[Octave-bug-tracker] [bug #41211] regexp: groups with '?' matching handled different between matlab and octave

[Michael C. Grant]

URL:
  <http://savannah.gnu.org/bugs/?41211>

                 Summary: regexp: groups with '?' matching handled different
between matlab and octave
                 Project: GNU Octave
            Submitted by: mcgrant
            Submitted on: Sun 12 Jan 2014 09:00:15 PM GMT
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Matlab Compatibility
                  Status: None
             Assigned to: None
         Originator Name: Michael C. Grant
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: dev
        Operating System: Any

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Details:

If a grouping construct is followed by '?', and that group is not matched,
Octave does not return an entry for that token. MATLAB, however, returns an
empty string. 

At first I thought this is probably not important, and I've already fixed it
in my code. Indeed, MATLAB's behavior might be considered *wrong*, since the
empty string will in many cases not be a valid match. 

But here's an example where I think the Octave behavior could be potentially
problematic. In this case, the final grouping is returned as token #2 in
Octave, and token #3 in MATLAB. It seems to me that MATLAB's behavior is
better here. After all, is that second token returned by Octave from group 2
or group 3? To determine this, you would have to do additional tests.

Octave:

octave:1> str = 'test::'
str = test::
octave:2> toks = regexp(str, '^([a-zA-Z]w*)((.*))?(.+)?$', 'tokens' )
toks = 
{
  [1,1] = 
  {
    [1,1] = test
    [1,2] = ::
  }
}


Matlab:

>> str = 'test::'
str =
test::
>> toks = regexp(str, '^([a-zA-Z]w*)((.*))?(.+)?$', 'tokens' )
toks = 
    {1x3 cell}
>> toks{:}
ans = 
    'test'    ''    '::'








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