[shootout/regex-dna] add version that works on Rakudo

the .subst() call is what causes the excessive memory consumption
of the original version

shootout/regex-dna.p6-v2.pl

@@ -0,0 +1,26 @@
+use v6;
+
+my $input = slurp;
+my $data = $input.lines.grep({ $_ !~~ /^ \>/}).join.lc;
+
+say $_ ~ ' ' ~ +$data.comb($_) for
+    /agggtaaa|tttaccct/             but 'agggtaaa|tttaccct',
+    /<[cgt]>gggtaaa|tttaccc<[acg]>/ but '[cgt]gggtaaa|tttaccc[acg]',
+    /a<[act]>ggtaaa|tttacc<[agt]>t/ but 'a[act]ggtaaa|tttacc[agt]t',
+    /ag<[act]>gtaaa|tttac<[agt]>ct/ but 'ag[act]gtaaa|tttac[agt]ct',
+    /agg<[act]>taaa|ttta<[agt]>cct/ but 'agg[act]taaa|ttta[agt]cct',
+    /aggg<[acg]>aaa|ttt<[cgt]>ccct/ but 'aggg[acg]aaa|ttt[cgt]ccct',
+    /agggt<[cgt]>aa|tt<[acg]>accct/ but 'agggt[cgt]aa|tt[acg]accct',
+    /agggta<[cgt]>a|t<[acg]>taccct/ but 'agggta[cgt]a|t[acg]taccct',
+    /agggtaa<[cgt]>|<[acg]>ttaccct/ but 'agggtaa[cgt]|[acg]ttaccct';
+
+say;
+
+my %iub = 'b' => '(c|g|t)', 'd' => '(a|g|t)', 'h' => '(a|c|t)',
+          'k' => '(g|t)',   'm' => '(a|c)',   'n' => '(a|c|g|t)',
+          'r' => '(a|g)',   's' => '(c|g)',   'v' => '(a|c|g)',
+          'w' => '(a|t)',   'y' => '(c|t)';
+
+my $output = $data.comb.map({ %iub{$^c} // $^c }).join;
+
+.chars.say for $input, $data, $output;