{ # Place this block at the top of the grammar, before any rules.
   sub dequote_double {
      local $_ = (@_ ? $_[0] : $_);
      $_ = substr($_, 1, -1);
      s/\\(.)/$1/sg;
   }

   # Like in Perl.
   # Unlike bash.
   sub dequote_single {
      local $_ = (@_ ? $_[0] : $_);
      $_ = substr($_, 1, -1);
      s/\\(['\\])/$1/sg;
   }
}

string : /"(?:[^\\"]|\\.)*"/  {[ $item[0], dequote_double($item[1]) ]}
       | /'(?:[^\\']|\\.)*'/  {[ $item[0], dequote_single($item[1]) ]}