title: Extracting an expression matching a pattern from a large expression
body: <p>I have a Mathematica expression that contains a single square root, schematically</p>
  
  <pre><code>expr = a / (b + Sqrt[c]);
  </code></pre>
  
  <p>where <code>a</code>,<code>b</code>,<code>c</code> are large expressions. I would like to extract the expression under the sqrt, for instance by matching to a pattern, something like</p>
  
  <pre><code>Match[expr,Sqrt[x_]] // should return c
  </code></pre>
  
  <p>Is there an easy way to do this?</p>