title: Why are lookbehind assertions not supported in Javascript?
body: <p>Recently I realized (<em>by some embarrassment</em>) that regex <code>lookbehind assertions</code> <strong><em>were not</em></strong> possible in <code>Javascript</code>. </p>
  
  <p><strong>What is the (factual) reason for the absence for this assertion so seemingly common?</strong> </p>
  
  <p><a href="https://stackoverflow.com/questions/7376238/javascript-regex-look-behind-alternative">I realize there are alternate ways to achieve the same thing perhaps</a>, although Is it the basic semantics at work which forbid the functionality, or what exactly?</p>
  
  <p>It also seems that some <a href="https://regex101.com/r/jR9uU5/1" rel="noreferrer">regex testing tools</a> out there that generate Javascript code from regex patterns seem to ignore this fact — which strikes me as a bit odd.</p>