title: Find fractions strictly not preceded by certain characters
body: <p>I'm trying to find fractions not preceded by a word followed by a /, for example, s2. My  code doesn't work for that one but it's able to capture for other ones. Can you please help modify the regex expression? The expected is given followed by _f.</p>
  <pre><code>s2=&quot;Virus/02/2013&quot;
  s3=&quot;Helium Nitrogen 10/10&quot;
  s4=&quot;Nitrogen Oxygen10/20&quot;
  s5=&quot;glycol,water,oxygen, 10/20/30&quot;
  concat=c(s2,s3,s4,s5)
  stringr::str_replace_all(concat,&quot;(?&lt;!\\/)(?&lt;!\\/)\\d*\\.?\\d+(\\s+)?\\/\\d*\\.?\\d+(\\s+)?(\\/\\d*\\.?\\d+(\\s+)?)?&quot;,&quot;&quot;)
  
  s2_f=&quot;Virus/02/2013&quot;
  s3_f=&quot;Helium Nitrogen&quot;
  s4_f=&quot;Nitrogen Oxygen&quot;
  s5_f=&quot;glycol,water,oxygen&quot;
  </code></pre>