update

I don't recommend the following code anymore

rather

    if ( my (@caps) = ($line =~ $re) ) {
        no warnings 'uninitialized';
        @caps = () if $caps[0] ne $1;           # reset pseudo capture
+s
        $cb->(@caps);
        last;
    }
[download]

/update

This should be backward compatible

    my (@matches) = ($line =~ $re)
    if (defined $&) {
        $cb->(@matches);
        last;
    }
[download]

use v5.12;
use warnings;

for my $str ("AB","") {
    say "****** str=<$str>";
    for my $re ( qr/../, qr/(.)(.)/, q/XY/, q/(X)Y/, q// ) {
        say "--- re=<$re>";
        my @captures = $str =~ $re;
        if ( defined $& ) {
            say "matched"
        } else {
            say "no match"
        }
        if (defined $1) {
            say "with captures <@captures>";
        } else {
            say "no captures";
        }
    }
}
[download]

But...

> See "Performance issues" above for the serious performance implications of using this variable (even once) in your code.

perlvar

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

>

    my (@matches) = ($line =~ $re)
    if (defined $&) {
        $cb->(@matches);
        last;
    }
[download]

nope, the real problem is that @matches = (1) if it matched without capture groups in $re

new attempt:

    
    if ( my (@caps) = ($line =~ $re) ) {
        @caps = () if $caps[0] ne $1;           # reset pseudo capture
+s
        $cb->(@caps);
        last;
    }
[download]

Full backwards compatible and no performance penalty.

OK?

Cheers Rolf
_{(addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)
Wikisyntax for the Monastery}