title: getting sh: =~: unknown operand in shell scripting
body: <p>I just started doing shell scripts and getting unknown operand error while using <code>regex</code> in if statement. i searched google but did not get anything</p>
  
  <pre><code>IP="172.21.1.1"
  if [[ "$IP" =~ /d ]] ; then
  echo "qqq"
  fi
  </code></pre>
  
  <p>Getting error as </p>
  
  <pre><code>sh: =~: unknown operand
  </code></pre>
  
  <p>Bash version is : BusyBox v1.19.3 (2012-01-31 08:57:52 PST) built-in shell (ash)</p>