title: What flavour of regular expression is grep?
body: <p>I'm guessing it's not a Perl Compatible Regular Expression, since there's a special kind of <code>grep</code> which is specifically PCRE. What's <code>grep</code> most similar to?</p>
  
  <p>Are there any special quirks of <code>grep</code> that I need to know about? (I'm used to Perl and the <code>preg</code> functions in PHP)</p>