I must say I have been puzzled too..... I have now run a test with an adaption of the short code I posted and got these results (I'll stick my test code at the bottom, its rough as a badgers arse) 

root@tivpre-master:/home/robinp # ./regexps
with eval "optimisation"
timethis 5000: 56 wallclock secs (52.42 usr +  0.00 sys = 52.42 CPU)
naive
timethis 5000: 16 wallclock secs (15.63 usr +  0.00 sys = 15.63 CPU)

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however the other results I posted were for real life code where I do get a magnitude better performance using the eval method. Unfortunately I have written the code for a client and can post neither it or the regex I am using (perhaps I can sanitise a couple of example regex). I suspect that the more complex regex I use and the large number make the saving in re-compilation overcome the cost of running eval. For simpler cases and less regex doing an eval probably costs more than the recompiling overhead.

Cheers,
R.

# foul dirty test code
#!/usr/local/bin/perl -w
use strict;
use integer;
use Benchmark;

my ($i, $compile_me, @names)=(1, "{my \@matches;", "no match");
my $count=5000;

my @DATA=(<DATA>);

foreach (@DATA) {
    next if /^\s*$/;
    last if /END CONFIG/;
    chomp;
    my ($name, $reg)=split;
    push @names, $name;
    $compile_me.="push \@matches, $i if /$reg/;";
    $i++;
}
$compile_me.="\@matches}";

print "with eval\n";
timethis ($count, \&with_eval);

sub with_eval {
    my ($data, $niks);
    foreach (@DATA) {
        next unless $data or (/END CONFIG/ and $data++);
        next if /^\s*$/;
        chomp;
        # print "\nmatches found for $_\n";
        my @matches=eval $compile_me;
        foreach (@matches) {
            # print $names[$_] , "\n"
            # we would do something here
            $niks=time;
        }
    }
}

my (@regex);
@names=();
foreach (@DATA) {
    next if /^\s*$/;
    last if /END CONFIG/;
    chomp;
    my ($name, $reg)=split;
    push @names, $name;
    push @regex, $reg;
}
print "naive\n";
timethis ($count, \&naive);
sub naive {
    my ($data, $niks);
    foreach my $line (@DATA) {
        next unless $data or ($line=~/END CONFIG/ and $data++);
        next if $line=~/^\s*$/;
        chomp $line;
        $i=0;
        # print "\nmatches found for $line\n";
        foreach (@regex) {
            # we would do something here
            # print $names[$i] , "\n" if $line=~/$_/;
            $niks=time if $line=~/$_/;
            $i++;
        }
    }
}

__DATA__
Fred_and_Friends fr.d
Paul_and_co     paul
some_numbers    \d{2}
freud_likes_fred        fr
complex         \d+.{2}\s*[r-x]+Thing\S

END CONFIG

freud
fred
NaNa
pauline
12312sdfsdf
2
34 xrstThingFlaFla
asdfasdf
asdgfasdfg
erwtwertert
wertewr
t asdf asdf sd fsd fasdh werhwearfuisa '[puf 768a\r4iluf7283 4
werter
twertewr
terwtewrtwer er woriqy rt 3245 44ugr qg5 t53q4iu 3q4a5
3qtr54
asdfo sdf sdf7-8afawbefagf we  r9 yeffawe
 dfhoa;ef8347r  94y3rgfawer89 7 ewr7 8yawe t

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In reply to Re^6: Matching against list of patterns by Random_Walk
in thread Matching against list of patterns by Eyck

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