title: Add some text using Regular Expression
body: <p>I have mobile number series like this</p>
  
  <pre><code>9777358594
  9861312901
  8895210866
  9861415461
  9178146387
  9853805726
  9040663961
  </code></pre>
  
  <p>And I want replace them with this</p>
  
  <pre><code>a,9777358594
  a,9861312901
  a,8895210866
  a,9861415461
  a,9178146387
  a,9853805726
  a,9040663961
  </code></pre>
  
  <p>What is the regular expression to do this?</p>