title: r regex Lookbehind Lookahead issue
body: <p>I try to extract passages like <code>44.11.36.00-1</code> (precisely, <code>nn.nn.nn.nn-n</code>, where <code>n</code> stands for any number from 0-9) from text in R. </p>
  
  <p>I want to extract passages if they are "sticked" to non-number marks: </p>
  
  <ul>
  <li><code>44.11.36.00-1</code> extracted from <code>nsfghstighsl44.11.36.00-1vsdfgh</code> is OK</li>
  <li><code>44.11.36.00-1</code> extracted from <code>fa0044.11.36.00-1000</code> is NOT</li>
  </ul>
  
  <p>I have read that <code>str_extract_all</code> is not working with <code>Lookbehind</code> and <code>Lookahead</code> expressions, so I sadly came back to <code>grep</code>, but cannot deal with it: </p>
  
  <pre><code>&gt; pattern1 &lt;- "(?&lt;![0-9]{1})[0-9]{2}\\.[0-9]{2}\\.[0-9]{2}\\.[0-9]{2}-[0-9]{1}(?![0-9]{1})"
  &gt; grep(pattern1, "dyj44.11.36.00-1aregjspotgji 44113600-1 agdtklj441136001 ", perl=TRUE, value = TRUE)
  
  [1] "dyj44.11.36.00-1aregjspotgji 44113600-1 agdtklj441136001 "
  </code></pre>
  
  <p>which is not the result I expected. </p>
  
  <p>I thought that: </p>
  
  <ul>
  <li><code>(?&lt;![0-9]{1})</code> means "match expression which is not preceeded by a number"</li>
  <li><code>[0-9]{2}\\.[0-9]{2}\\.[0-9]{2}\\.[0-9]{2}-[0-9]{1}</code> stands for the expression I seek for</li>
  <li><code>(?![0-9]{1})</code>  means "match expression which is not followed by a number"</li>
  </ul>