title: Extract variable names from the string expression using regex in python
body: <p>I have a algebraic expression in in string. I want to extract the variable names from the expression. The variable names should follow python variable naming rules. (should be combination of letter or numbers, should not start with a number, can have underscore etc)</p>
  
  <p>example:</p>
  
  <pre><code>formula = 'value1 * 5 + value_2 /4'
  </code></pre>
  
  <p>it should give a result of <code>['value1', 'value_2']</code></p>