The commenter also talked about &quot;RE engines&quot;.<p>Most RE engines that are described as being an NFA (examples include Perl&#x27;s, PCRE and Java&#x27;s) are implemented with backtracking exactly because you can easily add in a little state and suddenly you support backreferences.  Of course the language isn&#x27;t really now a proper regular expression, but everyone knows what you mean when you talk about them.<p>And yes, I&#x27;m well aware of how many real-world regex engines there are without backreferences.  However the ones with the most widespread use, do.