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Regular expressions in Perl are far more powerful than most people realize. The regex machine is a natural way of doing backtracking. That it also matches characters is just a free extra. But you can unleash its power by just matching against an empty string.

Below is a program that solves the N-queen problem (place N non-attacking queens on an NxN chess board). It isn't efficient, it could backtrack earlier in many cases, but this keeps the code simple.

Later this week, I hope to post an explanation of how the regex works, but if you print out the regex, it shouldn't be too difficult to figure it out.


#!/usr/bin/perl use strict; use warnings 'all'; use re 'eval'; my $nr_of_queens = $ARGV [0] || 8; my $nr_of_rows = $nr_of_queens; my $nr_of_cols = $nr_of_queens; sub attack { my ($q1, $q2) = @_; my ($q1x, $q1y, $q2x, $q2y) = (@$q1, @$q2); $q1x == $q2x || $q1y == $q2y || abs ($q1x - $q2x) == abs ($q1y - $ +q2y); } my $regex; foreach my $queen (1 .. $nr_of_queens) { local $" = "|\n "; my @tmp_r; foreach my $row (1 .. $nr_of_rows) { push @tmp_r => "(?{local \$q [$queen] [0] = $row})"; } $regex .= "(?:@tmp_r)\n"; my @tmp_c; foreach my $col (1 .. $nr_of_cols) { push @tmp_c => "(?{local \$q [$queen] [1] = $col})"; } $regex .= "(?:@tmp_c)\n"; foreach my $other_queen (1 .. $queen - 1) { $regex .= "(?(?{attack \$q [$other_queen], \$q [$queen]})x|)\n +"; } $regex .= "\n"; } $regex .= "\n"; $regex .= "(?{\@sig = sort map {chr (ord ('a') + \$_ -> [0] - 1) . \$_ + -> [1]}" . " \@q [1 .. $nr_of_queens];})\n"; $regex .= "(?{print qq !\@sig\n!})"; "" =~ /$regex/x;

In reply to Backtracking through the regex world by Abigail-II

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