Detect if a regexp is exponential

Question

This article show that there is some regexp that is O(2^n) when backtracking. The example is (x+x+)+y. When attempt to match a string like xxxx...p it going to

Answer 1

You could detect and reject nested repetitions using a regex parser, which corresponds to a star height of 1. I've just written a module to compute and reject start heights of >1 using a regex parser from npm.

$ node safe.js '(x+x+)+y'
false
$ node safe.js '(beep|boop)*'
true
$ node safe.js '(a+){10}'
false
$ node safe.js '\blocation\s*:[^:\n]+\b(Oakland|San Francisco)\b'
true