title: regular expression "?!" with variable
body: <p>I am testing a code for internet crawling.</p>
  <pre><code>def getExternalLinks(bs, excludeUrl):
     externalLinks = []
     #Finds all links that start with &quot;http&quot; that do
     #not contain the current URL
     for link in bs.find_all('a',
        href=re.compile('^(http|www)((?!'+excludeUrl+').)*$')):
        if link.attrs['href'] is not None:
           if link.attrs['href'] not in externalLinks:
              externalLinks.append(link.attrs['href'])
     return externalLinks
  </code></pre>
  <p>I cannot analysis the regular expression <strong>((?!'+excludeUrl+').)</strong> in re.compile('^(http|www)((?!'+excludeUrl+').)*$'))</p>