Make failed capturing parenthesis produce null instead of "".

Make failed capturing parenthesis produce null instead of "".

BUG=2068
Committed: https://code.google.com/p/dart/source/detail?r=5670

[siva, 2012-03-19 18:29:14]

lgtm

http://codereview.chromium.org/9718015/diff/3001/runtime/lib/regexp_jsc.cc
File runtime/lib/regexp_jsc.cc (right):

http://codereview.chromium.org/9718015/diff/3001/runtime/lib/regexp_jsc.cc#ne...
runtime/lib/regexp_jsc.cc:157: const int kMatchPair = 2;
ASSERT(retval <= (num_bracket_expressions + 1));

[ahe, 2012-03-19 20:59:55]

As best as I can tell, this still returns an empty string for unmatched groups
between other matches. For example:

$ ./xcodebuild/Release_ia32/d8 
V8 version 3.4.14.36 [console: dumb]
d8> var re = /(x)(y)?(x)/;
d8> var m = re.exec('xx');
d8> m 
xx,x,,x
d8> m[0]
xx
d8> m[1]
x
d8> m[2]
d8> m[3]
x
d8> typeof m[1]
string
d8> typeof m[2]
undefined
d8> typeof m[3]
string
d8> 

Notice that m[2] is undefined (which would be null in Dart), not an empty
string.

In Dart, this would be:

main() {
  var re = new RegExp('(x)(y)?(x)');
  var m = re.firstMatch('xx');
  print(m[0]);
  print(m[1]);
  print(m[2]);
  print(m[3]);
}

On the VM this currently prints:

xx
x

x

But it should print:

xx
x
null
x

As it does with "frog/frog.py --leg".

http://codereview.chromium.org/9718015/diff/3001/runtime/lib/regexp.dart
File runtime/lib/regexp.dart (right):

http://codereview.chromium.org/9718015/diff/3001/runtime/lib/regexp.dart#newc...
runtime/lib/regexp.dart:54: static final int _kMatchPair = 2;
Nit: we use HACKER_STYLE for constants in Dart.

[cshapiro, 2012-03-19 21:07:55]

Thanks for looking.  I had not considered the case of unmatched groups
intermixed with matched groups.  I will add code to correctly handle this.