Question: We need to mask credit card numbers.Masking all but last 4 digits. I am trying to use SED. As credit card number length varies from 12 digits to 19,I am trying to write regular expression.Following code will receive the String. If it contains String of the form "CARD\_NUMBER=3737291039299199", it will mask first 12 digits. 
Problem is how to write regular expression for credit card-12 to 19 digits long? If I write another expression for 12 digits, it doesn't work.that means for 12 digit credit card- first 8 digits should be masked. for 15 digit credit card, first 11 digits should be masked. 

```
 while read data; do
    var1=${#data}

    echo "Length is "$var1
    echo $data | sed -e "s/CARD_NUMBER=\[[[:digit:]]\{12}/CARD_NUMBER=\[\*\*\*\*\*\*\*\*/g"
    done

```

Answer: 
This does it in one `sed` command without an embedded newline:

```
sed -r 'h;s/.*([0-9]{4})/\1/;x;s/CARD_NUMBER=([0-9]*)([0-9]{4})/\1/;s/./*/g;G;s/\n//'

```

If your `sed` doesn't have `-r`:

```
sed 'h;s/.*\([0-9]\{4\}\)/\1/;x;s/CARD_NUMBER=\([0-9]*\)\([0-9]\{4\}\)/\1/;s/./*/g;G;s/\n//'

```

If your `sed` needs `-e`:

```
sed -e 'h' -e 's/.*\([0-9]\{4\}\)/\1/' -e 'x' -e 's/CARD_NUMBER=\([0-9]*\)\([0-9]\{4\}\)/\1/' -e 's/./*/g' -e 'G' -e 's/\n//'

```

Here's what it's doing:

* duplicate the number so it's in pattern space and hold space
* grab the last four digits
* swap them into hold space and the whole number into pattern space
* grap all but the last four digits
* replace each digit with a mask character
* append the last four digits from hold space to the end of the masked digits in pattern space (a newline comes along for free)
* get rid of the newline
I'm not much of a `sed` guru, and thus I cannot manage to do it in only one command, though there surely are ways. But with two sed commands, here is what I got:

```
sed -e 's/CARD_NUMBER=\([0-9]*\)\([0-9]\{4\}\)/\1\
\2/' | sed -e '1s/./x/g ; N ; s/\n//'

```

*Please note the embedded newline*.

Because sed works by lines, I first break the card number into the initial part and the last four digits, separating them by a newline (the first sed command). Then, I mask the initial part (`1s/./x/g`), and remove the new line (`N ; s/\n//`).

Good luck!