The parent comments are using the term &quot;NFA engine,&quot; not &quot;real world engines.&quot; NFA engines indeed cannot support backreferences for exactly the reason you specified. They simply aren&#x27;t powerful enough. (The top comment in this thread mischaracterized regex engines, which is a common confusion.)<p>And there are plenty of &quot;real world&quot; regex engines in wide use that don&#x27;t support backreferences.