code mentioned above can be tweaked with qr for a significant speed optimization. Change the line     my @patterns = ('B.B', 'CB')x10; to     my @patterns = map qr/$_/, ('B.B', 'CB')x10; in the &without subroutine.

Your benchmark doesn't show any support for compiled patterns, and that's due to an optimization I've already described here.

Cheers,
-Anomo

I ran a few benchmarks here on a "contrived" large regex and large dataset; although the speed increase through qr// isn't what you'd call large, it is at least significant. Should be noted, though, that the the so-called "evil" /o was the winner in this case.

At any rate, thats not to say qr// is bad. Personally, I think its great because it encourages modularity and readability in regular expressions.