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

[download]

In
    @caps = () if $caps[0] ne $1;
$1 will be undefined if there is no capture group 1 or it is something like (...)? that fails to match (but an overall match can still occur). Warnings will be generated.

Although it's only compatible back to version 5.6, an alternative would be 

c:\@Work\Perl\monks>perl

use strict;
use warnings;

use Data::Dump qw(pp);

my $cb = sub { print "matched,  captured ", pp @_; };

for my $str ("AB","") {

    for my $re (qr/../, qr/(.)(.)/, qr/(X)?(.)/, qr/XY/, qr/(X)Y/, qr/
+/) {

        print "str <$str>  re $re  ";

        if ( my (@caps) = ($str =~ $re) ) {
          # @caps = () if $caps[0] ne $1;  # reset pseudo capture
            $#caps = $#- - 1;  # reset pseudo capture
            $cb->(@caps);
            }
        else {
            print 'NO match';
            }

        print "\n";

        }

    }

^Z
str <AB>  re (?-xism:..)  matched,  captured ()
str <AB>  re (?-xism:(.)(.))  matched,  captured ("A", "B")
str <AB>  re (?-xism:(X)?(.))  matched,  captured (undef, "A")
str <AB>  re (?-xism:XY)  NO match
str <AB>  re (?-xism:(X)Y)  NO match
str <AB>  re (?-xism:)  matched,  captured ()
str <>  re (?-xism:..)  NO match
str <>  re (?-xism:(.)(.))  NO match
str <>  re (?-xism:(X)?(.))  NO match
str <>  re (?-xism:XY)  NO match
str <>  re (?-xism:(X)Y)  NO match
str <>  re (?-xism:)  matched,  captured ()

[download]

Update: But see haukex's reply about the preference for $#+ versus $#- (@+ versus @-) regex special variables.

Give a man a fish:  <%-{-{-{-<

In reply to Re^5: How to know that a regexp matched, and get its capture groups? by AnomalousMonk
in thread How to know that a regexp matched, and get its capture groups? by Anonymous Monk

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