#!/usr/bin/perl -w -i -n

BEGIN {
  undef $/;
  use strict;
  use vars qw($basename);

  ($basename = $0) =~ s!.*/!!;

  sub usage () {
    return <<USAGE;
Usage:
  $basename file.bib [...]
Takes a set of bibtex files, and sorts them by label.  Duplicate labels
are not eliminated.
USAGE
  }

  if (@ARGV == 0 || $ARGV[0] eq '--help') {
    warn usage;
    exit(0);
  }
}

my %Entries;

while (/^(\@\w+{(.*?),)/mg) {
  my $Entry = $1;
  my $Label = $2;
  my $Count = 1;
  while (/\G(.*?(?<!\\)(?:\\\\)*([{}]))/gs) {
    $2 eq '{' ? $Count++ : $Count--;
    $Entry .= $1;
    last if ($Count == 0);
  }

  if (exists($Entries{$Label})) {
    if (ref($Entries{$Label})) {
      push @{$Entries{$Label}}, $Entry;
    } else {
      $Entries{$Label} = [$Entries{$Label}, $Entry];
    }
  } else {
    $Entries{$Label} = $Entry;
  }
}

foreach my $Label (sort keys %Entries) {
  print +(ref($Entries{$Label}) ?
	  join("\n\n\n", @{$Entries{$Label}}) :
	  $Entries{$Label}), "\n\n\n";
}