title: regular expression in sed for masking credit card
body: <p>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. </p>
  
  <pre><code> while read data; do
      var1=${#data}
  
      echo "Length is "$var1
      echo $data | sed -e "s/CARD_NUMBER=\[[[:digit:]]\{12}/CARD_NUMBER=\[\*\*\*\*\*\*\*\*/g"
      done
  </code></pre>