my @o;
foo();
foo();
foo();

sub foo {
	my $window = "a b X20 c X5 d e X17 X12";
	@::o = ();
	my @m = ($window =~ m/(X\d+(?{push @::o, pos()}))/g);
	print join(" ", "Matches:", @m, "\n");
	print join(" ", "Offsets:", @::o, "\n\n");
}