Interesting.  This feature was probably implemented without a lot of checking for Matlab compatibility.  It should be relatively easy to return a struct array, rather than a scalar structure.

The file to modify is regexp.cc in libinterp/corefcn.

There are two functions, octregexp and octcellregexp, which probably need modification.  The relevant code for octregexp is shown below.

  // Converted the linked list in the correct form for the return values

  octave_idx_type i = 0;
  octave_scalar_map nmap;

  retval.resize (7);

  if (sz == 1)
    {
      string_vector named_tokens = rx_lst.begin ()->named_tokens ();

      for (int j = 0; j < named_pats.numel (); j++)
        nmap.assign (named_pats(j), named_tokens(j));

      retval(5) = nmap;
    }
  else
    {
      for (int j = 0; j < named_pats.numel (); j++)
        {
          Cell tmp (dim_vector (1, sz));

          i = 0;
          for (octave::regexp::match_data::const_iterator p = rx_lst.begin ();
               p != rx_lst.end (); p++)
            {
              string_vector named_tokens = p->named_tokens ();

              tmp(i++) = named_tokens(j);
            }

          nmap.assign (named_pats(j), octave_value (tmp));
        }

      retval(5) = nmap;
    }

As you can see, in the case of multiple return arguments, it creates a temporary cell and stuffs the results into that cell array, and then assigns to the structure, under the ptn name, the temporary cell array.

Octave's output differs from Matlab's when using the 'names' flag and nothing matches.

Octave:

>> r = regexp('nothing','(?<file>\S+) (?<id>[0-9]+)','names')
r =

  scalar structure containing the fields:

    file = {}(1x0)
    id = {}(1x0)

>> size(r)
ans =

   1   1

and Matlab:

>> r = regexp('nothing','(?<file>\S+) (?<id>[0-9]+)','names')
r =
  0×0 empty struct array with fields:
    file
    id
>> size(r)
ans =
     0     0