title: Meaning of regular expressions like - \\d , \\D, ^ , $ etc
body: <p>What do these expressions mean? Where can I learn about their usage?</p>
  
  <pre><code>\\d 
  \\D 
  \\s 
  \\S 
  \\w 
  \\W
  \\t 
  \\n 
  ^   
  $   
  \   
  |  etc..
  </code></pre>
  
  <p>I need to use the <code>stringr</code> package and i have absolutely no idea how to use these .</p>