title: What is 'the regular expression library supplied by [my] system'?
body: <p>The <em>man</em> page for GNU's <em>less</em> utility says the following about searching:</p>
  
  <pre><code>/pattern
      Search forward in the file for the N-th line containing the pattern.  N
      defaults to 1.  The pattern is a regular expression, as recognized by the
      regular expression library supplied by your system.
  </code></pre>
  
  <p>I use <em>less</em> on all sorts of systems: my personal Ubuntu laptop, my CentOS cloud server, under Cygwin at work, etc. I keep wanting to do things like negative look-aheads and other fancy stuff, but I don't know what regex syntax to use. How do I find out?</p>