On 30th of October, Tom Lane committed patch:
Implement lookbehind constraints in our regular-expression engine. A lookbehind constraint is like a lookahead constraint in that it consumes no text; but it checks for existence (or nonexistence) of a match *ending* at the current point in the string, rather than one *starting* at the current point. This is a long-requested feature since it exists in many other regex libraries, but Henry Spencer had never got around to implementing it in the code we use. Just making it work is actually pretty trivial; but naive copying of the logic for lookahead constraints leads to code that often spends O(N^2) time to scan an N-character string, because we have to run the match engine from string start to the current probe point each time the constraint is checked. In typical use-cases a lookbehind constraint will be written at the start of the regex and hence will need to be checked at every character --- so O(N^2) work overall. To fix that, I introduced a third copy of the core DFA matching loop, paralleling the existing longest() and shortest() loops. This version, matchuntil(), can suspend and resume matching given a couple of pointers' worth of storage space. So we need only run it across the string once, stopping at each interesting probe point and then resuming to advance to the next one. I also put in an optimization that simplifies one-character lookahead and lookbehind constraints, such as "(?=x)" or "(?<!\w)", into AHEAD and BEHIND constraints, which already existed in the engine. This avoids the overhead of the LACON machinery entirely for these rather common cases. The net result is that lookbehind constraints run a factor of three or so slower than Perl's for multi-character constraints, but faster than Perl's for one-character constraints ... and they work fine for variable-length constraints, which Perl gives up on entirely. So that's not bad from a competitive perspective, and there's room for further optimization if anyone cares. (In reality, raw scan rate across a large input string is probably not that big a deal for Postgres usage anyway; so I'm happy if it's linear.)
Continue reading Waiting for 9.6 – Implement lookbehind constraints in our regular-expression engine.