title: Summary of differences in regular expression syntax for various tools and languages?
body: <p>I can never remember the differences in regular expression syntax used by tools like <a href="http://en.wikipedia.org/wiki/Grep" rel="noreferrer">grep</a> and <a href="http://en.wikipedia.org/wiki/AWK" rel="noreferrer">AWK</a>, or languages like Python and PHP. Generally, Perl has the most expansive syntax, but I'm often hamstrung by the limitations of even <code>egrep</code> ("extended" <code>grep</code>).</p>
  
  <p>Is there a site that lists the differences in a concise and easy-to-read fashion?</p>