title: R Regexp - extract number with 5 digits
body: <p>i have a string <code>a</code> like this one:</p>
  
  <blockquote>
    <p>stundenwerte_FF_00691_19260101_20131231_hist.zip</p>
  </blockquote>
  
  <p>and would like to extract the 5-digit number "00691" from it.</p>
  
  <p>I tried using <code>gregexpr</code> and <code>regmatches</code> as well as <code>stringr::str_extract</code>
  but couldn't figute out the right rexexp.
  I came as far as:</p>
  
  <blockquote>
    <p>gregexpr("[:digits{5}:]",a)</p>
  </blockquote>
  
  <p>Which should return 5-digit-numbers and i dont understand how to fix it.<br>
  This does not work propperly :(</p>
  
  <pre><code>m &lt;- gregexpr("[:digits{5}:]",a)
  regmatches(a,m)
  </code></pre>
  
  <p>Thanks for your help in advance!</p>