title: Extracting specific elements from a character vector
body: <p>I have a character vector </p>
  
  <pre><code>a=c("Mom", "mother", "Alex", "Betty", "Prime Minister")
  </code></pre>
  
  <p>I want to extract words starting with "M" only (upper and lower both)</p>
  
  <p>How to do this?</p>
  
  <p>I have tried using <code>grep()</code>, <code>sub()</code> and other variants of this function but I am not getting it right.</p>
  
  <p>I expect the output to be a character vector of "Mom" and "mother"</p>