putting named captures to awesome use

brian_d_foy on 2008-01-18T06:47:00

Well, I think the use I put them to is awesome, but it's part of an IRC interface to something, so there's sort of a maximum cap on awesomeness.

Anyway, here's some slightly simplified code (using MooseX::POE):

my @MSG_PATTERNS = (
  qr/item\s+(?<x>-?\d+)\s*,\s*(?<y>-?\d+)\s*/ => 'item_report',
  qr/grid/                                    => 'grid_summary',
);

event irc_public => sub {
  my ($self, $kernel, $nick, $msg) = @_[OBJECT, KERNEL, ARG0, ARG2];

  for (my $i = 0; $i < @MSG_PATTERNS; $i += 2) {
    my ($pattern, $method) = @MSG_PATTERNS[ $i, $i + 1 ];
    next unless $msg =~ /\A$pattern\z/;
    my $result = $self->$method({%+});
    if (ref $result) {
      $self->privmsg($self->channel => $_) for @$result;
      return;
    } else {
      return $self->privmsg($self->channel => "$nick: $result");
    }
  }
};

The important points are:

1. the regex pattern with named captures like: (?<x>-?\d+)
2. the method call that passes in { %+ }

See, if the pattern matches, all the named captures end up in the magic variable %+. That means that after this line:

"item (3, -1)" =~ qr/item\s+(?<x>-?\d+)\s*,\s*(?<y>-?\d+)\s*/

That variable contains:

(x => 3, y => -1)

Basically, by using named captures, I can allow these rules to set up named arguments for methods! I don't need to futz around with converting from a list of positional matches into anything else. I don't even really need to validate (for this application) because I've already matched them against a validating regular expression. I know this is a very simple application of named captures, but the amount to which it simplified this code is amazing. It turned a tedious three step process (extract, translate, dispatch) into an elegant two step process, eliminating translation.

I look forward to future applications of 5.10's fun new features.