> This seems like a quibble, but putting the emphasis on "using an appropriate engine" in parentheses at the end is burying the lede. Sticking to regular langua

> This seems like a quibble, but putting the emphasis on "using an appropriate engine" in parentheses at the end is burying the lede. Sticking to regular languages will not, in itself, help. Yes. I attempted to make this point at the end of my article, while avoiding phrases like "regular languages". > There are quite a number of backreferences out there that work this way Yes, this appears to be a common usage of backreferences. In the regex corpus from the FSE'19 Lingua Franca paper, many backreference-using regexes are of this form. I believe a loose bound on the worst-case time complexity for regexes with backreferences is n^{mX} where X is the number of backreferenced capture groups. The goal is to describe the number of distinct values the capture groups can take on. If a capture group can take on arbitrary sub-strings (e.g. .(.)\1) then m=2; if it is fixed but ambiguous (e.g. .*(')\1) then m=1; if it is unambiguous then m=0. Often still a polynomial though. The exception proves the rule; I've seen regexes with 20 backreferences, many of them ambiguous, but the vast majority cover simple use cases and are quite sane.