allowed when using the Perl regular expression syntax. This change has
been added for Perl compatibility, when the new [syntax_option_type]
['no_empty_expressions] is set then the old behaviour is preserved and
empty expressions are prohibited. This is issue
[@https://svn.boost.org/trac/boost/ticket/1081#1081].
Fixes#1081.
[SVN r50374]
Add optional support for marked sub-expression location information.
Add support for ${n} in format replacement text.
Fixes#2556.
Fixes#2269.
Fixes#2514.
[SVN r50370]
