title: Difference between grep, grep -E, grep -P, grep -e
body: <p>I'm a newbie on regular expression and want to do some practicing with the 'grep' command. However, I have noticed that people have been using the following commands for rgx. </p>
  
  <pre><code>grep -E &lt;regular expression pattern&gt;
  grep -P &lt;regular expression pattern&gt;
  grep -e &lt;regular expression pattern&gt;
  grep &lt;regular expression pattern&gt;
  </code></pre>
  
  <p>Can some one tell me what's the difference between them? I have look them up in "man" command and some simple googles, but either is too brief or too overwhelming to understand (e.g. what is 'perl style' or extended regular expression?).</p>