$" = '';
$\ = "\n";

chomp( my @edges = map /--/
                       ? join( '', sort /\w/g )
                       : (),
                   <DATA> );
my @nodes = sort { $a cmp $b }
            uniq( map /\w/g,
                  @edges );

for $node ( @nodes ) {
    my $edge_rx = qr/$node/;
    my @other_nodes = map /$edge_rx/
                          ? grep( $_ ne $node, # ne vs gt
                                  split( //, $_ ) )
                          : (),
                      @edges;
    
    use re 'eval';
    my $third_edge
      = qr/([@other_nodes])
           .*
           ([@other_nodes])
           (??{push @TRI, join(" - ",sort($node,$1,$2))})/x;

    # removed the gt. Now nodes are unordered in the triangle. All triangles are visited three times, not just once.
    grep ! /$node/
         && /$third_edge/,
        @edges;
}
print for uniq( sort @TRI );

sub uniq {
    my %seen;
    grep !$seen{$_}++, @_
}
__DATA__
graph triangles {
  A -- B
  A -- C -- F -- I
  A -- D -- G -- J
  A -- E
  B -- C -- D -- E
  B -- F -- H -- J
  B -- I
  E -- G -- H -- I
  E -- J
  I -- J
}

##</code><code>##

$" = '';
$\ = "\n";

chomp( my @edges = map /--/
                       ? join( '', sort /\w/g )
                       : (),
                   <DATA> );
my @nodes = sort { $a cmp $b }
            uniq( map /\w/g,
                  @edges );

for $node ( @nodes ) {
    my $edge_rx = qr/$node/;
    my @other_nodes = map /$edge_rx/
                          ? grep( $_ ge $node, split( //, $_ ) )
                          : (),
                      @edges;
    
    use re 'eval';
    my $node_rx = qr/$node/;
    my $third_edge_rx = qr/[@other_nodes].*[@other_nodes]/;
    my $last_step
      = qr/([@other_nodes])
           .*
           ([@other_nodes])
           (??{print "$node -- $1 -- $2"})/x;

    grep $_ ge $node
         && ! /$node_rx/
         && /$last_step/,
        @edges;
}

sub uniq {
    my %seen;
    grep !$seen{$_}++, @_
}
__DATA__
graph triangles {
  A -- B
  A -- C -- F -- I
  A -- D -- G -- J
  A -- E
  B -- C -- D -- E
  B -- F -- H -- J
  B -- I
  E -- G -- H -- I
  E -- J
  I -- J
}

##</code><code>##

$" = '';
$\ = "\n";

chomp( my @edges = map /--/
                       ? join( '', sort /\w/g )
                       : (),
                   <DATA> );
my @nodes = sort { $a cmp $b }
            uniq( map /\w/g,
                  @edges );

for $node ( @nodes ) {
    my $edge_rx = qr/$node/;
    my @other_nodes =  map /$edge_rx/
                          ? grep( $_ ge $node, split( //, $_ ) )
                          : (),
                      @edges;

    my $last_step = qr/(.).*(.)(?{print "$node -- $1 -- $2"})(?!)/;
    my $node_rx = qr/$node/;
    my $third_edge_rx = qr/[@other_nodes].*[@other_nodes]/;

    grep $_ ge $node
         && ! /$node_rx/
         && /$third_edge_rx/
         && /$last_step/,
        @edges;
}

sub uniq {
    my %seen;
    grep !$seen{$_}++, @_
}
__DATA__
graph triangles {
  A -- B
  A -- C -- F -- I
  A -- D -- G -- J
  A -- E
  B -- C -- D -- E
  B -- F -- H -- J
  B -- I
  E -- G -- H -- I
  E -- J
  I -- J
}

##</code><code>##

$" = '';

chomp( my @edges = grep /\w -- \w/, <DATA> );
my @nodes = sort { $a cmp $b }
            uniq( map /\w/g,
                  @edges );

for my $node ( @nodes ) {
    my $edge_rx = qr/$node -- (\w)/;
    my @other_nodes = map /$edge_rx/
                          ? $1
                          : (),
                      @edges;

    my $third_edge = qr/[@other_nodes] -- [@other_nodes]/;
    for ( grep /$third_edge/, @edges ) {
        my @nodes = sort { $a cmp $b }
                    $node, /\w/g;
        $triangles{"@nodes"} = undef;
    }
}

for ( sort keys %triangles ) {
    print join( " -- ", split // ), "\n";
}

sub uniq {
    my %seen;
    grep !$seen{$_}++, @_
}
__DATA__
graph triangles {
  A -- B
  A -- C
  A -- D
  A -- E
  B -- A
  B -- C
  B -- D
  B -- E
  B -- F
  B -- H
  B -- J
  B -- I
  C -- A
  C -- B
  C -- D
  C -- E
  C -- F
  C -- I
  D -- A
  D -- B
  D -- C
  D -- E
  D -- G
  D -- I
  E -- A
  E -- B
  E -- C
  E -- D
  E -- G
  E -- H
  E -- I
  E -- J
  F -- B
  F -- C
  F -- H
  F -- I
  F -- J
  G -- A
  G -- D
  G -- E
  G -- H
  G -- I
  G -- J
  H -- B
  H -- E
  H -- F
  H -- G
  H -- I
  H -- J
  I -- B
  I -- C
  I -- E
  I -- F
  I -- G
  I -- H
  I -- J
  J -- A
  J -- B
  J -- D
  J -- E
  J -- F
  J -- G
  J -- H
  J -- I
}