Re^3: A regex that only matches at offset that are multiples of a given N?

[tobyink]

Ah, you're using /g... in that case I'd just...

use v5.12;

my $a = join '', map{ ('a'..'j')[rand 10] } 1 .. 1000;

while ($a =~ m[(aa..)]g)
{
    next if pos($a) % 4;
    say "Match at ", pos($a), ": ", $1;
}

It's possible that (?(cond)yes-expr|no-expr) might be able to do what you want, but I've not had much luck with that.

Update: with the hints about pos % 4 below, I've managed to get (?(cond)yes-expr|no-expr) to work. Not sure how well it goes performance-wise in practice, but:

use v5.12;

my $a = join '', map{ ('a'..'j')[rand 10] } 1 .. 1000;;

while ($a =~ m[(?(?{ pos() % 4 })(*F)|(aa..))]g)
{
    say "Match at ", pos($a), ": ", $1;
}

[BrowserUk]

Ah, you're using /g...

Yes. Sorry about failing to mention that. It was late and I was tired {excuse, excuse, excuse...}

in that case I'd just..

That is essentially what I doing (except with index) in Re: Store larg hashes more efficiently (10e6 md5s in 260MB at 4µs per lookup), because I couldn't work out how to get a regex to do it for me. Hence the question.

I'd rather push the check inside the regex engine if I could; and it seems like it should be a reasonable ask; but it has me totally stumped.

Even johngg's solution is flawed. :(

[BrowserUk]

Update: with the hints about pos % 4 below, I've managed to get (?(cond)yes-expr|no-expr) to work. Not sure how well it goes performance-wise in practice,

Hm. I cannot make it work for my application; but on the basis of the failure I think my previous conclusion that putting code in regexes is always going to be dog slow:

C:\test>junk39 -N=1
#! perl -slw
use strict;
use Digest::MD5 qw[ md5 ];
use Benchmark qw[ cmpthese ];

our $data = pack '(Va16)*', map{
    $_, md5( $_ )
} 1 .. 1000; ## 1000 data items

our $N //= -1;

cmpthese $N, {
    a => q[
        my $c = 0;
        for my $i ( 1 .. 2000 ) { ## half should pass; half fail.
            my $iBin = pack( 'V', $i );
            my $md5 = md5( $i );
            my $p = 0;
            while( $p = 1+index $data, $iBin, $p ) {
                next if ( $p - 1 ) % 20;
                ++$c, last if substr( $data, $p+3, 16 ) eq $md5;
            }
        }
        print "a: $c" if $N == 1;
    ],
    b => q[
        my $c = 0;
        for my $i ( 1 .. 2000 ) { ## Finds 10??
            my $iBin = pack( 'V', $i );
            my $md5 = md5( $i );
            while( $data =~ m{\G(?:.{20})*?(?=\Q$iBin\E(.{16}))}g ) {
                 pos( $data ) += 20;
                 ++$c, last if $1 eq $md5;
            }
        }
        print "b: $c" if $N == 1;
    ],
    c => q[
        use re 'eval';
        my $c = 0;
        for my $i ( 1 .. 2000 ) { ## finds none??
            my $iBin = pack( 'V', $i );
            my $md5 = md5( $i );
            while( $data =~ m[(?(?{ pos( $data ) % 20 })(*F)|\Q%iBin\E
+(.{16}))]g ) {
                 ++$c, last if $1 eq $md5;
            }
        }
        print "c: $c" if $N == 1;
    ],
};

__END__
C:\test>junk39 -N=1
a: 1000
b: 10
c: 0
     Rate      c      a      b
c 0.111/s     --   -99%  -100%
a  21.3/s 19117%     --   -68%
b  66.7/s 60113%   213%     --

I also can't get smls' version to work for this either?