title: How to form a regex to recognize correct declaration of variable names
body: <p>I would like to form a regex ro recognize the declaration of a variable name. User will enter a string that they would like as a variable name, and the program has to check whether the variable is valid.</p>
  
  <ol>
  <li>The first character of the variable name must either be alphabet or underscore. It should not start with the digit.</li>
  <li>No commas and blanks are allowed in the variable name.</li>
  <li>No special symbols other than underscore are allowed in the variable name.</li>
  </ol>
  
  <p>I have been trying for the whole day and couldn't get the correct one. </p>