[Patch] Fix regex `nosubs` correctly

[Jonathan Wakely]

On 20/01/14 17:43 -0500, Tim Shen wrote:
>The semantic of `nosubs` should simply be that `let all parentheses
>not be a subexpression and do not capture it`.

It's not clear to me whether the standard says that nosubs implies
mark_count() == 0 or that the mark count remains the same, but no
sub-expression matches are stored in the results.

What does Boost.Regex do?