ocaml-re VS recross-coq

Note that it's not difficult to (lazily or not) build a NFA using derivatives as well (with Antimirov's construction).

[1]: https://github.com/ocaml/ocaml-re/

I'm familiar with standard (compact) regex syntax, but I've been using the above syntax recently in a couple small places. I'm a bit on the fence as to which is "better". The compact syntax is, of course, more compact. I think it's a very similar comparison between APL (which I've not used) and most other common programming languages.

One advantage of the expanded syntax is that it's a bit nicer to incorporate a string variable, e.g. "str some_string" vs. "/#{Regexp.escape(some_string)}/" (to borrow Ruby's syntax).

[1] https://github.com/ocaml/ocaml-re

I'm currently building a couple of regexp engines:

One, that's a formalization[0] in Coq with big-step semantics, which uncommonly has the intersection operator, and includes several equivalence relations and a proof of the pumping lemma, excepting one case (more on that below).

As a learning exercise and for historical reasons, I've also mostly ported Rust Cox's re1 engine to Rust[1], which includes VM matchers in the style of Henry Spencer, Ken Thompson, and Rob Pike. I also plan to port Doug McIlroy's engine[2], which is interesting for having intersection and complement and special handling for sublanguages, all the way down to just concatenation matched with Knuth-Morris-Pratt. I also want to examine the Rust (thanks burntsushi!), RE2, and Plan 9 engines in more depth.

Once I have time to get back to the project, I want to get back to my regular expression crossword puzzle solver. For that, I'm converting the hint regexps to DFAs, that match strings of some fixed length, and concatenating and intersecting them, until a single regexp is yielded, which should be a string literal, if the puzzle has a single solution. For backreferences, it's more tricky, but I plan on rewriting backreferences to the captured expression, where the lengths of both match, then either executing it with a stack like a pushdown automata or constructing a set of constraints on the characters by index.

As an aside: In my proof of the pumping lemma[3], I got stuck on the case for intersection and I'd love insight. Regular languages are closed under intersection, so the pumping lemma should hold for my implementation. I need to prove that if s =~ re1 and s =~ re2 can be pumped, then so can s =~ And re1 re2. My problem is that re1 and re2 split s into different substrings s = s11 ++ s12 ++ s13 = s21 ++ s22 ++ s23, then state that (forall n, s11 ++ repeat s12 n ++ s13 =~ re1) and (forall n, s21 ++ repeat s22 n ++ s23 =~ re2). My intuition is that s11 = s21, s12 = s22, and s13 = s23, because they both match for the intersection, but I'm not convinced of that and haven't been able to formulate a proof for that.

0: https://github.com/thaliaarchi/recross-coq