use v5.12;
use warnings;
# https://perlmonks.org/?node_id=11141581

my $in = "anXwxdXdzd";
my $pi = "1415926535";

my $mask = $in =~ tr/Xa-z/\xff\x00/r;
my $re   = $pi & $mask;
$re=~ tr/\x00/./;

$re = "/bin/grep $re /path/to/some/file";

say $re;


##</code><code>##

/bin/grep ..1...6... /path/to/some/file