print "$_ => ", extract($_)//'invalid', "\n"
	for qw/ xxx x.x.x x12x..x x123x...x x1x2x...x
	x123x.x.x x12x1x ax1x2xbx34x56xc /;

sub extract {
	my ($x) = shift=~/(x.*x.*x)/;
	return unless length($x)%2
		&& substr($x,(length($x)-1)/2,1) eq 'x';
	return $x;
}

__END__

xxx => xxx
x.x.x => x.x.x
x12x..x => x12x..x
x123x...x => x123x...x
x1x2x...x => x1x2x...x
x123x.x.x => x123x.x.x
x12x1x => invalid
ax1x2xbx34x56xc => x1x2xbx34x56x