class Rubocop::Cop::Style::HashSyntax

A separate offence is registered for each problematic pair.
when the 1.9 syntax is applicable as well.
This cop checks for uses of the Ruby 1.8 hash literal syntax,

def on_hash(node) 

def on_hash(node)
  pairs = *node
  sym_indices = pairs.all? { |p| word_symbol_pair?(p) }
  if sym_indices
    pairs.each do |pair|
      if pair.loc.operator && pair.loc.operator.is?('=>')
        add_offence(:convention,
                    pair.loc.expression.begin.join(pair.loc.operator),
                    MSG)
      end
    end
  end
  super
end

def word_symbol_pair?(pair) 

def word_symbol_pair?(pair)
  key, _value = *pair
  if key.type == :sym
    sym_name = key.to_a[0]
    sym_name =~ /\A\w+\z/
  else
    false
  end
end