There is a persistent meme out there that matching regular expressions with back-references is NP-Hard. There is a post about this and the claim is repeated by Russ Cox so this is now part of received wisdom.

None of these claims are false; they just don’t apply to regular expression matching in the sense that most people would imagine (any more than, say, someone would claim, “colloquially” that summing a list of N integers is O(N^2) since it’s quite possible that each integer might be N bits long). It depends on the generally unfamiliar notion that the regular expression being *matched *might be arbitrarily varied to add more back-references.

These constructions rely on being able to add more things to the regular expression as the size of the problem that’s being reduced to ‘regex matching with back-references’ gets bigger.

Suppose, instead, as per more common practice, we are considering the difficulty of matching a fixed regular expressions with one or more back-references against an input of size N.

Is **this** task is in P? That is, is there a polynomial-time algorithm in the size of the input that will tell us whether this back-reference containing regular expression matched?

Note that back-references in a regular expression don’t “lock” – so the pattern /((\wx)\2)z/ will match “axaxbxbxz” (EDIT: sorry, I originally fat-fingered this example). So, sadly, we can’t just enumerate all starts and ending positions of every back-reference (say there are k backreferences) for a bad but polynomial-time algorithm (this would be O(N^2k) runs of our algorithm without back-references, so if we had a O(N) algorithm we could solve it in O(N^(2k+1)). Unfortunately, this construction doesn’t work – the capturing parentheses to which the back-references occur update, and so there can be numerous instances of them.

Note that even a lousy algorithm for establishing that this is possible suffices. So if there’s a construction that shows that we can match regular expressions with k backreferences in O(N^(100k^2+10000)) we’d still be in P, even if the algorithm is rubbish. I’ve read that (I forget the source) that, informally, a lousy poly-time algorithm can often be improved, but an exponential-time algorithm is intractable. So knowing that this problem was in P would be helpful.

So I’m curious – are there any either (a) results showing that *fixed* regex matching with back-references is also NP-hard, or (b) results, possibly the construction of a dreadfully naive algorithm, showing that it can be polynomial?