title: more than one number or 'g' in substitute flags
body: <p>when I use sed in mac OS, I can't use substitute flags like this:</p>
  
  <pre><code>sed 's/xx/yy/3g' my.txt 
  </code></pre>
  
  <p>what's the replacement for this command?
  I cannot use "3g" together here. This use case is in the reference documentation.</p>
  
  <p>I use BSD sed. It show error:</p>
  
  <pre><code>more than one number or 'g' in substitute flags
  </code></pre>