`(?{})`constructs, and I was finally able to understand how Abigail does it..

Then, on a regex kick, I discovered Abigail's pure regex 3-SAT reduction. If you haven't already seen it, it's the coolest couple of lines on the planet. With all this regex excitement, I decided to try my hand at solving some NP-complete problem(s) with pure regexes.

I tried a handful of different NP-complete problems, but after a few emails with MJD, I understood why these attempts went wrong. I kept getting hung up on the string and regex size being exponential on the size of input (not meaning my solution was incorrect, but invalidating its use as an NP-completeness reduction proof). Finally, I think I may have gotten it right with Hamiltonian Circuits. Abigail has done this before using extended regexes, but claimed to be stuck on a pure regex solution that wasn't exponential.

So without further ado, here's my solution. Given a (simple, undirected) graph with E edges and V vertices, it finds a Hamiltonian Circuit (a cycle that visits every vertex exactly once, starting and ending in the same place). The size of the string it creates is bounded by O(V^4), and the size of the regex it creates is bounded by O(V^2).

Now the dirty details.. Here's the value ofmy @E = ([1,3],[1,5],[2,3],[2,4],[2,5],[4,5]); my $V = 5; my $verbose = 1; my @all_edges = map { my $x = $_; map { [$x, $_] } $x+1 .. $V } 1 .. $ +V-1; my $string = (join(' ', 1 .. $V) . "\n") x $V . "\n" . (join(' ', map { join "-", @$_ } @all_edges ) . "\n") x @all_edges . "\n" . (join(' ', map { join "-", @$_ } @E ) . "\n") x $V; my $regex = "^ " . ".* \\b (\\d+) \\b .* \\n\n" x $V . "\\n\n" . join("", map { my ($x, $y) = @$_; ".* \\b (?: \\$x-\\$y | \\$y-\\$x ) \\b .* +\\n\n" } @all_edges) . "\\n\n" . join("", map { my ($x, $y) = ($_, $_+1); ".* \\b (?: \\$x-\\$y | \\$y-\\$x ) \\b .* +\\n\n" } 1 .. ($V-1)) . ".* \\b (?: \\$V-\\1 | \\1-\\$V ) \\b .* \\n \$\n"; print "'$string' =~ /\n$regex\n/x\n" if $verbose; if (my @c = $string =~ /$regex/x) { local $" = " -> "; print "Hamiltonian circuit: [ @c -> $1 ]\n"; } else { print "No Hamiltonian circuit\n"; }

`$string`and

`$regex`for the example graph included:

Here's how it works:$string = q[ 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 1-3 1-5 2-3 2-4 2-5 4-5 1-3 1-5 2-3 2-4 2-5 4-5 1-3 1-5 2-3 2-4 2-5 4-5 1-3 1-5 2-3 2-4 2-5 4-5 1-3 1-5 2-3 2-4 2-5 4-5 ]; $regex = q[^ .* \b (\d+) \b .* \n .* \b (\d+) \b .* \n .* \b (\d+) \b .* \n .* \b (\d+) \b .* \n .* \b (\d+) \b .* \n \n .* \b (?: \1-\2 | \2-\1 ) \b .* \n .* \b (?: \1-\3 | \3-\1 ) \b .* \n .* \b (?: \1-\4 | \4-\1 ) \b .* \n .* \b (?: \1-\5 | \5-\1 ) \b .* \n .* \b (?: \2-\3 | \3-\2 ) \b .* \n .* \b (?: \2-\4 | \4-\2 ) \b .* \n .* \b (?: \2-\5 | \5-\2 ) \b .* \n .* \b (?: \3-\4 | \4-\3 ) \b .* \n .* \b (?: \3-\5 | \5-\3 ) \b .* \n .* \b (?: \4-\5 | \5-\4 ) \b .* \n \n .* \b (?: \1-\2 | \2-\1 ) \b .* \n .* \b (?: \2-\3 | \3-\2 ) \b .* \n .* \b (?: \3-\4 | \4-\3 ) \b .* \n .* \b (?: \4-\5 | \5-\4 ) \b .* \n .* \b (?: \5-\1 | \1-\5 ) \b .* \n $ ]; __OUTPUT__ Hamiltonian circuit: [ 5 -> 4 -> 2 -> 3 -> 1 -> 5 ]

- In the first "paragraph",
`$1`through`$5`each get a vertex assigned to them. This paragraph of`$string`is bounded by O(V^2), and`$regex`by O(V).

- The second "paragraph" is the ugly bit. We have to make sure all the vertices chosen are different. This could have been done by originally choosing
`$1`through`$5`to be any permutation from an exhaustive list of permutations, but such a list would have been exponential in size.

The way I checked is by picking any 5 vertices, then ensuring that they are all pairwise distinct. In`$string`, we have a repeated list of all "valid" (distinct) vertex pairs. In`$regex`, we make sure that every pair of two vertices in {`$1`,..,`$5`} shows up in that list.This isn't exponential, because there are V(V-1)/2 "valid" (distinct) pairs, and V(V-1)/2 pairs to verify. So this paragraph of

`$string`is bounded by O(V^4), and`$regex`by O(V^2).

- The third "paragraph" is where we verify that our distinct vertex set has edges from
`$1`to`$2`... to`$5`and then back to`$1`. This paragraph of`$string`is bounded by O(E*V) = O(V^3), and`$regex`by O(V).

If all these conditions matched, then the regex matches, and`$1`through`$5`must be the vertices of a Hamiltonian Circuit.

There you have it. Your feedback is welcome. I really hope I haven't made any mistakes in my analysis. This code isn't one-tenth as beautiful and elegant as Abigail's 3-SAT reduction, and perhaps it could be improved upon. But hey, we all have to start somewhere `;)`

**Update:** modified code so that `$string` is a little clearer to read. Instead of comma-separating the edge and vertex lists, they are separated by spaces. `$regex` is modified accordingly, matching on `\b` where appropriate, instead of a comma.

blokhead