sub foo {
    my $window = "a b X20 c X5 d e X17 X12";
    my( @o, @m );
    while( $window =~ m/(X\d+)/g ) {
        push @m, $1;
        push @o, $+[0];
    }
    
    print "Matches: @m";
    print "Offsets: @o";
    print " ";
}