title: Using a string variable in expression for R for subscript/superscript
body: <p>We can use subscript or superscript in a plot title in R using the following</p>
  
  <pre><code>plot(1,1, main=expression('title'^2))  #superscript
  
  plot(1,1, main=expression('title'[2])) #subscript
  </code></pre>
  
  <p>However, what if I want to use a string variable in expression. For example,</p>
  
  <pre><code>my_string="'title'^2"
  
  plot(1,1, main=expression(my_string))
  </code></pre>
  
  <p>Clearly, this doesn't work and the plot title just becomes my_string rather than  title^2.</p>
  
  <p>Is it possible to use a string variable inside expression?</p>
  
  <p>Thanks,
  Brij.</p>