use strict;
use warnings;

$_ = 'stuffABC Xmorestuff';
if (/((A(?:B(?:C(?:\s+(?:X(?:Y(?:Z)?)?)?)?)?)?))/) {
    print "Searched for 'ABC\\s+XYZ' but only found $1";
}

##</code><code>##

use strict;
use warnings;

$_ = 'stuAffABC XmoresABtuff';
my $longest = '';
while (/((A(?:B(?:C(?:\s+(?:X(?:Y(?:Z)?)?)?)?)?)?))/g) {
    $longest = $1 if length($1) > length($longest);
}
print "Searched for 'ABC\\s+XYZ' but only found '$longest'\n";
__END__
Searched for 'ABC\s+XYZ' but only found 'ABC X'