title: Invert match with regexp
body: <p>With PCRE, how can you construct an expression that will only match if a string is <em>not</em> found.  </p>
  
  <p>If I were using grep (which I'm not) I would want the -v option.</p>
  
  <p>A more concrete example:  I want my regexp to match iff string <code>foo</code> is <em>not</em> in the string.  So it would match <code>bar</code> would but not <code>foobar</code>.</p>