title: How do (*SKIP) or (*F) work on regex?
body: <p>I'm learning an advanced usage of regex and noticed that many posts use <code>(*SKIP)</code> or <code>(*F)</code> in it.</p>
  
  <p>I posted a question where the idea was to match lines that don't have <code>yellow</code> but has <code>blue</code> only if <code>brown</code> exists after blue. And the right answer was:</p>
  
  <pre><code>.*yellow.*(*SKIP)(*F)|^.*\bblue\b(?=.*brown).*$
  </code></pre>
  
  <p>I also have tried lookaround expressions like below but haven't worked for all the cases:</p>
  
  <pre><code>^((?!yellow).)*blue(?=.*brown).*$
  </code></pre>
  
  <p>I had no idea about these <code>(*SKIP)(*F)</code> flags, so the question is, how do these flag works? What do they do? And are there other flags like these?</p>
  
  <p>Thanks.</p>