http://www.perlmonks.org?node_id=970770


in reply to numbers masking

$ perl -le'
my @numbers = ( "541321069873210200", "5413 21069873210200" );
my $start_at = 6;
my $end_digits = 4;

for ( @numbers ) {
    print;
    my $digit_count = tr/0-9//;
    my $replace_count = $digit_count - $end_digits;
    my $count = -1;
    s{ ( [0-9] ) }{ ++$count; $count >= $start_at && $count < $replace
+_count ? "X" : $1 }xeg;
    print;
    }
'
541321069873210200
541321XXXXXXXX0200
5413 21069873210200
5413 21XXXXXXXX0200

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In Section Seekers of Perl Wisdom